Black hole accretion and X-ray spectra of QSOS

Changes in Eddington accretion ratios are thought to result in X-ray spectral index changes in Galactic binary black hole systems. Objects with higher Eddington ratios have softer X-ray spectra. Can we apply this result to much more massive black hole systems such as QSOs? If so, X-ray observations will give us valuable insight into the physics of QSOs. Among QSOs, X-ray spectral index is part of a large set of correlated optical and UV observational properties, especially optical Fe II and [O III] strengths in the Hβ region. To investigate whether this set of correlations is related to Eddington ratio, we use as probes, BALQSOs that have been suggested to be youthful super-accretors. We conducted infrared spectroscopy of the Hβ rest wavelength region for a sample of BALQSOs and compared line measurements with those for high and low redshift non-BAL QSOs. Hβ line widths and bolometric luminosity are used to calculate QSO black hole masses and Eddington ratios. Our results support the hypothesis that optical Fe II and [O III] line strengths are Eddington ratio indicators in QSO central engines. A possible explanation is that strong Fe II and weak [O III] indicate abundant cold gas that could fuel near Eddington accretion.     

X-ray observations of black-hole accretion disks

Our current theoretical and observational understandings of the accretion disks around Galactic black-holes are reviewed. Historically, a simple phenomenological accretion disk model has been used to interpret X-ray observations. Although such a phenomenological interpretation is still useful, high quality X-ray data from contemporary instruments allow us to test more realistic accretion disk models. In a simple and ideal case, the standard optically thick accretion disk model is successful to explain observations, such that the inner disk radius is constant at three times the Schwarzschild radius over large luminosity variations. However, when disk luminosity is close to or exceeds the Eddington luminosity, the standard disk model breaks, and we have to consider the “slim disk” solution in which radial energy advection is dominant. Recent observations of Ultra-luminous X-ray sources (ULXs), which may not be explained by the standard disk model, strongly suggest the slim disk solution. We compare theoretical X-ray spectra from the slim disk with observed X-ray spectra of ULXs. We have found that the slim disk model is successful to explain ULX spectra, in terms of the massive stellar black-holes with several tens of solar mass and the super-Eddington mass accretion rates. In order to explain the large luminosities (>10⁴⁰ ergs s⁻¹) of ULXs, “intermediate black-holes” (>100M) are not required. Slim disks around massive stellar black-holes of up to several tens of solar mass would naturally explain the observed properties of ULXs.     

Observational appearances of isolated stellar-mass black hole accretion – Theory and observations

General properties of accretion onto isolated stellar-mass black holes in the Galaxy are discussed. An analysis of plasma internal energy growth during the infall is performed. Adiabatic heating of collisionless accretion flow due to magnetic adiabatic invariant conservation is 25% more efficient than in the standard non-magnetized gas case. It is shown that magnetic field line reconnections in discrete current sheets lead to significant nonthermal electron component formation, which leads to a formation of a hard (UV, X-ray, up to gamma), highly variable spectral component in addition to the standard synchrotron optical component first derived by Shvartsman generated by thermal electrons in the magnetic field of the accretion flow. Properties of accretion flow emission variability are discussed. Observation results of two single black hole candidates – gravitational lens MACHO-1999-BLG-22 and radio-loud X-ray source with featureless optical spectrum J1942+10 – in optical band with high temporal resolution are presented and interpreted in the framework of the proposed model.     

Composite profile of the Fe Kα spectral line emitted from a binary system of supermassive black holes

We used a model of a relativistic accretion disk around a supermassive black hole (SMBH), based on ray-tracing method in the Kerr metric, to study the variations of the composite Fe Kα line emitted from two accretion disks around SMBHs in a binary system. We assumed that the orbit of such a binary is approximately Keplerian, and simulated the composite line shapes for different orbital elements, accretion disk parameters and mass ratios of the components. The obtained results show that, if observed in the spectra of some SMBH binaries during their different orbital phases, such composite Fe Kα line profiles could be used to constrain the orbits and several properties of such SMBH binaries.     

Self-similar problems of the time-dependant discs accretion and the nature of the temporary X-ray sources

Self-similar solutions, obtained by means of non-linear hydrodynamic equations, describing the picture of the time-dependant discs accretion. Some of the solutions describe the evolution of the accumulating disc, which is created around the neutron star with a strong magnetic field. Others give the time evolution of the initial narrow material ring, thrown at a certain distance around the gravitation centre. Practically all problems of the time-dependant disc accretion, asymptotically come to the above mentioned solutions.It is shown that the time-dependant disc accretion of the gas of the neutron star from the accumulating disc cause recurrent X-ray outbursts. The fall of the X-ray luminosity at the maximum stage depends of the time to a certain degree; is near the ones which are observed in the time-dependant X-ray sources. The model of the time-dependant X-ray sources is explained in the framework of the time-dependant disc accretion of magnetized neutron star with a strong magnetic field.     

Determination of K,Ar, Cl,S, Si and Al flare abundances from RESIK soft X-ray spectra

The RESIK is a high sensitivity, uncollimated bent crystal spectrometer which successfully operated aboard Russian CORONAS-F solar mission between 2001 and 2003. It measured for the first time in a systematic way solar soft X-ray spectra in the four wavelength channels from 3.3 Å to 6.1 Å. This range includes characteristic strong lines of H- and He-like ions of K, Ar, Cl, Si, S and Al in the respective spectral channels. A distinguishing feature of RESIK is its possibility of making reliable measurements of the continuum radiation in flares. Interpretation of line and the continuum intensities observed in vicinity of respective strong lines provides diagnostics of plasma temperature and absolute abundances of K, Ar, Cl, S, Si and Al in several flares. We analyzed the observed intensities of spectral lines and the nearby continuum using the CHIANTI v5.2 atomic data package. A specific, so-called “locally isothermal” approach has been used in this respect allowing us to make not only flare-averaged abundance estimates, but also to look into a possible variability of plasma composition during the course of flares.