ProgRAMME

No abstract available.

No abstract available.

No abstract available.

Hard X-ray radiation in magnetic Cataclysmic Variables (mCVs) arises in post-shock regions (PSRs) near magnetic poles of magnetized white dwarfs (WDs). Here, accreted plasma falling along magnetic field lines meets the surface of a WD. Models of the PSRs are simple enough and can describe observed hard X-ray spectra of mCVs, allowing to determine the WD masses and magnetospheric radii in some cases. Distances to most mCVs are also known from Gaia EDR3 and, therefore, their luminosities, mass accretion rates, and magnetic field strengths on the WD surface can be also determined. These values were found before for the brightest 35 intermediate polars (IPs) observed by NuSTAR and Swift/BAT observatories. We added to the list all the mCVs detected in the 105-month BAT Catalogue and performed a joint analysis for all the objects. We estimated the average WD mass to 0.8 +/- 0.2 solar masses, which is in perfect agreement with earlier estimates in the literature. Most of the mCVs were found to have mass accretion rates in the range 10^{-8} – 10^{-9} solar masses per year. We find no evidence for an increase of the mass accretion rate with the orbital period in contradiction with the theoretical predictions. The average value of the magnetic field strength found for the sample is <log B> =6.6^{+0.4}_{-0.4} in MG. We also derived, by using the information about all the CVs in BAT Catalogue, that the local number density of hard X-ray emitting CVs per solar mass is 1.37^{+0.3}_{−0.16} × 10^{−5} and the corresponding luminosity density per solar mass is 8.95^{+0.15}_{−0.1} × 10^{26} erg/s/M_sun. The integrated Galactic ridge X-ray emission and nuclear stellar cluster luminosities computed using these values coincide with the observed values in good accuracy. We also found that IPs dominate at luminosities L > 10^{33} erg/s, whereas non-magnetic CVs and polars are significant at lower luminosities.

This presentation will cover most aspects of my PhD, which has focused on accreting binary systems and will discuss the varying accretion behaviours seen at different magnetic field strengths in these systems. I will begin by giving an overview of GOTO (Gravitational wave Optical Transient Observer), which have been heavily involved in this work. I will then go onto discuss accretion behaviours starting at low magnetism, starting with VY Scl and AM CVn systems. I will then, increasing in magnetism, discuss accretion behaviours in the Intermediate Polar, DW Cnc. Finally, and at the most magnetic end of the spectrum, I will discuss the accretion behaviours seen Polars. The presentation will conclude by comparing and contrasting the behaviour of these systems and which factors affect the accretion behaviours which we see.

I will present a geometrical model for the synchrotron emissions from the AR Sco and RXJ1912 systems. By reproducing the orbit, spin and beat morphology of the optical polarized emission, the model puts constraints on the nature and origin of the synchrotron emission.

I will describe the observational properties of LAMOST J024048.51+195226.9. The system contains the fastest spinning white dwarf and, combined with its magnetic field, the white dwarf ejects gas donated from its companion star. The system is seen at high inclination so that the secondary eclipses the interaction region and the ejected gas is seen in absorption over half the binary orbital period. Recent HST observations will be discussed.

Observational properties of magnetized accreting stars are determined by the magnetic field and rotation rate of the star, properties of the accretion disc, and the processes at the disc-magnetosphere boundary. I will show the results of the 3D MHD simulations of accretion onto a magnetized star at different sizes of the magnetosphere and fastness parameters and will compare the light curves with the light curves of CTTSs. I will also discuss the possible application of models to Dwarf Novae. In the second part of the talk, I will show the properties of the propeller-driven outflows and the application of the propeller model to AE Aqr.

AGNs and black hole X-ray binaries are two sources that exhibit the energetic phenomena known as relativistic jets and disc-winds (BH-XRBs). Many aspects of jet launching, especially the jet-disc connection in these sources, remain poorly understood despite recent observational advances in unraveling the region near the black hole. In this talk, the impact of the underlying accretion disc properties on jet launching will be discussed. In particular, we employ an adaptive mesh refinement GRMHD framework and initialise our simulations with a thin and truncated accretion disc in hydrostatic equilibrium. From our axi-symmetric simulations, we obtain a structured wind comprising of several types including Blandford & Znajek (BZ) jet, Blandford & Payne (BP) disc-wind, and toroidal magnetic field dominated disc-wind. We further discover that the dynamical characteristics of jet and disc-winds including the variability are significantly influenced by the aspect ratio of the underlying accretion disc and inner radius of the accretion disc. In particular, the transition of underlying accretion disc to magnetically arrested disc (MAD) and generation of transient shocks and the their role in governing the variability will be discussed. Additionally, role of truncated accretion disc flows will be described in context of quasi-periodic oscillations from BH-XRBs.

Using equations of modified magnetic hydrodynamics, we developed a three-dimensional MHD model to study the flow structure in close binary stars. The model takes into account the processes of radiation heating and cooling, heating due to current dissipation, as well as magnetic field diffusion. Application of the model both for intermediate polars and polars allows us to clarify main details of the flow structures obtained in previous studies, and to get some new features that were partly confirmed by observations. Authors: Dmitry Bisikalo , Andrey Zhilkin, Andrey Sobolev

The magnetic field is a critical player in the dynamics of accretion disks. I will address two questions: 1. How to study magnetic field in circumstellar accretion discs from observations? 2. How can a magnetic field affect circumstellar accretion disks? To address the first question, I shall discuss polarization studies’ limitations arising from the grain alignment theory and introduce two new ways of magnetic field studies, one employing velocity gradients and another with atomic/ionic ground state alignment. To address the second question, I will describe the effects of turbulent reconnection that a. induces efficient magnetic field diffusion out of the disk, b. changes the topology of the magnetic field from the split monopole to the dipole, significantly reducing the momentum loss of the disk material. Finally, I will describe the synergetic use of velocity gradients and polarization for exploring the accretion of matter on central black holes of nearby galaxies.

An understanding of spin frequency evolution (ν) of neutron stars in the low-mass X-ray binary (LMXB) phase is essential to explain the observed ν-distribution of millisecond pulsars (MSPs), and to probe the stellar and binary physics, including the possibility of continuous gravitational wave emission. I will discuss the crucial effects of transient accretion on the spin evolution of neutron stars. Then, using numerical computations I will conclude that ν can evolve in two distinctly different modes in a way which is counter-intuitive. This implies that the traditional way of ν-evolution computation is inadequate in most cases.

Outburst of a new BeXRB transient Swift J0243.6+6124 in 2017 became one of the brightest ever observed, and has been extensively observed with multiple facilities. With peak luminosity of 10^39 erg/s this system is truly unique, and can be considered as a first Galactic ultra luminous X-ray source. Here we report on observations of Swift J0243.6+6124 with HXMT-Insight and NuSTAR throughout the outburst and into quiescence, which cover a range of over five orders of magnitude in accretion rate. As a result, we find evidence for a radiative-pressure-dominated accretion disc close to the peak of the outburst, a first-ever for strongly magnetised system, and discuss implications of this finding in context of basic properties of the system such as magnetic field measured independently through detection of a cyclotron line. Finally, we briefly discuss the results in relation to pulsating Ultra-luminous X-ray sources found in other galaxies.

Millisecond pulsars in binary systems play a substantial role in the astrophysics of neutron stars. We usually observe them as either radio/gamma-ray pulsars powered by the rotation of their magnetic field or as X-ray pulsars that accrete mass transferred by the companion star. According to a long-standing paradigm, these two mechanisms are mutually exclusive. However, in transitional millisecond pulsars, variations of the mass accretion rate produce swings between a radio pulsar regime and an X-ray pulsar state. These systems showcase the different possible outcomes of the interaction between a quickly spinning magnetized NS and the accretion disk matter as they unfold over timescales accessible to human life. I will review the rich and complex phenomenology unveiled by a decade of observations of transitional millisecond pulsars. In particular, I will focus on the peculiar state achieved when the accretion luminosity is comparable to the pulsar spin-down power. In this context, the recent discovery of optical and UV pulsations from a transitional (and later from an accreting) millisecond pulsar suggests that magnetospheric particle acceleration can proceed even when an accretion disk surrounds a pulsar. The dichotomy between radio and X-ray millisecond pulsars might be less pronounced than commonly assumed, with accretion and rotation-powered processes coexisting under certain circumstances.

The neutron star low-mass X-ray binary SWIFT J1749.4-2807 is an accreting millisecond X-ray pulsar. (AMXP) which underwent a 2-week-long outburst in 2021. We conducted a thorough analysis of all the available X-rays observations, including XMM-Newton, NICER and NuSTAR. The outburst was relatively faint, reaching at the peak only about 1% of the Eddington luminosity and the system was found in the hard state, as typically observed for AMXPs. The significant detection of a blue-shifted Fe XXVI absorption line at ~7 keV indicates weakly relativistic X-ray disc winds, which are typically absent in the hard state of X-ray binaries. The detection of these winds defies typical paradigms of disc winds formation in X-ray binaries and may imply the existence of propeller-driven outflows. I will discuss the discovery in the context of unexpected disc winds and powerful outflows in AMXPs, potentially shedding light on the role of the rapidly spinning magnetic field in powering them.

The recent years have seen significant progress in our understanding of the radiation processes of strongly magnetized (B~10^12 G) neutron stars in X-ray binaries due to both, improvements in the modeling of radiative transfer and through new X-ray observations, including polarization. These are among the few neutron stars where the magnetic field can be directly measured through the Cyclotron Resonance Scattering Features present in their X-ray spectra.I will discuss new observations of these objects, focusing on the pulse profile evolution with luminosity, and present first results of the modeling of the X-ray emission from their accretion columns that includes polarization. For the latter I will focus on the diagnostically important regime of low luminosity accretion, where the X-ray spectrum changes from a power law with an exponential cutoff to a double humped spectrum.

I will report on the X-ray observations of two different samples of neutron-star Be X-ray binaries, which are at the two opposite extremes of matter accretion: the persistent, low-luminosity and long-period pulsars and the transient, short-period pulsars during the type-II outburst phases. I will show how these two classes of sources are characterized by different timing and spectral properties, which allow us to infer how the matter accretion is affected by the NS magnetic field. In the case of the persistent sources, the pulsed fraction does not vary with the photon energy. The spectra show a hot (kT = 1-2 keV) blackbody component which contributes for 30-40 % to the total flux and has a size consistent with the estimated polar-cap size of the NS. In these sources the wide orbit (Porb > 100 d) and the low mass-transfer rate hampers the formation of an accretion disk and the matter is accreted directly from the wind of the companion Be star. The accretion stream is channeled by the magnetic field onto the NS polar caps, but with a rate low enough to make them directly visible. In the case of the transient sources the pulse profile is double peaked and the pulsed fraction increases with the photon energy. The spectra show a soft (kT = 0.1-0.2 keV) blackbody component wide a large size, comparable with the magnetospheric radius; this component is variable along the pulse and contributes only marginally to the total flux. In these cases the narrow orbit (a few days) and the large mass-transfer rate leads to the formation of an accretion disk, which is truncated at the magnetospheric radius. Also in this case the matter accretion is driven by the magnetic field, but with such a high rate that the polar caps are not visible. The the primary X-ray emission has a “fan-beam” geometry and is reprocessed at the inner edge of the accretion disk, thus generating the observed BB component.

I will review the measurement of magnetic fields and accretion in T Tauri stars and how the observational results fit the proposed models of star-disk interaction.

In the framework of the ERC SPIDI project (“Star-Inner Disk-Planets Interactions”, see https://www.spidi-eu.org), whose long-term goal is to search for compact planetary systems orbiting at the inner edge of circumstellar disks around young stars, I will review our current understanding of the magnetospheric accretion process and present the results of recent multi-site campaigns aimed at monitoring the variability of young systems by combining interferometry, spectropolarimetry, high-resolution spectroscopy, and multi-color photometry. I will also highlight the development of MHD and radiative transfer models of the magnetic star-disc interaction region, whose predictions can be directly confronted to observations.

EX Lupi is the touchstone star for the family of EXors. They are Low-Mass Young Stellar Objects (YSOs) undergoing episodic mass-accretion events in short time-scales (few months to years). Astronomers have monitored ExLupi undergoing accretion outbursts several times in the last 7-8 decades. The astrophysics which drives this episodic accretion phenomenon is still not well understood. A few months ago, in March 2022, ExLupi went into its latest outburst. During the evolution of the outburst, for the first time we obtained high cadence, high resolution spectra using HRS on SALT. The spectra showed a plethora of emission lines originating near the magnetosphere. These lines showed multi-component structures, asymmetry and short timescale variability in line profiles. In this talk, I shall present the results from line profile modeling and the inferences we can make about the magnetospheric accretion channel and the inner disk-region. I shall also connect the photometric variability with the spectroscopic variability in terms of stellar hot-spots and clumps in the inflow and outflow channels in the magnetosphere.

Radio pulsars, discovered more than half a century ago, remain one of the profound mysteries of modern astrophysics. There is still no reliable quantitative model explaining pulsar emission mechanisms. However, with the advent of powerful computers, significant progress has been made in understanding physical processes in pulsar magnetospheres giving us hope to solve the problem of pulsar emission mechanism in the foreseeable future. In this talk I will briefly review our current understanding of the physics of pulsar magnetospheres and describe possible ways to a solution of the pulsar emission mechanism problem.

Invited talk about ULX pulsars

Using Ultra-Luminous X-ray (ULX) sources as progenitors of compact binary systems, we estimate the coalescence rate of compact binary systems. ULXs are extra-galactic X-ray sources with apparent luminosity > 10^{39} erg/s. They are thought to be accreting NS or BH. The mechanism of how these systems reach such high luminosity is still unknown. 10^{39} erg/s is above the Edington accretion limit for a 10M_{\odot} BH. There are a few explanations for such high luminosities: (1) NS/BH accreting at super-Eddington accretion rate, (2) highly beamed emission, and (3) emission from an intermediate-mass black hole with a mass of 100M_{\odot} to 10^5M_{\odot} at sub-Eddington accretion. Observation of some ULX systems suggests NSs accreting with beamed emission with massive companions. Eventually, some of these companion stars will become compact objects, forming compact binary systems which are potential progenitors of gravitational waves and short GRBs. Assuming all merging compact binary systems undergo a ULX phase, we simulate a population of binary stars and follow their evolutionary history. We compute the merger rate from the compact binaries that went through a ULX phase.

A talk on the paper https://ui.adsabs.harvard.edu/abs/2022MNRAS.516.1601B/abstract. We report on the significant reflected emission on the super-Eddington state of the ultraluminous X-ray pulsar Swift J0243.6+6124. We perform phase-resolved spectral analysis and find that the reflected emission amplitude changes weakly as the pulsar rotates, and that the direct emission pulsates with larger amplitude. We propose the accretion geometry envisioned in the theoretical works: the neutron star finds itself in the centre of the well formed by the inner edge of the geometrically thick super-Eddington accretion disc truncated by the magnetic field of the pulsar. We discuss the emission geometry and speculate about the dipole magnetic field of the pulsar, favouring a rather low magnitude of the field. Abstract is below: We report the discovery of the bright reflected emission component in the super-Eddington state of the ultraluminous X-ray pulsar Swift J0243.6+6124, based on the NuSTAR observations of the source during its 2017 outburst. The flux of the reflected emission is weakly variable over the pulsar phase while the direct emission shows significantly larger pulsation amplitude. We propose that in this system the neutron star finds itself in the centre of the well formed by the inner edge of the geometrically thick super-Eddington accretion disc truncated by the magnetic field of the pulsar. The aspect ratio of the well is H/R ~ 1. The inner edge of the truncated disc is continuously illuminated by the emission of the accretion column giving rise to the weakly variable reflected emission. As the neutron star rotates, its emission sweeps through the line of sight, giving rise to the pulsating direct emission. From Doppler broadening of the iron line, we measure the truncation radius of the accretion disc ~50 Rg. The inferred dipole component of the magnetic field is consistent with previous estimates favouring a not very strong field. The uniqueness of this system is determined by its moderately super-Eddington accretion rate and the moderate magnetic field so that the inner edge of the truncated geometrically thick accretion disc is seen from the neutron star at a large solid angle.

The configuration and magnitude of the magnetic field are the key parameters determining the physics of accretion on a strongly magnetized neutron star. The only direct way to measure the magnitude of the magnetic field in the radiation formation zone is to study the energy spectra for the presence of cyclotron absorption features. Thus, the magnetic field for several dozen pulsars was determined. However, we found that the depth of the absorption line may depend on the phase of rotation of the pulsar. This means that the line can be washed out on the average energy spectrum. In such cases, phase-resolved spectral analysis becomes a powerful tool for searching for cyclotron features. I plan to present results of NuSTAR observations of two X-ray pulsars GROJ2058+42 and SWIFTJ1626.6-5156 and discovery of the transient cyclotron lines (features was detected only in narrow phase bin) in them. Results have been published in two papers: Molkov et al. 2021, The Astrophysical Journal Letters, Volume 915, Issue 2, L27 Molkov et al. 2019, The Astrophysical Journal Letters, Volume 883, Issue 1, L11 The study was financially supported by the Russian Science Foundation (grant 19-12-00423).

Black hole X-ray binaries are accreting objects that launch powerful relativistic jets. These binary systems evolve through bright outburst phases on rapid timescales of days to months, allowing us to probe jet/accretion phenomena in real time. While emission from the accretion flow dominates at shorter wavelengths (X-ray), synchrotron jet emission, which can be strongly polarized, dominates at longer wavelengths (radio, submm, infrared/optical). As such, polarization measurements can allow us to extract much more information about these jets than we can infer from just the total intensity of the emission (e.g., orientation of the jet, magnetic field strength and structure of both the jet and the surrounding medium, shock and plasma conditions in the jet flow). However, to date, such jet polarization studies are still quite limited, and are often hampered by complicating external effects (Faraday rotation, time variability). In this talk, I will review the state of the field on X-ray binary jet polarization studies, discuss some of the latest exciting results working to overcome the above mentioned issues, and discuss how current and next generation instrumentation can open up a new polarimetric viewpoint on X-ray binary jets that has yet to be thoroughly explored.

In 1996, O’Donoghue & Charles demonstrated that the outburst optical light-curves of the then half-dozen BH X-ray transients all did in fact display superhumps with properties that seemed similar to those seen in CVs. This was a surprising discovery, because superhumps are explained in CVs as being due to periodic tidal energy dissipation in the accretion disc as it precesses, on a period slightly longer than orbital, and this would not be detectable in BH XRTs as their optical luminosity is ~100-1000x greater than CVs, driven entirely by the intense X-ray flux. Within a few years, an explanation appeared (Haswell et al 2001) where the associated variation in disc area through the precession could lead to the observed “superhump”. Now, 25 years after that initial work, we know of a dozen such systems displaying this effect, but one (MAXI J1820+070) shows a superhump modulation with an amplitude much greater (almost 10x) than ever seen before. This calls into question the entire nature of this remarkable property and has fundamental implications for how discs precess and warp.

The first QPOs from X-ray binaries have been discovered in the 1980s. Since then, the known phenomenology has increased impressively and models independent of the nature of the compact objects have been proposed. I will review the main observational properties and discuss the current standpoint, with a view to the opportunities offered by current and future X-ray missions.

These days jets are observed on very different scales: from few gravitational radii to a few kiloparsecs. The variety of scales and variability times tells us that there are few dissipation mechanisms that transform electromagnetic energy into the observed radiation. In my talk I will discuss two possible mechanisms: (1) the energy dissipation in current sheets in the vicinity of the black hole and (2) the current-driven kink instability as a source of energy dissipation on the scales of 10^5-10^7 gravitational radii from the central engine.

Recent studies confirmed that Swift J1728.9-3613 is a Black hole X-ray binary (BHXRB). Hardness intensity diagram indicates that it is a high inclination source. During an outburst black hole X-ray binary undergoes a transition from hard state to soft state and soft to hard state. They exhibit a short lived intermediate state during state transition. The behaviour of accretion disk during intermediate state is not well understood. We analysed the data for BHXRB Swift J1728.9-3613 during the intermediate state. We observe drastic change in the flux without much change in hardness. We did the spectral analysis to see the variation in parameters as flux changes. We found that the change in flux is due to the variation in coronal behaviour. This sudden change of coronal power law can be explain with magnetic field that is most accepted heating mechanism for corona. In this work, I will discuss the observed behaviour in analysed BHXRB system within the framework of magnetically influenced accretion.

We analyze a Neutron Star Interior Composition Explorer (NICER) observation of the black hole X-ray binary MAXI J1820+070 during a transition from type-C to type-B quasi-periodic oscillations (QPOs). We find that below ~2 keV, for the type-B QPOs the rms amplitude is lower and the magnitude of the phase lags is larger than for the type-C QPOs. Above that energy, the rms and phase-lag spectra of the type-B and type-C QPOs are consistent with being the same. We perform a joint fit of the time-averaged spectra of the source, and the rms and phase-lag spectra of the QPOs with the time-dependent Comptonization model (vkompth) to study the geometry of the corona during the QPO transition. We find that the data can be well-fitted with a model consisting of a small and a large corona that is physically connected. The size of the small and large corona increases gradually during the type-C QPO phase, whereas they decrease abruptly at the transition to type-B QPO. At the same time, the inner radius of the disc moves inward at the QPO transition. Combined with the simultaneous radio data that show that a radio flare happens around the time of the QPO transition, we propose that a corona that expands horizontally during the type-C QPO phase, from ~10^4 km to ~10^5 km overlying the accretion disc, suddenly transforms into a vertical jet-like corona extending over ~10^4 km during the type-B QPO phase.

In my talk I will present recent results on the gas and dust dynamics and their evolution in young protoplanetary disks. Using 3D radiation MHD simulation we investigate the detailed gas and dust kinematics and their structure evolution ranging from the inner regions where the dust is sublimated until the outer regions of protoplanetary disks close to the disk outer edge.

The nature and the accretion regulation mechanism in the inner parts of protoplanetary discs (whether it is magnetospheric accretion or boundary layer accretion or something else) still remains a major open problem. This critical phase of the formation which impacts the pre-main-sequence stellar evolution tracks, as well as the modelling of the formation environments of the exoplanets, is still poorly understood. Our long term optical and near infrared spectroscopic monitoring of episodic accretion events (a.k.a. FUors and EXors) using various telescope facilities, including SALT has provided significant insights on the outflow driving mechanism as well as some clues on the accretion regulation. In this talk, I shall present some of these exciting inferences on the inner disc accretion. Towards the end of the talk, I shall also present a new kind of multi-object spectrograph we are currently building to conduct the world’s largest optical to near infrared spectroscopic survey of protoplanetary disc systems.

Rossby wave instability (or, Rossby vortices) is observed in many rotating systems. Examples are the Great Red Spot on Jupiter, vortices in the Earth’s atmosphere, and oceans which help to mix cold and warm air and water. They also form in astrophysical accretion discs in situations where an initial bump of the density/pressure is present, or when the density sharply drops, such as at the edges of cavities. I will discuss the physics of Rossby Vortices and their application to planet formation in protoplanetary discs and the formation of high-frequency QPOs in disc-accreting compact objects.

Accretion burst is now taunted as the verified means by which massive protostellar objects acquire their large masses. Single-dish maser monitoring observations are extremely useful in detecting the onset of this burst events. Follow-up centimeter and millimeter interferometric observations reveal the significant changes in the masers spatial distribution, dust and millimeter line properties of the object. I will present the results of the extensively studies accretion burst objects in NGC6334I, S255 and G358 massive star-forming regions.

This talk will be an observational retrospective on the formation and ejection of “slingshot prominences”, which form near the corotation radii of young rapidly-rotating FGKM dwarfs in the field and young star forming regions. I’ll also discuss their possible relationship to the “dusty dippers” and “scallop-shell stars” observed among M dwarfs in star-forming regions and young open clusters.

This is an invited talk where I’ll discuss the evidence indicating that CVs launch jets and the outstanding issues/questions of this model.

The magnetic cataclysmic variables (MCVs) are stellar binary systems composed by an white dwarf (WD) with an intense magnetic field (over 1 MG) and by a red dwarf (RD) that transfer mass to its companion through Roche’s lobe overflow. The MCVs can be classified as polars, that have the WDs spin period (Pspin) equal to the orbital period of the system (Porb), or as intermediate polars (IPs), that have typically Pspin/Porb of the order or equal to 0.1. Among the few hundreds of MCVs known today, there are six IPs with Pspin/Porb > 0.1, that we call near synchronous IPs, and four polars with Pspin different from Porb, that we call asynchronous polars (APs). In addition to those ten objects, there are four systems showing differences between Pspin and Porb of around 3% to 20% . There are uncertainties in the classification of those last four as APs or as near synchronous IPs. We present new optical data for two of those systems, 1RXS J083842.1-282723 and IGR J19552+0044, obtained at Pico dos Dias Observatory (Brazil), using the IAGPOL polarimeter. In this work we will show results from the analysis of these data relative to the search of periodicities in those photometric and polarimetric time series, to the detection of polarized emission and the classification of those objects. The reduction was performed using a new software for the processing of polarimetric data in Python language called Astropop.

Flickering is a stochastic variability that can be detected on all timescales – from milliseconds to hours. Flickering is always associated with accretion and can be detected in cataclysmic variables as well as in active galactic nuclei. While the physical origin of flickering is not known it has been hypothesized that it may originate in accretion discs. Here I will present a study of flickering in cataclysmic variables without accretion disc – polars. This study will help us understand the location of flickering source in these systems.

We present time-resolved optical photometry, polarimetry, and spectroscopy as well as X-ray XMM spectrum of the magnetic cataclysmic variable, MCV, 1RXS J174320.1-042953. The orbital period of the system is 0.086507(4) d, placing it below the period gap. The flux and circular polarization show a clear variation with the orbital phase, allowing us to confirm the system as a polar, a MCV in which both stars rotates synchronously with the orbital period. The circular polarization can be as high as 40% in V and R bands. The modelling of post-shock region using optical and X-ray data indicates a high-mass white dwarf.

The lowest accretion rate dwarf novae exhibit a complex light curve morphology consisting of a sharp, deep “dip” and subsequent multiple “echo-outbursts” on their super-outburst decline in their path to quiescence. As the standard disk instability model fails to predict this peculiar decline-phase behaviour, we investigate that these abrupt changes between faint and bright states represent transitions into and out of a magnetic propeller state. We exploit the distinctive UV spectrum of the prototypical magnetic propeller white dwarf system AE Aqr as a template for the presence of these spectroscopic signatures and we test whether the observed spectroscopic features of a propeller are present and limited to the faint states during which the propeller is thought to operate. In this talk, I will present the results of our search for a propeller signature in time-resolved UV spectroscopy taken just before, during and after the dip in WZ Sge’s 2001 super-outburst. Finally, I will discuss the implications of these findings to the broader image we have for the outburst mechanisms in accreting binaries.

We have carried detailed time-resolved timing analysis of an intermediate polar V709 Cas, using the long-baseline, short cadence optical photometric data from the Transiting Exoplanet Survey Satellite(TESS). We found an orbital period of 5.33306 ± 0.00004 hr, a spin period of 312.748 ± 0.002 sec and a beat period of 317.927 ± 0.002 sec, which are similar and more precise than the earlier published results. From the continuous data, we report the system’s accretion geometry as disc overflow with disc-fed dominance with some part of it being also stream-fed. The double peaked pulse profile nature shows it being a two pole accretor.

In intermediate polars, the temperature of the plasma in the region where the accreted material impacts the white dwarf surface is thought to depend only on the white dwarf mass. The main cooling mechanism is assumed to be the emission of X-rays. Hence, many authors have derived estimates of the white dwarf masses in these systems from X-ray spectral modelling. However, this technique is known to suffer from significant model- and instrument-dependent systematic effects. To overcome these shortcomings, we are currently performing dynamical studies of intermediate polars using optical and near-infrared data obtained with the 10.4-m Gran Telescopio Canarias and other telescopes. In this talk we present the dynamical white dwarf masses of GK Per and XY Ari, and compare our results with several estimates from X-ray spectral modelling. We find that a major revision of the cooling models currently assumed for the accretion in intermediate polars is needed.

I will discuss several aspects of observations and theory of disc-jet connection in black-hole binaries, concentrating on constraints on the magnetic fields in the disc and jets. After an overall review, I will cover results obtained for the parameters, composition, power and of the jets in MAXI J1820+070, MAXI J1348-630, Cyg X-1, Cyg X-3 and GRS 1915+105.

In recent years, the advancement in radio telescope capabilities have allowed for the study of jets launched by strongly-magnetised neutron stars in X-ray binaries. Most of our understanding about the inflow-outflow (i.e. accretion-jet) connection in such systems was based on a single, super-Eddington accreting source. In this presentation, I will discuss our long-running efforts to extend such inflow-outflow studies to a larger sample. I will show how this campaign now strongly suggest the existence of a standard accretion-jet coupling in these strongly-magnetised neutron star sources, and discuss what this result means for neutron star jet launch models.

I will summarise the state of the observations across the electromagnetic spectrum and beyond, of the moments when jets are launched from black holes and neutron stars. Combined with our best estimates of the energetics and composition of these jets, and the direct imaging of the event horizon with EHT in two cases, we may try and piece together the empirical constraints on the process of relativistic jet formation.

Different classes of outflows are likely to be associated with the magnetospheric activity of accreting classical TTauri stars. Stellar winds are accelerated along the open magnetic field lines emerging from the stellar surface; disk-winds (extended or X-winds) can be launched along the open magnetic surfaces threading the accretion disk; another type of ejection can arise from the region of interaction of the closed magnetosphere with the surrounding accretion disk (magnetospheric ejections, conical winds). Taking advantage of numerical models of magnetospheric accretion and ejection in classical TTauri stars, I will describe the main dynamical properties of these classes of outflows. I will pay specific attention to some open issues: the connection of the magnetospheric outflows with the origin of protostellar jets; the impact of these ejection phenomena on the angular momentum evolution of the protostar; the possibility of extracting synthetic observables from the models to be directly compared with observations.

FN Sgr is a well studied symbiotic system at a distance of about 7 kpc, composed of an M5 giant and a white dwarf of about 0.7 solar masses. The orbital period is 567.3 days. The V and I band light curves showed a phenomenon never before observed with such recurrence in any symbiotic system, namely short outbursts, starting between orbital phase 0.3 and 0.5 and lasting about a month, with a fast rise and a slower decline, and amplitude of 0.5-1 mag. In the Kepler high cadence light curve we discovered three frequencies. We attribute a stable frequency of 127.5/day (corresponding to a 11.3 minutes period) to the white dwarf rotation. We suggest that this detection probably implies that the white dwarf accretes through a magnetic stream, like in intermediate polars. The small outbursts may be ascribed to localized thermonuclear burning, perhaps confined by the magnetic field, like recently inferred in intermediate polars, albeit on different timescales. We measured also a second frequency around 116.9/day ( (corresponding to about 137 minutes), which is is much less stable and has a drift. We discuss possible interpretations of this rocky detritus around the white dwarf, or to inhomegeinity in an accretion disk. Finally, there is a third frequency close to the first one, that appears to correspond to the beating between the rotation frequency and the second one.

The accreting black holes in X-ray binaries (XRBs) and Active Galactic Nuclei (AGN) are well established to exhibit the highly energetic ionized environment around them. They also show sub-relativistic or relativistic outflows, vigorously moving outwards, having significant imprints of the ionizing radiation, environment, the origin of the outflows. It is postulated that in case of extremely high degree of collimation, as observed in a class of AGNs, these outflows can form relativistic jets. The possible connections between the warm outflows and jet can be addressed by a thorough study of UV and X-ray spectrum of a sample of XRBs and AGNs.

We present the results from a recent survey of Southern hemisphere magnetic CVs where we employed the Sutherland High-speed Optical Cameras on the SAAO 1.9m telescope to study the rapid variability in these fascinating astrophysical objects. We highlight the discovery of Quasi-periodic Oscillations (QPOs) in the two new sources named above, presenting their light curves, periodograms and dynamic power spectra. We compare the results for the two new polars to the previously known QPO sources and finally discuss what insights we might gain into the magnetic accretion process through studying this interesting phenomenon.

AR Sco is an intriguing binary system that contains both a white and red dwarf. The spin rate of the white dwarf has been observed to slow down with time, analogous to rotation-powered radio pulsars; it has thus been dubbed a “white dwarf pulsar”. We previously fit the traditional radio pulsar rotating vector model to linearly polarized optical data from this source, constraining the system geometry and white dwarf mass. Next, using a much more extensive dataset from the South African Astronomical Observatory (SAAO) HIPPO Polarimeter on their 1.9-m telescope, we also explored the application of the same geometric model to the orbitally phase-resolved optical polarimetric data. The optical emission is thought to be due to non-thermal synchrotron radiation. We constrained the magnetic inclination angle and the observer angle at different orbital phases. Now, we have constructed a much more sophisticated emission model, solving the particle dynamics from first principles, including a generalized radiation reaction force, and implementing similar techniques to what were used in a pulsar emission code developed by A.K. Harding and collaborators to produce sky maps, light curves and spectra. We present our results of the particle pitch-angle evolution and Lorentz-factor evolution for different scenarios, as well as studying the impact of using generalised dynamical equations vs. a super-relativistic approximation only since our equations can also be applied to non-relativistic motion. Additionally, we investigate a magnetic mirror scenario, similar to that of Takata et al. (2017), and show the importance of not being constrained by assumptions of super-relativistic particles with small pitch angles. We also present some test cases that confirm the accuracy of our calculations. Finally, we discuss how we calculated the curvature and synchrotron radiation to obtain our emission maps, light curves and spectra as well as our future plans to calculate the phase-resolved polarisation properties of AR Sco.

In This work we present the preliminary of the observational study of the candidate for the magnetic cataclysmic variable of the polar type CRTS J035758.7+ 102943 (CSS0357+10), with spectroscopy, photometry, and polarimetry data, obtained in the SOAR and OPD observatories, and TESS space telescope. The average spectrum is dominated by emission lines, mainly Balmer and HeII 4686 A, with the latter almost as intense as Hbeta. The fit made to the most prominent lines identified two components, one with a semi-amplitude of 720 km/s, and the other with 270 km/s. One of the components has a maximum redshift close to phase 0.3, while in the another this point is at phase nearly 0.5, evidence that each one of them originates in different parts of the system. The polarimetric analysis showed that the CSS0357+10 system has a large amount of circularly polarized light that varies with the orbital period, reaching a maximum of nearly 40% in filter V, and nearly 30% in R and I, definitely classifying it as polar. the light curve has a modulation with an amplitude of nearly 0.75 mag, with its minimum coinciding with maximum of the circular polarization curve, indicating that our line of sight is along the magnetic field, and its maximum with the minimum of circular polarization, moment at which the magnetic field is parallel to the sky plane. In search of periods, the Lomb-Scargle technique was applied to various combinations of data, resulting in a period of 0.0791810(8) days, reducing the uncertainty of the period published in the literature by 20%. In short, it can be concluded that the CSS0357+10 system is in fact a polar-type mVC with a period slightly below the period gap.

Cataclysmic Variables (CVs) consist of pairs of stars, one of them being a mass-donating secondary and another one being a white dwarf primary. The material transferred from the secondary flows through the inner Lagrangian point and orbits around the primary, forming an accretion disk in non-magnetic systems. However, if the magnetic field strength of the WD is large enough, an accretion disk can form far from the WD, but the inner disk disrupts at the magnetospheric radius, and the field forces material to fall onto the poles of the WD. These systems are known as Intermediate Polars. Alternatively, if the magnetic field of the WD is sufficiently strong, the magnetic pressure exceeds the ram pressure, preventing the formation of an accretion disk and accreted matter follows the magnetic field lines. These binary systems are known as polars. Most of the CV candidates discovered from the various surveys require a detailed investigation and provide us a unique opportunity to unravel their all defining characteristics as well as help to enhance the sample of CVs. In this context, we will discuss the detailed optical properties of two poorly studied CVs namely, RBS 0490 and SDSS J075939.79 +191417.3. Based on the optical photometric and spectroscopic observations, we have found that the characteristic features of RBS 0490 seem to favor low-field polars, while SDSS J075939.79+191417.3 appears to be similar to non-magnetic systems.

In this work, I will present my master’s research on the characterization of CRTSJ091936.6-055519, a polar cataclysmic variable candidate with previously unknown orbital period, using spectroscopy, photometry and polarimetry data obtained from SOAR, TESS and Pico dos Dias Observatory.

The unusual white dwarf GD 356 was discovered in 1985, and hosts a spectrum characterized only by Balmer features in Zeeman-split triplet emission. However, GD 356 shows no signatures of an interacting stellar companion in its spectral energy distribution, and an explanation for the source of the emission remains elusive. In the following decades, it was proposed that the emission may be driven by the orbit of a conducting planet through the stellar magnetosphere which excites the stellar atmosphere via a unipolar inductor mechanism, similar to how Io is known to impart a signal on Jupiter. Finally, in 2020, the discovery of SDSS J1252-0234 established these hydrogen-atmosphere magnetic emission line systems as a new class: the DAHe white dwarfs. This class has since grown to 5 members and counting, and discerning the nature of its emission mechanism is at the forefront of white dwarf science today. We present an overview of the five DAHe white dwarfs discovered to date, which share remarkably similar properties, and speculate as to their origins and the source of their spectral activity.

Accretion of matter is a fundamental astrophysical process, however the mechanisms responsible for the radial transport of angular momentum driving accretion remain the topic of significant investigation. In order to understand the dynamical evolution of accretion disks and their connection to powerful jet outflows it is critically important to understand both the processes responsible for the outward flux of angular momentum and the transport of magnetic flux in the disk. In this talk I focus on the evolution of truncated accretion disks around black holes, with a focus on angular momentum and magnetic flux transport.

Ultra-luminous X-ray sources (ULXs) are very bright, point-like X-ray sources, whose luminosity exceeds the Eddington luminosity of the stellar mass black hole. Recent X-ray observations reveal that some of ULXs originate from neutron stars, but their energy production mechanism is still veiled in mystery. We have performed the general relativistic radiation magnetohydrodynamics (GR-RMHD) simulations of super-Eddington accretion flows around neutron stars with dipole magnetic fields, as a model of neutron-star-powered ULXs. Our simulations show accretion disks outside the magnetosphere, accretion columns near the magnetic poles and powerful outflows launched from accretion disks. The resulting gas temperature (~<1E+7K) and blackbody radius (>100 km) of the winds are consistent with the observation of galactic neutron-star-powered ULX (Swift J0243.6+6124, Tao et al. 2019) in the case of relatively high mass accretion rate models (> 5E+19 g/s). Our results indicate that Swift J0243.6+6124 is powered by super-Eddington accretion flows onto a magnetized neutron star, whose dipole magnetic fields are weaker than 1E+12 G. This is because the magnetospheric radius should become smaller than the spherization radius, within which radiation-driven outflows are launched, and mass accretion rate should be larger than 5E+19 g/s to produce such optically thick outflows.

Based on our paper currently submitted https://arxiv.org/abs/2207.12312

The astrophysical jets in active galactic nuclei (AGNs) and microquasars originate from the inner part of the accretion disc. The radiation field of the accretion disc interacts with the jet material as the jet ploughs through the radiation field. Various steady-state investigations have shown that the radiation field can play a crucial role in the acceleration and collimation of the jets. It can also produce steady shocks very close to the jet base. However, the numerical simulations of the radiatively driven jets are very limited. In this work, we have made an attempt to study the dynamics of the jet under the influence of a time-dependent radiation field of the accretion disc. Along with the relativistic equations of motion we use a relativistic equation of state (EoS) for multispecies fluid which enables us to study the effect of composition on jet dynamics. Starting from very low injection velocities, the jets achieve high Lorentz factors. For sub-Eddington luminosities, lepton-dominated jets can be accelerated to Lorentz factors > 60. The radiation field can also generate shocks in the jet and depending upon the disk oscillation frequency, amplitude, and jet parameters these shocks can collide with each other and may trigger shock cascades.

This report presents an interpretation of synchronous multi-wave observations of one of the brightest gamma- ray bursts GRB 160625B with detailed continuous fast optical photometry of its optical counterpart. It was obtained by the MASTER with hard X-ray and gamma-ray emission, obtained by the Lomonosov and Konus- Wind spacecraft. Detailed photometry led us to the detection of quasi-periodic components of radiation in the internal optical radiation. As a result of our analysis of synchronous multi-wave observations, we propose three- the scenario of a stage collapse for this long and bright GRB. We assume that quasi-periodic fluctuations can be related with forced precession of a self-gravitating rapidly rotating superdense body (spinar), the evolution of which is determined by a powerful magnetic field. The spinar’s mass allows it to collapse into a black hole at the end of evolution. In addition, based on the spinar model, several other interesting properties of gamma-ray bursts are considered: the time shift between the precursor and the burst, as well as the fundamental plane of gamma-ray bursts.

Galactic black hole transients (GBHTs) show distinct X-ray spectral states at different X-ray luminosities in their outbursts. The state transitions are considered to be associated with the change in the structure of the accretion flows, caused by the change in the mass accretion rate. A narrow distribution of transition luminosity in terms of the Eddington ratio has been found in previous studies of GBHTs based on RXTE/PCA 2–20 keV data (Macarone 2003; Vahdat et al.2019) and this Eddington ratio at the transitions is often used in recent studies with instruments covering softer energy bands, such as Swift/XRT and NICER/XTI, covering energies below 1 keV to 10 keV. However, the X-ray states characterized by the spectral parameters may have different definitions depending on the energy ranges adopted in the spectral analysis, leaving the question whether the distribution of transition luminosity obtained with RXTE remains the same when we use the instruments covering softer energy bands. In this work, we investigated the state transitions and the variations of luminosities of 8 outbursts of 7 GBHTs. Our results show that the bolometric luminosity of the power-law component is tightly constrained to 1% Eddington luminosity at index transition when the photon index of that component starts to decrease towards the hard state. This is consistent with the conclusions from the previous RXTE results (Vahdat et al. 2019; Kalemci et al. 2013). Moreover, our results suggest that the disk truncation starts after bolometric disk luminosity drops below 1% Eddington luminosity. We would be very happy if audience from theoretical side could give hints to answer the question: how accretion processes contribute to result in such luminosity distributions at transition phase.

I will present results of our ongoing GRB follow-up campaign with MeerLICHT with a look forward towards O4 gravitational wave follow-up set to begin in March 2023.

In this work, the Fermi-LAT multiband selected sample of active galactic nuclei (AGN) was used to investigate the emission spectra of blazars and radio galaxies. We computed the broadband emission spectra from the low energy components of radio to X-ray, radio to γ-ray and the high energy component of X-ray to γ-ray bands. Our findings from the distributions of the continuous spectra clearly indicate that radio galaxies form the tail of the distributions in the low energy components, overlapping in a well-determined range (up to 4 orders of magnitude) in the high energy spectrum. A two-sample Kolmogorov-Smirnov test (K-S test) of the computed spectra showed that radio galaxies differ from blazars in the low energy components while there is no clear difference between them in the high energy component, which implies that high energy emissions in radio galaxies and blazars may be as a result of the same emission mechanism. There is a regular sequence of distributions on the continuous spectra planes for radio galaxies and blazar subsamples. Simple linear regression analyses yield significant positive correlations (r ≥ 0.60) within the low-energy components. This upturns into anti-correlation (r > – 0.60) in for high energy component. These results are not only consistent with a unified scheme for blazars and radio galaxies but also show that the emission mechanisms of these sources are similar.

We present long-term monitoring program of Blazars in Abastumani observatory using 70-cm meniscus 1.25m telescopes and CCD cameras Apogee Ap6E and SBIG St-6. The program was started in 1997 and over 4000 observing nights were conducted up to date. During this period over 400000 frames were obtained and about 70 selected sources were studied. We participated in many international campaigns with space and ground based telescopes such as FERMI/Lat, MAGIC, HESS, VERITAS, radio and optical telescopes. Over 100 papers were published in collaboration with western institutes and observatories.

The main objective of the project is to perform a systematic study of circumstellar discs of Be X-ray binaries with the goal of demonstrating observational signatures of the Kozai-Lidov mechanism. Through the analysis of the double-peaked H-alpha emission line of a sample of Galactic Be X-ray binaries using data obtained from various telescopes including the Southern African Large Telescope (SALT) and the Liverpool Telescope, I obtain a quantitative measure of the disc parameters that have previously been elusive from observational studies, such as the eccentricity and inclination. These is used together with long-term archival X-ray lightcurves from the Rossi X-ray Timing Explorer All-Sky monitor and the Swift Burst Alert Telescope to study the relationship between the various disc properties and X-ray outbursts.

Gamma-ray bursts (GRBs) are highly energetic impulses of gamma rays that are classified into two major categories, namely long and short GRBs. In this work, the so called Norris function is used to fit short GRB pulses which allows the study of the sources. This will further distinguish short GRB sources from long ones as the duration of the sources is not enough to completely differentiate the two. Despite making a distinction between long and short GRBs, the Norris function can be used to further make a distinction in another class of short GRBs, Magnetar Giant Flares (MGFs). Magnetars are highly magnetised Neutron stars which produce short lived gamma-ray transients called MGFs. These sources can be easily mistaken for cosmological short GRBs hence the aim to study these two sources using the Norris function parameters. In particular the rising and falling times of the function are used to understand these intriguing sources much better. MGFs arise from nearby star forming galaxies while cosmological short GRBs are from compact binary mergers. Without the redshift, one cannot easily tell if a short GRB or MGF has been detected based only on the duration of the source hence the function will allow an easy way to differentiate the sources. As a result, the distinction between the two sources will enable a much better way to study GRBs hence of compact binary mergers.

Studying the rapid variability of accreting sources is a powerful way to investigate the underlying processes that drive them; however, a mixture of variable telescope quality and spotty availability, as well as the typically faint and unpredictable nature of these sources, make this a tricky task. Thus, we here describe CorrSim, a newly-developed code that aims to help both observation planning and analysis; given a temporal and Fourier model of a system (i.e. Power Spectra, Coherence, and Lags), CorrSim will return a simulated multiwavelength observation, including effects of noise, telescope parameters, and finite sampling. The goals of this are: (i) To simulate a potential observation (in order to aid in planning, and inform decisions about an observation’s feasibility); and (ii) To investigate how different Fourier models affect the variability of the system (e.g. how altering the frequency-dependent coherence or Fourier lags between bands can affect data products like cross-correlation functions). In this talk, I will present the methodology behind CorrSim, show how a variety of parameters (e.g. noise sources, time resolution, and telescope choice) can affect an observation, and demonstrate example uses of the program in observation planning and data interpretation.

The third observation run (O3) of Advanced LIGO and Advanced Virgo started in April 2019 and ended in March 2020. The science results achieved during the O3 run include the GWTC-2, GWTC-2.1, GWTC-3 catalogs of compact binary mergers and several exceptional events. The presentation will review the catalogs of mergers, some exceptional events and will summarize the related multi-messenger observations.

In our recent work devoted to identifying the new members of the Lupus I cloud, we identified four new accretors in the region. For selecting the new members of Lupus I, we used both Gaia catalogs and the OmegaCAM survey that has access to H_alpha narrow band filter. We understood that the synergy among various surveys for identifying the new members of a stellar association is crucial, and, surprisingly, one of our accretors had escaped H_alpha surveys. The interesting result of our study is that accretors are scattered across both the main filament of Lupus I and around the main filaments of the cloud, and also, the strongest accretors are found to form both in the disk of a companion star, or in total isolation.

Galaxy evolution strongly depends on the physics of the interstellar medium (ISM). The ISM is permeated by B-fields, in which magnetic energy is in close equipartition with the thermal energy. This physical condition makes the B-fields dynamically important at several stages of galaxy evolution affecting gas flows in the ISM and driving gas inwards toward the galaxy’s centers, and outwards toward the intergalactic medium via galactic outflows. Thus, B-fields remain an important, but still highly ignored, ingredient to understanding the evolution of galaxies across cosmic time. SOFIA has been key to providing a complete picture of extragalactic magnetism by doing what only HAWC+ can do: measuring magnetic fields in the densest areas of the universe. Using FIR and radio polarimetric observations, in combination with the kinematics of the neutral and molecular gas, the three dimensional structure of the B-field in galaxies is characterized for the first time. I will present the results of the first data release of the SOFIA Legacy Program on extragalactic magnetism. I will present the polarization properties of 14 nearby galaxies observed in the wavelength range of 50-220 um. These results open a new window of exploration on galaxy evolution and provide the building blocks for scientific cases of future IR polarimetric missions.

This study focuses on the origin of gamma-ray emission from blazars and the connection between different components of an AGN, such as accretion disk, jet and broad line region (BLR). The origin of the gamma-ray emission from blazars is still debated, in particular whether it is produced by leptonic or hadronic processes. In our study, we are testing the leptonic scenario for the Flat Spectrum Radio Quasar (FSRQ) 3C 279, assuming that the gamma-ray emission is generated by inverse Compton scattering of external target photons from Broad Line Region (IC-BLR scenario). We use a 10-year data set of the source consisting of the optical spectroscopy data from the Steward Observatory blazar monitoring program and Fermi-LAT gamma-ray data. We search for possible correlations between various observational parameters, such as non-thermal synchrotron continuum flux, emission line flux, gamma-ray flux and the Compton dominance parameter, with a goal to disentangle how these quantities are interconnected between each other, and reveal which parameters contribute the most to long-term variations of the gamma-ray flux and the Compton dominance.

We present the study of the correlations between Active Galactic Nuclei (AGN) variability and rest-frame wavelength. We studied a sample of optical light curves from the Zwicky Transient Facility Data Release 6 for Active Galactic Nuclei (AGN) in the g-band (4722.74 Å) and r-band (6339.61 Å). We use a homogeneous analysis of SDSS DR14 quasars by Rakshit et al. (2020) to define a sample with well-measured Black Hole mass (M_{BH}) and Eddington ratio/accretion rate in a well-defined redshift bin. Our sample sources have 0.1 <= redshift <= 0.8, and 10^{8.0} <= M_{BH}/M_solar <= 10^{8.5}, while the Eddington ratio range is between 10^{-1.3} and 10^{-0.8}. This sample was selected because the emission from quasars is highly variable, and variability is a potential key to understanding the accretion process. We corrected the light curves to prevent biases from redshift effects as time dilation. Then we calculate the amplitude of variability (or variance) at different time scales in the power spectrum using the ‘Mexican-Hat’ filter (Arévalo et al. 2012) for 3054 sources in the g-band and 3407 sources in the r- band. Redshift is used as a tool to probe different rest-frame wavelengths. In this work, we studied variance on 300 and 75-day timescales and found a strong anti-correlation between rest-frame wavelengths and median variance. This anti-correlation suggests that short and long timescales optical fluctuations are less in the outer annuli than in the inner annuli if the rest-frame wavelength represents the radius of the accretion disk. We also find that the variance ratio weakly correlates with rest-frame wavelength. To test the Corona-heated Accretion-disk (CHAR) model of (Sun et al. 2020), we compare the optical variability of the ZTF quasars with the CHAR model. The CHAR model can predict the observed variance vs. rest-frame wavelength for Blackhole Mass 10^{8} M_solar, and Eddington ratio of 10^{-1}. Our research demonstrates that the CHAR model can be applied to constrain the temperature variations in the accretion disk.

In this invited talk, I will present an overview of particle acceleration up to very high energies (VHE) by magnetic reconnection in the magnetically dominated regions of relativistic jets, showing results of relativistic MHD simulations with test particles that probe the efficiency of the process. I will also discuss applications of these results to explain observed variable gamma-ray and neutrino emission in Blazar AGN jets.

Short outbursts in IPs are too short to be explained by the disk instability model. I’ll discuss briefly discuss the micronova model and intend to focuss on the magnetic gating instability.

We present phase-resolved snapshot circular spectropolarimetry of a sample of eleven cataclysmic variables (CV) stars obtained with SALT. Our results show that eight of the CVs studied show either positive or negative circular polarization ranging from +7% to −25%. The majority of these systems show not more than one cyclotron harmonic in their cyclotron spectra with the exception of RX J1313.2-3259 and UW Pic where at least two resolved cyclotron humps are observed. These features are understood as cyclotron harmonics due to cyclotron emission from the hot plasma and modelling them gives an idea of the strength of the magnetic field amongst other properties.

Intermediate polars (IPs) are the low magnetic field strength ( B ~10^6 –10^7 G) subclass of magnetic cataclysmic variables (MCVs). IPs are asynchronous systems, and they follow the asynchronism relation as the spin period of the white dwarf (WD) is relatively less than the orbital period of the binary system. Most IPs have orbital periods longer than the ‘period gap’ of 2-3 h. However, there is a special class of IPs known as nearly synchronous IPs for which the spin period of WD is approximately in the range of (0.7-0.9) times the orbital period of the WD. These systems are thought to be in the process of attaining synchronism and evolving into polars. There is only one confirmed system, ‘Paloma’, which belongs to this class and lies in the period gap. Within this frame of reference, we will discuss the X-ray and optical properties of only other nearly synchronous IP, namely SWIFT J0503.7-2819. The X-ray and optical variations of this target have been found to occur at the period of ∼65 min, which we propose as the spin period of the white dwarf. The energy-dependent modulations at this period, which are due to the photoelectric absorption in the accretion flow, also assure this conjecture. If the proposed spin period is indeed the actual period, then SWIFT J0503.7-2819 could be the first nearly synchronous intermediate polar below the period gap.

Magnetic cataclysmic variables (CVs) are known to transition between high and low accretion states which are visible in long-term light curves from optical transient surveys. We investigated magnetic CVs from the Catalina Real-time Transient Survey (CRTS) with respect to the long-term photometric properties of the global population, as well as the photometric and spectroscopic follow-up of individual candidate magnetic CVs selected from the CRTS. I will present the various long-term photometric patterns which we identified from the long-term light curves of previously known magnetic cataclysmic variables and some of the follow-up results of individual magnetic CV candidates using the Southern African Large Telescope (SALT) and the 1.0/1.9-m telescopes situated at the South African Astronomical Observatory in Sutherland, South Africa.

Helium-dominated cataclysmic variables (AM CVn-type binary systems) are a class of ultracompact system in which a white dwarf accretes hydrogen-depleted material from an evolved donor star, with an orbital period in the range 5-70 minutes. They may descend from a double white dwarf binary or from a cataclysmic variable with an evolved donor. In many ways such systems exhibit similar behaviour to typical non-magnetic cataclysmic variables, such as the presence of dwarf novae. However, of the 100 known AM CVn-type binary systems, no system has a detected magnetic field and there has never been a detected helium-dominated polar. This is a stark contrast to the typical hydrogen-rich cataclysmic variables, of which one third are polars or intermediate polars. This represents an additional constraint on the formation of magnetic fields in cataclysmic variables. I will discuss the observational state of play for AM CVn binaries (including the robustness of the non-detection of magnetic fields) and several hypotheses to explain for the absence of magnetic fields.

I discuss an intriguing mCV that challenges the once-obvious dichotomy between intermediate polars and polars. This system, SDSS J134441.83+204408.3, was previously classified as a synchronous polar, but its TESS light curve shows it to be asynchronous, with a likely P_spin/P_orb ratio of 0.89. Although there is some ambiguity as to the orbital period, my colleagues and I argue that it is either 114 min or 102 min, with the former being preferred. Most interestingly of all, the cyclotron humps in J1344’s SDSS spectrum enable us to measure its magnetic-field strength to be B=56(+/-2) MG, making it one of the few asynchronously rotating mCVs for which the field strength can be measured directly. Collectively, J1344’s highly asynchronous rotation, its short orbital period, and its high surface field strength are difficult to reconcile with the canonical expectation that mCVs will rapidly synchronize if B > ~10 MG. I also discuss two other mCVs for which P_spin/P_orb ~ 0.8-0.9: Paloma and the recently reidentified Swift J0503.7-2819 (Halpern 2022; Rawat et al. 2022).

Nova Her 2021 (V1674 Her) was the first classical nova that self-identified as an intermediate polar within one week of eruption. We have tracked the 501-second pulsation continuously over at least 5 magnitudes of decline, and find a period change ~100x faster than usually seen in IPs. We explore the hypotheses that (1) this simply reflects elevated accretion torques from very high mass transfer… and (2) this is what happens to all IPs in the millennia after their nova eruptions.

I will summarise the current status of GRB research in the multi-messenger era, discuss new emerging discoveries into the physics of GRB progenitors and classifications that are challenging the standard models and highlight the opportunities for future study given new ground- and space-based missions in planning or coming online in the next decade. I will highlight breakthroughs in studying the early time light caught soon after the initial explosion that have been enabled by new autonomous robotic technologies and hint at the discovery landscape in the era of future wide-field capabilities that facilities such as the Rubin Observatory, LUVOIR, Roman telescopes may provide. I will emphasis the importance of measuring polarisation for breaking model degeneracies and probing the magnetic field properties of high-energy sources to complement observations from the high-energy and neutrino detectors such as Cerenkov Telescope Array and IceCube.

Gamma-ray bursts (GRBs) are the most powerful stellar explosions detected in electromagnetic wavebands. The two varieties, the long and short bursts last for less than and more than two seconds, respectively and are of different origins. Core collapse of massive stars give births to the long bursts while the short bursts arise from coalescence of binary neutron stars or a neutron star and blackhole pairs. The exact mechanism for energy extraction from the central engine of a GRB is not known but recent observations give clues to emission mechanisms from ultra-relativistic jets produced by them. In this talk I will give a brief overview of GRB observations and theoretical understanding of the prompt and afterglow emissions in radio to very high-energy gamma rays. I will also discuss GRBs as potential sources of ultrahigh-energy cosmic rays and probes of fundamental physics based on recent detection of GRB 221009A.

Long-duration Gamma Ray Bursts (GRBs) are a result of the collapse of massive stars accompanied by relativistic outflows. The initial gamma-ray flash of a GRB is accompanied by a long-lasting afterglow visible from X-ray to radio wavelengths. The rate of radio afterglows detection is $\sim$ 30\%. The early evolution of radio afterglows (below 4 GHz) is through the optically thick regime. Therefore, the light curve peak corresponds to the transition from an optically thick to a thin regime. Hence, radio frequencies are unique in probing the evolution of the self-absorption frequency ($\nu_a$) which in turn can constrain the physical parameters. Due to the long-lived nature of radio afterglows, they serve as an excellent probe of GRB energetics and their environments. In this work, I will present the results of our efforts in observing the radio afterglows of GRBs at low frequencies with the Giant Meterwave Radio Telescope (GMRT). Multi-wavelength numerical modeling performed by combining data at all available wavelengths has allowed us to put constraints on the ambient medium density, collimation angle, shock microphysical parameters, and kinetic energy of the burst. I will also highlight the importance of future sensitive radio telescopes which will increase the detection rate significantly and would be able to answer some of the important issues related to afterglow calorimetry, emission mechanisms, and environments around the massive stars exploding as GRBs in the early Universe.

We present a detailed prompt emission and early optical afterglow analysis of the two very high energy (VHE) detected bursts GRB 201015A and GRB 201216C, and their comparison with a subset of similar bursts. Time-resolved spectral analysis of multi-structured GRB 201216C using the Bayesian binning algorithm revealed that during the entire duration of the burst, the low energy spectral index (α) remained below the limit of the synchrotron line of death. However, statistically some of the bins supported the additional thermal component. Additionally, the evolution of spectral parameters showed that both peak energy (E) and α pt tracked the flux. These results were further strengthened using the values of the physical parameters obtained by synchrotron modeling of the data. Our earliest optical observations of both bursts using FRAM-ORM and BOOTES robotic telescopes displayed a smooth bump in their early optical light curves, consistent with the onset of the afterglow due to synchrotron emission from an external forward shock. Using the observed optical peak, we constrained the initial bulk Lorentz factors of GRB 201015A and GRB 201216C to Γ0 = 204 and Γ0 = 310, respectively. The present early optical observations are the earliest known observations constraining outflow parameters and our analysis indicates that VHE-detected bursts could have a diverse range of observed luminosity within the detectable redshift range of present VHE facilities.

I will review the current status of our understanding of the physics of gamma-ray bursts, including progenitors, central engines, jet composition and radiation mechanisms. I will also present two recent special events: a long GRB with kilonova association (GRB 211211A) and the bright-of-all-time (BOAT) GRB 221009A, and discuss how they teach us more things about GRBs.

In this talk I will review our current understanding of the magnetic fields of young stars, focussing on stars with dynamo-generated magnetic fields.

The presence of a strong magnetic field is a feature common to a significant fraction of degenerate stars, yet little is understood about the field’s origin and evolution. New observational constraints from volume-limited surveys point to a more complex situation than a single mechanism valid for all stars. We show that in high-mass white dwarfs, which are probably the results of mergers, magnetic fields are extremely common and very strong and appear immediately in the cooling phase. These fields may have been generated by a dynamo active during the merging. Lower-mass white dwarfs, which are often the product of single-star evolution, are rarely detectably magnetic at birth, but fields appear very slowly, and very weakly, in about a quarter of them. What we may see is an internal field produced in an earlier evolutionary stage that gradually relaxes to the surface from the interior. The frequency and strength of magnetic fields continue to increase to eventually rival those of highly massive stars, particularly after the stars cool past the start of core crystallization, an effect that could be responsible for a dynamo mechanism similar to the one that is active in Earth’s interior.

My talk will focus on massive star formation and three questions: what are the mechanisms behind accretion and ejection processes in massive star formation ? Why do massive stars form more ofte in multiple systems than low-mass stars ? As I will show, using numerical simulations incorporating non-ideal MHD and radiative transfer, magnetic effects play a crucial role in answering those three questions.

We model helium-rich stars with solar metallicity (X = 0.7, Z = 0.02) progenitors that evolve to form AM Canum Venaticorum systems through a helium-star formation channel, with the aim to explain the observed properties of Gaia14aae and ZTFJ1637+49. We show that semi-degenerate, H-exhausted (X ⩽ 10 −5 ), He-rich (Y ≈ 0.98) donors can be formed after a common envelope evolution (CEE) phase if either additional sources of energy are used to eject the common envelope, or a different formalism of CEE is implemented. We follow the evolution of such binary systems after the CEE phase using the Cambridge stellar evolution code, when they consist of a He-star and a white dwarf accretor, and report that the mass, radius, and mass-transfer rate of the donor, the orbital period of the system, and the lack of hydrogen in the spectrum of Gaia14aae and ZTFJ1637+49 match well with our modelled trajectories wherein, after the CEE phase Roche lobe overflow is governed not only by the angular momentum loss (AML) owing to gravitational wave radiation (AML GR ) but also an additional AML owing to α − Ω dynamos in the donor. This additional AML is modelled with our double-dynamo (DD) model of magnetic braking in the donor star. We explain that this additional AML is just a consequence of extending the DD model from canonical cataclysmic variable donors to evolved donors. We show that none of our modelled trajectories matches with Gaia14aae or ZTFJ1637+49 if the systems are modelled only with AML GR .

In this talk, I will present an overview of the structure of Jupiter’s magnetosphere, and how the local ambient magnetic field at Ganymede’s orbit typically varies with time. This variation leads to an induced field response from a subsurface conducting shell at Ganymede, evidence for which has arisen from previous analysis of the Ganymede flybys from the Galileo mission. The induced field is one contribution among many others in Ganymede’s complex electromagnetic environment, which includes continuous merging and disconnection of the Jovian and Ganymedean field lines. The upcoming JUICE (Jupiter Icy moon Explorer) mission will have a dedicated Ganymede orbital phase designed to acquire optimum-accuracy monitoring of the magnetic environment at Ganymede, with a view to producing the most accurate mapping of the induced field. This in turn will allow us to better infer the depth, thickness and conducting properties of the subsurface ocean.

I will review recent progresses in the study of neutron star mergers including gamma-ray bursts, kilonovae and gravitational wave sources.

In recent times, emissions at TeV energies have been detected from Gamma Ray Bursts (GRBs) and their afterglows. The TeV emissions have been detected using the HESS and the MAGIC telescopes. In this work, I will review the properties of the TeV-bright long GRBs both in the prompt and afterglow emission phases. I will focus on the multi-wavelength modelling of the afterglow of GRB 190114C with an emphasis on its low-frequency evolution and the nature of the shock microphysical parameters.

The contemporaneous detection of gravitational waves and gamma rays from the GW170817/GRB 170817A confirmed compact binary neutron-star mergers as progenitors of short-duration gamma-ray bursts (GRBs). However, the nature and lifespan of the merger remnant powering these bright gamma-ray flashes remain largely debated. I would like to present our recently published results on GRB 180618A. Our multiwavelength observations suggest a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly spinning and highly magnetized neutron star (i.e. a millisecond magnetar). These findings suggest such neutron stars can survive the collapse to a black hole on timescales much larger than the expected few hundred milliseconds after the merger, and power the GRB itself through accretion.

Gamma-ray bursts (GRBs) comprise of short, bright, energetic flashes of emission from extragalactic sources followed by a longer afterglow phase of decreased brightness. Recent discoveries of GRB 180720B and GRB 190829A afterglow emission up to very-high-energy $\gamma$-rays by H.E.S.S. have raised important questions regarding the emission mechanism responsible. We interpret these observed afterglows to be the result of external Compton emission of ultrarelativistic electrons, and present predictions of spectra corrected for the $\gamma$-ray attenuation by absorption of photons through their interaction with the extragalactic background light (EBL). Thus, we fit an attenuated model $dN/dE={(dN/dE)_{\rm EC}}e^{-\tau(E,z)}$, where $(dN/dE)_{\rm EC}$ is the intrinsic external Compton spectrum, the exponential term corresponding to the attenuation, and $\tau$ is the energy-dependent optical depth for a source at redshift $z$, to the data. This will enable us to constrain the GRB environment assuming an external Compton emission mechanism which mitigates the particle energy requirements for the emission observed at late times and has consequences for the future observations of GRBs at these extreme energies.

The astrophysical origin of the rapid neutron capture (r-process) remains an outstanding mystery in nuclear astrophysics. I will review our evolving understanding of potential r-process sites, particularly those associated with neutron-rich outflows from the magnetized accretion disks generated following the merger of binary neutron stars and the core collapse of massive rotating stars. I will describe how these diverse channels are directly probed by the thermal kilonova or r-process-enriched supernova emission which follows these events, highlighting also the connection with the gravitational wave emission detected by LIGO/Virgo. Time permitting, I will describe a new type of stellar explosion – a “super-kilonova” – produced by the birth of the most massive spinning black holes, which may be discovered following gamma-ray bursts or by future infrared observations with the Roman Space Telescope.

The South African Radio Astronomy (SARAO) commissioned a 64 antenna array for the MeerKAT radio telescope in July 2018. The new array, being a significant upgrade from the previous phase of MeerKAT, was inaugurated alongside a detailed, high resolution image of Sagittarius A*. Today, MeerKAT continues to be a vital instrument for astronomers performing continuum and spectral line radio observations of various objects within our Galaxy and the rest of the Universe. Due to its exceptional sensitivity, MeerKAT has produced detailed images of Galactic plane revealing the presence of mysterious radio arcs, magnetised filaments, and supernova remnants in unprecedented detail. For large survey projects such as ThunderKAT, MeerKAT has provided precise measures of periodicity in pulsars and neutron stars detected within the local Universe. With these crucial observations, our window of understanding for the physical mechanisms underlying transient phenomena continues to be widened.

The Spektr-Roentgen-Gamma (SRG) Observatory has been successfully operating in orbit since 2019. During this time, it conducted several all sky surveys, as well as a significant number of observations of the most interesting regions and objects of the Universe. I will present an overview of the results obtained by the Mikhail Pavlinsky ART-XC telescope onboard SRG. They include all sky survey, gamma-ray bursts detections, follow-up observations of GW events detected during O3 run. Particular attention will be paid to the deep survey of our Galaxy currently being carried out by ART-XC, the discovery and study of new and transient objects, presumably magnetic white dwarfs and neutron stars, performed through the multiwavelenght observations together with other space and ground instruments. This work is supported by the Russian Foundation for Basic Research grant 19-29-11029.

Analysis of the polarization of electromagnetic radiation, or polarimetry, is a unique tool that allows us to obtain information about astrophysical objects that cannot be obtained in other ways, for example, regarding their geometry. With the launch of the IXPE (Imaging X-ray Polarimetry Explorer) mission at the end of last year, this instrument became available in the X-ray range as well. In my talk I will give a brief overview of the results obtained during the first year of IXPE observations of accreting X-ray pulsars (XRPs). It was found that in all observed XRPs, the measured value of the degree of polarization is below 15%, which is much less than the theoretically predicted values. In some pulsars, it was possible to study in detail the variations in the degree of polarization and the polarization angle as a function of the rotation phase of the neutron star, which, in turn, made it possible to determine the geometric parameters of the system. I will briefly discuss physical mechanisms that could potentially explain the relatively low degree of polarization in XRPs.

Optical follow-up observations of optical afterglows of Gamma-ray bursts (GRBs) are crucial to probe the geometry of outflows, emission mechanisms, energetics, and burst environments. Considering the longitudinal advantage of India, we initiated exploring these exciting and explosive astronomical sources using recently commissioned India’s largest optical telescope, i.e., 3.6m Devasthal Optical Telescope (DOT) at Devasthal observatory of Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. In a short period of the proposed target of opportunity (ToO) observations (since cycle 2020-C2), DOT discovered many interesting results such as the detection of long GRB (GRB 211211A) from the binary merger (kilonova emission), the discovery of most delayed optical flare (GRB 210204A) observed from any GRB so far, discoveries of dark and orphan afterglows, observations of optical counterparts of very high energy detected burst, the detection of host galaxies of peculiar GRBs, etc. In this talk, I will briefly summarize the recent discoveries and observations of GRBs using 3.6m DOT. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients as a part of time-domain astronomy in the future.

eROSITA on the SRG mission (Spektrum-Roentgen-Gamma) has performed 4 complete X-ray all-sky surveys with an imaging telescope array in the energy range 0.2-10 keV. It has a much improved spatial and spectral resolution compared to ROSAT, a significantly extended energy range, and the surveys have a longer exposure per sky pixel. In addition, repeated all-sky survey observations uncover strongly variable sources. The sensitivity between 0.3 and 2.3 keV is similar to that of XMM-Newton. All these properties make eROSITA the ideal discovery machine of magnetic cataclysmic variables. The talk will present the basics of the mission and the mission profile, reports the first serendipitous discoveries made and then describes the target selection and first results of systematic and comprehensive optical follow-up programmes that were initiated.

I want to present work that I have been doing recently on particle acceleration and gamma-ray emission in magnetic powered fast rotating white dwarfs like AE Aquarii and AR Scorpii. I will look at the possibility of curvature radiation and electron-positron production in the magnetospheres of these two white dwarfs and the influence it will have on gamma-ray emission as well as gamma-ray opacity in the system and the possibility of pair avalanches being produced, which will have consequences for synchrotron emission in the magnetosphere up to the corotation radius. This may be an interesting vehicle to explain the pulsar-like emission in the magnetosphere of both these systems.

Symbiotic stars are strongly interacting binaries composed of the white dwarf accreting from its red giant companion. (e.g., Munari & Renzini, 1992). We study five symbiotics stars: BD Cam, V1261 Ori, NQ Gem, CD -27 8661, CD -36 8436 using observations from the Swift/XRT and XMM-Newton satellites in X-rays and from TESS in optical. The X-ray spectra were fit with absorbed optically thin thermal plasma models, with either single- or multi- temperature. The spectra are all compatible with that arising from the most internal region of the accretion disk; the boundary layer. The TESS observations of these five systems show the presence of flickering, which is related with the presence of an accretion disk. These five symbiotics thus belong to the accretion-powered type, a finding supported by their low X-ray luminosity as well as the presence of flickering in their light curves.

The emission of hard X-rays around white dwarfs (WD) is generated by the presence of a stellar companion either by their coronal emission or by an acretion disk formed by the material stripped from the companion. In the recent years, studies has proven that a Jupiter-like planet can be the donor of material to create a disk of acretion and generate hard X-ray emission. With the GUACHO code, we reproduce the conditions of the scenario WD-planet and research the conditions and features for the hard X-ray emission observed and this variability. With the example of KPD\,0005+5106 we explore a first approximation for a future network of simulations. We observe that the material stripped from the planet, congregate around a WD and reach high enough temperatures to generate hard X-rays, similar to the process generate by a stellar companion.

Following 50 years of X-ray studies, we are at the threshold of a new era of fast multiwavelength timing studies of X-ray binaries. The optical and infrared regimes can directly measure the peak emission of the jet and hot flow in many accretion systems which, in turn, is determined by the plasma magnetic field. When combined with simultaneous X-ray observations, they can be a powerful tool to probe the accretion/outflow connection in ‘real-time’ and to measure key physical parameters of the various binary components. This field has long been handicapped by the lack of suitable detectors and the difficulty of multiwavelength coordination of observations, but this coordination possibilities are improving. I will review advances made in this field, concentrating on results from multiwavelength observations of black hole binaries in the hard state. I will also discuss prospects from some upcoming missions and mention what will still remain to be tested beyond these.