We formulate and solve the equations governing the transonic behaviour of a general relativistic black-hole accretion disc with non-zero advection velocity. We demonstrate that a relativistic Rankine-Hugoniot shock may form leading to the formation of accretion powered outflow. We show that the critical points of transonic discs generally do not coincide with the corresponding sonic points. The collection of such sonic points forms an axisymmetric hypersurface, generators of which are the acoustic null geodesics, i.e. the phonon trajectories. Such a surface is shown to be identical with an acoustic event horizon. The acoustic surface gravity and the corresponding analogue horizon temperature TAH at the acoustic horizon are then computed in terms of fundamental accretion parameters. Physically, the analogue temperature is associated with the thermal phonon radiation analogous to the Hawking radiation of the black-hole horizon.Thus, an axisymmetric black-hole accretion disc is established as a natural example of the classical analogue gravity model, for which two kinds of horizon exist simultaneously. We have shown that for some values of astrophysically relevant accretion parameters, the analogue temperature exceeds the corresponding Hawking temperature. We point out that acoustic white holes can also be generated for a multi-transonic black-hole accretion with a shock. Such a white hole, produced at the shock, is always flanked by two acoustic black holes generated at the inner and the outer sonic points. Finally, we discuss possible applications of our work to other astrophysical events which may exhibit analogue effects.

We investigate the origin of the soft X-ray excess emission from narrow-line Seyfert 1 galaxies Akn 564 and Mrk 1044 using XMM-Newton observations. We find clear evidence for time delays between the soft and hard X-ray emission from Akn 564 based on a ∼ 100 ks long observation. The variations in the 4 − 10 keV band lag behind that in the 0.2 − 0.5 keV band by 1768 ± 122 s. The full band power density spectrum (PDS) of Akn 564 has a break at ∼ 1.2 × 10 −3 Hz with power-law indices of ∼ 1 and ∼ 3 below and above the break. The hard (3 − 10 keV) band PDS is stronger and flatter than that in the soft (0.2 − 0.5 keV) band. Based on a short observation of Mrk 1044, we find no correlation between the 0.2 − 0.3 keV and 5 − 10 keV bands at zero lag. These observations imply that the soft excess is not the reprocessed hard X-ray emission. The high resolution spectrum of Akn 564 obtained with the reflection grating spectrometer (RGS) shows evidence for a highly ionized and another weakly ionized warm absorber medium. The smeared wind and blurred ionized reflection models do not describe the EPIC-pn data adequately. The spectrum is consistent with a complex model consisting of optically thick Comptonization in a cool plasma for the soft excess and a steep power-law, modified by two warm absorber media as inferred from the RGS data and the foreground Galactic absorption. The smeared wind and optically thick Comptonization models both describe the spectrum of Mrk 1044 satisfactorily, but the ionized reflection model requires extreme parameters. The data suggest two component corona -a cool, optically thick corona for the soft excess and a hot corona for the power-law component. The existence of a break in the soft band PDS suggests a compact cool corona that can either be an ionized surface of the inner disk or an inner optically thick region coupled to a truncated disk. The steep power-law component is likely arising from an extended region.

Multi-wavelength observations of Galactic black hole candidate sources indicate a close connection between the accretion disk emission and the jet emission. The recent discovery of an anti-correlated time lag between the soft and hard X-rays in Cygnus X-3 (Choudhury & Rao 2004) constrains the geometric picture of the diskjet connection into a truncated accretion disk, the truncation radius being quite close to the black hole. Here we report the detection of similar anti-correlated time lag in the superluminal jet source GRS 1915+105. We show the existence of the pivoting in the X-ray spectrum during the delayed anti-correlation and we also find that the QPO parameters change along with the spectral pivoting. We explore theoretical models to understand this phenomenon.

We present the results of an analysis of data from XMM-Newton and Chandra observations of the highluminosity narrow-line quasar PG 1404ϩ226. We confirm a strong soft X-ray excess in the X-ray spectrum, and we find rapid variability (a factor of 2 in about 5000 s). When the X-ray spectrum is fitted with a two-component model that includes a power-law and a blackbody component, we find that low-energy absorption lines are required to fit the data. If we interpret these lines as due to highly ionized species of heavy elements in an outflowing accretion disk wind, an outflow velocity of ∼26,000 km s Ϫ1 could be derived. One interesting feature of the present observation is the possible detection of variability in the absorption features: the absorption lines are visible only when the source is bright. From the upper limits of the equivalent widths of the absorption lines during the low-flux states and also from the model-independent pulse-height ratios, we argue that the strength of absorption is lower during the low-flux states. This constrains the physical size of the absorbing medium within 100 Schwarzschild radius (R S ) of the putative supermassive black hole. We also find a marginal evidence of a correlation between the strength of the absorption line and the X-ray luminosity.

We report the discovery of hard X-ray delays in the X-ray emission of the Seyfert 1 galaxy Mrk 110, based on a long XMM-Newton observation. Cross correlation between the X-ray light curves of different energy bands reveals an energy dependent delay ranging from a few minutes to an hour. We find that the energy spectrum can be modeled by Comptonization of disk blackbody photons. The energy dependent delay can be modeled as due to the effect of Comptonization in a hot plasma confined within 10 Schwarzschild radius of the black hole. We discuss our results in the context of inverse Comptonization of the soft photons by highly energetic plasma.