V. Karas

AN EXTENDED SCHEME FOR FITTING X-RAY DATA WITH ACCRETION DISK SPECTRA IN THE STRONG GRAVITY REGIME

Accreting black holes are believed to emit X-rays, which then mediate information about strong gravity in the vicinity of the emission region. We report on a set of new routines for the XSPEC package for analyzing X-ray spectra of black-hole accretion disks. The new computational tool significantly extends the capabilities of the currently available fitting procedures that include the effects of strong gravity and allows one to systematically explore the constraints on more model parameters than previously possible (e.g., black-hole angular momentum). Moreover, axial symmetry of the disk intrinsic emissivity is not assumed, although it can be imposed to speed up the computations. The new routines can be used also as a stand-alone and flexible code with the capability of handling time-resolved spectra in the regime of strong gravity. We have used the new code to analyze the mean X-ray spectrum from the long XMM-Newton 2001 campaign of the Seyfert 1 galaxy MCG -6-30-15. Consistent with previous findings, we obtained a good fit to the broad Fe K line profile for a radial line intrinsic emissivity law in the disk that is not a simple power law, and for near maximal value of black hole angular momentum. However, equally good fits can be obtained also for small values of the black hole angular momentum. The code has been developed with the aim of allowing precise modeling of relativistic effects. Although we find that current data cannot constrain the parameters of black-hole/accretion disk system well, the code allows, for a given source or situation, detailed investigations of what features of the data future studies should be focused on in order to achieve the goal of uniquely isolating the parameters of such systems.

Effects of Kerr space-time on spectral features from X-ray illuminated accretion discs

ABSTRA C T We performed detailed calculations of the relativistic effects acting on both the reflection continuum and the iron line from accretion discs around rotating black holes. Fully relativistic transfer of both illuminating and reprocessed photons has been considered in Kerr space‐time. We calculated overall spectra, line profiles and integral quantities, and present their dependences on the black hole angular momentum. We show that the observed EW of the lines is substantially enlarged when the black hole rotates rapidly and/or the source of illumination is near above the hole. Therefore, such calculations provide a way to distinguish between different models of the central source.

Evidence for a relativistic iron line in GRS 1915+105

We report on the discovery of a relativistic iron K fluorescent emission line in the BeppoSAX spectrum of the microquasar GRS 1915+105, taken on April 19, 1998, when the line was unusually intense. The feature is broad and skewed, clearly indicating emission from the innermost regions of the accretion disc. The inner emitting orbit is larger than the innermost stable orbit, even in Schwarzschild metric: thus, a non-zero BH spin is not required by these data.

Non-Linear Resonance in Nearly Geodesic Motion in Low-Mass X-Ray Binaries

We have explored the ideas that parametric resonance affects nearly geodesic motion around a black hole or a neutron star, and that it may be relevant to the high-frequency (twin) quasi-periodic oscillations that occur in some low-mass X-ray binaries. We have assumed the particles or uid elements of an accretion disc to be subject to an isotropic perturbation having a hypothetical but rather general form. We nd that the parametric resonance is indeed excited close to the radius where epicyclic frequencies of the radial and meridional oscillations are in a 2 : 3 ratio. The location and frequencies of the highest amplitude excitation vary with the strength of the perturbation. These results agree with actual frequency ratios of twin kHz QPOs that have been reported in some black hole candidates, and they may be consistent also with correlation of the twin peaks in Sco X-1.

Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares

Context. Recent near-infrared polarization measurements of Sgr A* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ∼20 min in agreement with previous observations. Aims. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. Methods. We discuss the significance of the 20 min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Results. Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values ≥35° on a 3σ level.

On the origin of the broad, relativistic iron line of MCG–6-30-15 observed by XMM-Newton

The relativistic iron line prole recently observed by XMM-Newton in the spectrum of the Seyfert 1 galaxy MCG{6-30-15 (Wilms et al. 2001) is discussed in the framework of the lamp-post model. It is shown that the steep disc emissivity, the large line equivalent width and the amount of Compton reflection can be self-consistently reproduced in this scenario.

K-band polarimetry of an Sgr A* flare with a clear sub-flare structure ⋆

Context. The supermassive black hole at the Galactic center, Sgr A*, shows frequent radiation outbursts, often called ’flares’. I n the near-infrared some of these flares were reported as showing i ntrinsic quasi-periodicities. The flux peaks associated wi th the quasi-periodic behavior were found to be highly polarized. Aims. The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation fr om Sgr A* and to again provide strong support for the quasi-periodicity of∼18± 3 min reported earlier. Methods. Observations were carried out at the European Southern Observatory’s Very Large Telescope on Paranal, Chile. We used th e NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetr ic lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray -tracing code in combination with a simple hot-spot/ring model. Results. We report on new polarization measurements of a K-band flare f rom the supermassive black hole at the Galactic center. The data provide very strong support for a quasi-periodicity of 15.5± 2 min. The mean polarization of the flare is consistent with th e direction of the electric field vector that was reported in previous obser vations. The data can be modeled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i> 20 ◦ on a 3σ level, and the dimensionless spin parameter of the black hole is derived to be a⋆> 0.5.

The structure and radiation spectra of illuminated accretion disks in AGN - II. Flare/spot model of X-ray variability

We discuss a model of the X-ray variability of active galactic nuclei (AGN). We consider multiple spots that originate on the surface of an accretion disk following intense irradiation by coronal flares. The spots move with the disk around the central black hole and eventually decay while new spots continuously emerge. We construct time sequences of the spectra of the spotted disk and compute the corresponding energy-dependent fractional variability amplitude. We explore the dependence on the disk inclination and other model parameters. AGN seen at higher inclination with respect to the observer, such as Seyfert 2 galaxies, are expected to have a fractional variability amplitude of the direct emission that is by a factor of a few higher than objects seen face on, such as Seyfert 1s.

Polarization signatures of strong gravity in active galactic nuclei accretion discs

The effects of strong gravity on the polarization of the Compton reflection from an X-ray-illuminated accretion disc are studied. The gravitational field of a rotating black hole influences Stokes parameters of the radiation along the propagation to a distant observer. Assuming the lamp-post model, the degree and the angle of polarization are examined as functions of the observers inclination angle, of the height of the primary source and of the inner radius of the disc emitting region. It is shown that polarimetry can provide essential information on the properties of black holes sources, and it is argued that time variation of the polarization angle is a strong signature of general-relativity effects. The expected polarization degree and angle should be detectable by new-generation polarimeters, such as that planned for the Xeus mission.

Gravitating discs around black holes

Fluid discs and tori around black holes are discussed within different approaches and with the emphasis on the role of disc gravity. First reviewed are the prospects of investigating the gravitational field of a black hole–disc system using analytical solutions of stationary, axially symmetric Einstein equations. Then, more detailed considerations are focused to the middle and outer parts of extended disc-like configurations where relativistic effects are small and the Newtonian description is adequate.Within general relativity, only a static case has been analysed in detail. Results are often very inspiring. However, simplifying assumptions must be imposed: ad hoc profiles of the disc density are commonly assumed and the effects of frame-dragging are completely lacking. Astrophysical discs (e.g. accretion discs in active galactic nuclei) typically extend far beyond the relativistic domain and are fairly diluted. However, self-gravity is still essential for their structure and evolution, as well as for their radiation emission and the impact on the surrounding environment. For example, a nuclear star cluster in a galactic centre may bear various imprints of mutual star–disc interactions, which can be recognized in observational properties, such as the relation between the central mass and stellar velocity dispersion.