We present the long term evolution of the power spectrum of the black hole candidate Cygnus X-1 in the 0.002– 128 Hz frequency range as monitored from 1998 to 2001 with the Rossi X-ray Timing Explorer (RXTE). For most of this period the source was in its hard state. The power spectral density (PSD) is well modeled as the sum of four Lorentzians, which describe distinct broad noise components. B...

We present a model for the short timescale spectral variability of accreting black holes. It describes well the time-averaged spectra as well as temporal characteristics such as powerdensity spectrum, time/phase lags, and coherence function of Cygnus X-1. We assume that X/γ-rays are produced in compact magnetic flares at radii <∼ 100GM/c from the central black hole. The tendency for magnetic lo...

With their relatively fast variability time-scales, Galactic X-ray binaries provide an excellent laboratory to explore the physics of accretion and related phenomena, most notably outflows, over different regimes. After comparing the phenomenology of jets in black hole systems to that of neutron stars, here I discuss the role of the jet at very low Eddington ratios, and present preliminary resu...

Four XMM-Newton observations of the core of M31, spaced at 6 month intervals, show that the brightest point X-ray source, RXJ0042.6+4115, has a 0.4–10 keV luminosity of ∼5 10 erg s−1, and exhibits significant variability in intensity and X-ray spectrum over a time scale of ∼100 s including hard flares; such behaviour is only observed in Z-sources and transient black hole binaries in our Galaxy....

The Galactic center provides a unique astrophysical laboratory for us to study various astrophysical processes. In this paper, we review and outline the latest results from observations of Sgr A∗ in terms of source structure and variations in flux density. Sgr A∗ phenomenon represents a typical case of low radiative efficiency accretion flow surrounding a supermassive black hole in low luminosi...

We recorded spectra of the highly variable Seyfert 1 galaxy Mrk 110 in a variability campaign with the 9.2m Hobby-Eberly Telescope at McDonald Observatory in order to study the detailed line profile variations of the broad emission lines. Here we show that only an AGN model predicting the formation of the broad Hβ line emission in the wind of an accretion disk matches the observed 2-D variabili...

Variability of active galactic nuclei is now a well-known phenomenon. This remains to be fully explained by a theoretical model of the central engine. Time scales of AGN variability seem to range continuously from hours up to months. The short time scale variability must be related to the phenomena on the event horizon of the black hole, while the long one to those in the accretion disk or surr...

We report a correlation between velocity offset (β = v/c) of strong Mg II absorption systems and the bolometric luminosity (Lbol) of quasars in SDSS-DR7. We find that β shows a powerlaw increase with Lbol, with a slope ∼1/4. We find that such a relation of β with Lbol is expected for outflows driven by scattering of black hole radiation by dust grains, and which are launched from the innermost ...

Despite all the already existing observational data, current models still cannot explain completely the excessive energy output and the time variability of GRB. One of the reasons for this is the lack of a good model of the central engine of GRB. A major problem in the proposed models with a black hole (BH) in the center is that they don’t explain the observed evidences of late time activity of...

The relationship between the black hole mass and velocity dispersion indicated with [O III] line width is investigated for a sample of 87 flat-spectrum radio quasars (FSRQs) selected from SDSS DR3 quasar catalogue. We found the Mbh − σ[OIII] relation is deviated from Tremaine et al. relation for nearby inactive galaxies, with a larger black hole mass at given velocity dispersion. There is no st...