Related papers: Extreme AGN variability: evidence of magnetically …
X-ray obscuration of active galactic nuclei (AGNs) is considered in the context of ionized winds of stratified structure launched from accretion disks. We argue that a Compton-thick layer of a large-scale disk wind can obscure continuum…
The standard accretion discs are known to be thermally and viscously unstable over a certain range of temperatures. In the inner disc regions there may develop radiation pressure driven instability, which is possibly related to the rapid…
We have observed with XMM-Newton four radiatively efficient active type 1 galaxies with black hole masses < 10^6 Msun, selected optically from the SDSS. We show here that their soft X-ray spectrum exhibits a soft excess with the same…
We model the broadband (optical/UV and X-ray) continuum spectrum of the "changing-look" active galactic nucleus (AGN) Mrk 1018 as it fades from Seyfert 1 to 1.9 in $\sim 8$ years. The brightest spectrum, with Eddington ratio $L/L_{\rm…
The $\alpha$ turbulent viscosity formalism for accretion discs must be interpreted as a mean field theory. The extent to which the disc scale exceeds that of the turbulence determines the precision of the predicted luminosity $L_\nu$. The…
The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to…
When Beppo-SAX measured the 0.1--12 keV spectrum of RE J1034+396, observations in the optical, UV and EUV were also taken within a few weeks. This multiwavelength spectrum placed very strong constraints on its unusually hot big blue bump…
Accretion disc outbursts are re-occurring events observed in various astrophysical systems, including X-ray binaries and cataclysmic variables. These outbursts are characterized by a sudden increase in luminosity due to various…
We present a study of X-ray ionization of magnetohydrodynamic (MHD) accretion-disk winds in an effort to constrain the physics underlying the highly-ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various…
X-ray observations of active galactic nuclei (AGN) show variability on timescales ranging from a few hours up to a few days. Some of this variability may be associated with occultation events by clouds in the broad line region. In this…
The presence of the warm absorber of considerable optical depth is seen in many AGN. We show that this medium may affect significantly the optical/UV spectrum of an AGN by backscattering a fraction of the total radiation flux towards the…
The origin of the soft X-ray excess in AGN has been a mystery ever since its discovery. We present how the time variability of this spectral component can point towards its origin. Using the powerful technique of multi-epoch fitting, we…
(Abridged) Recent XMM-Newton studies of X-ray variability in the hard states of black hole X-ray binaries (BHXRBs) imply that the variability is generated in the 'standard' optically-thick accretion disc. The variability originates in the…
We present a systematic, analytical study of geometrically thin, optically thick accretion disc solutions for magnetized turbulent flows, with an alpha-like viscosity prescription. Under the only assumptions that (1) Magneto-Rotational…
We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor…
We collate active galactic nuclei (AGN) with reported detections of both relativistic reflection and ultra-fast outflows. By comparing the inclination of the inner disc from reflection with the line-of-sight velocity of the outflow, we show…
It is now widely accepted that most ultraluminous X-ray sources (ULXs) are binary systems whose large (above $10^{39}$ erg s$^{-1}$) apparent luminosities are explained by super-Eddington accretion onto a stellar-mass compact object. Many…
We present a review of the topics of X-ray stellar tidal disruption events (TDEs) and changing-look active galactic nuclei (AGN). Stars approaching a supermassive black hole (SMBH) can be tidally disrupted and accreted. TDEs were first…
Recurring outbursts associated with matter flowing onto compact stellar remnants (black-holes, neutron stars, white dwarfs) in close binary systems, provide strong test beds for constraining the poorly understood accretion process. The…
Rapid progress in multi-wavelength observations of Seyfert Galaxies in recent years is providing evidence that X-ray emission in these objects may be produced by magnetic flares occurring above a cold accretion disk. Here we attempt to…