The entropy increases enormously when a star collapses into a black hole. For the radiation dominated star, the temperature has decreased from the almost gravitational equilibrium one to the so-called black hole temperature. If we interpret this temperature decrease as the volume increase of the black hole, the entropy increase could be understood as the free expansion of radiation throughthe increased volume. The expected volume size is tentatively estimated. Under this interpretation, it is derived that the entropy is proportional to the horizon surface area and the microscopic states of the black hole entropy could be understood as the statistical states of the enlarged phase space.

By a simple physical consideration and uncertain principle, we derive that temperature is proportional to the surface gravity and entropy is proportional to the surface area of the black hole. We apply the same consideration to de Sitter space and estimate the temperature and entropy of the space, then we deduce that the entropy is proportional to the boundary surface area. By the sameconsideration, we estimate the temperature and entropy in the uniformly accelerated system (Rindler space). The cases in higher dimensions are considered.

We study linear polarization from scattering of light on a cloud of relativistic electrons. We assume that the cloud is hovering above an accretion disc, or it is in an accelerated motion underthe combined influence of the radiation and gravitational fields near an accreting black hole. Atfirst we derive simple and general analytical formulae for the Stokes parameters. These formulae are then used in calculations of the temporal evolution of the observed signal.

We find that higher-order images can significantly enhance the observed flux. Possible targets where the effect should be searched are accreting super-massive black holes and Galactic microquasars exhibiting episodic accretion/ejection events.

We apply the Boltzmann-Plot (BP) method to the Balmer lines to estimate the physical propertiesin the Broad Line Region of Active Galactic Nuclei. We study the Balmer lines of a sample of 90 AGN from Sloan Digital Sky Survey database, as well as the time variability of the BP parameter A of NGC 5548.

Fe K emission line is a powerful probe of the inner part of an accretion disk. We analyze X-ray spectra of 43 Palomar-Green (PG) quasars taken from Boroson & Green (1992) observed with XMM-Newton and make an averaged Fe K line profile. We study the Eddington ratio dependence of the Fe K line profile. The width of the Fe line becomes broader (σ=0.1 to 0.7 keV) and its peak energy becomes higher (6.4 to 6.8 keV) as the Eddington ratio gets larger.

These results indicate that the physical state of the accretion disk, such as the geometrical structure and/or ionization state, changes with the Eddington ratio.

We analyze the asymptotic directional structure and properties of gravitational field of specific exact solutions belonging to the large family of black hole spacetimes of type D found by Plebański and Demiański. We discuss the structure in the case of de Sitter (Λ>0) and anti–de Sitter (Λ<0) conformal infinity. With the aim of further interpretation, we have found the relation between the structure of the sources (the mass m, the electric e and magnetic g charges, the NUT parameter l, the rotational parameter a, the acceleration α) and the properties of radiation generated by them. In particular, we studied the amplitude of radiation and illustrated it in several figures.

Selected results of 1994-2004 optical photometric and high-resolution spectral observations including those obtained in the frame of a coordinated CIS countries campaign “Optical Monitoring of Unique Astrophysical Objects” and their comparison with ASM/RXTE X-ray data are briefly reviewed.

The process of accretion onto the isolated black holes under the various conditions of the ISM and its observational manifestations are discussed. For the majority of the Galaxy volume the accretion rate is as low as 10−6–10−9 of Eddington one, and the accretion is spherically-symmetric. Such objects manifest itself as a weak optical and x-ray sources with featureless spectra and the significant variability of the emission.

For the BH located inside the dense molecular cloud the regime of accretion depends on its massand velocity. A massive (100–1000M⊙) BH born in the cloud or having low relative velocity, may manifest itself as a ultra-luminous x-ray source (ULX).

After discovering microquasars – active stellar systems with relativistic outflows (Mirabel et al. 1992) – many papers bringing the observational arguments on similar nature of accreting systems with different BH masses (from stellar to galactic nuclei) have appeared. It became clear that a level of activity in such systems could depend on few parameters: BH mass and angular momentum; accretion rate and magnetization parameter of accretion disk (AD); relative orientation of BH and AD angular momentum. The bolometric luminosity and characteristic variation time in such systems is proportional to BH masses, which span an interval in 5–8 decades.

The formation of the relativistic jets and a non-thermal emission from the collapsing magnetized stars with dipole magnetic fields and the heterogeneous particles distribution are investigated. These polar jets are formed when the stellar magnetosphere compress during collapse its magnetic field increases considerable. The electric field is produced in magnetosphere, which the charged particles will be accelerated. As follow from the calculation, the jets can be formed from collapsing stars already the explosion of supernova stars without shock waves. These jets will generate the non-thermal radiation. The radiation flux depends on the distance to the star, its magnetic field and the particle spectrum in the magnetosphere. This flux can be observed near Earth by means of modern telescopes in the form of the radiation pulse with duration equal to time collapse.

We give a brief summary of the ongoing Abastumani Active Galactic Nuclei Monitoring Program started in the May 1997. More than 110000 frames are obtained during more than 1300 nights of observations for about 50 target objects, among them gamma-ray, X-ray and optical blazars. All observations were done in the BVRI bands using ST-6 CCD based photometer attached to the Newtonian focus of 70-cm meniscus telescope.

We present optical R band photometry of nine X-ray selected BL Lac objects: 1ES 0229+200, 1ES 0323+022, 1ES 502+675, 1ES 0647+250, 1ES 0806+524, 1ES0927+500, 1ES 1028+511, 1ES 1959+650, 1ES2344+514.

Variability on long time scales within one magnitude in R band was found for all of the observed objects, except 1ES 0229+200 and 1ES0927+500. Largest variation was detected for 1ES 0502+675 and equals to 1.07 mag. Only few objects show statistically significant variation on intra-day scale.

We study the possibility of pure general relativistic models without exotic matter to describe the observed flattening of the rotation curves for stars moving in circular orbits in a galaxy disk. In particular we consider the dragging of inertial frames (rotation of the source) and the presence of a Taub–NUT (Newman–Unti–Tamburino) “charge”, the gravitational equivalent to a magnetic monopole in electrodynamics.

Since the mass-radius relation is quite different for a main sequence (MS) star and a giant (G) star, we find that the radiation efficiencies in the star capture processes by a black hole (BH) are also very different. This may provide a useful way to distinguish the capture of MS and G stars. Comparing with observations of the very high energy (VHE) gamma-ray emissions, we argue the event that triggers the gamma-ray emission in the energy range 4–40 TeV should be a G star capture. On the other hand, the capture of MS stars by the massive BH is required when the measured spectrum of VHE gamma-rays extends from 109 to 1015 eV.

We present the results of global 3D MHD simulations of optically thin black hole accretion flows. The initial disk is embedded in a low density, spherical, isothermal halo and threaded by weak (β ≡ Pgas/Pmag = 100) toroidal magnetic field. General relativistic effects are simulated by using the pseudo-Newtonian potential. When the Maxwell stress in the innermost region of the disk is reduced due to the loss of magnetic flux or by decrease of disk temperature, inner torus is created around 4 – 10rs. We found that in such an inner torus, one-armed (m = 1) density enhancement grows and that the inner torus oscillates quasi-periodically. The oscillation period is about 0.1s when we assume a 10M⊙ black hole. This frequency agrees with the low-frequency QPOs observed in low/hard state of black hole candidates. The disk ejects winds whose opening angle is about 30 degree. The maximum velocity of the wind is about 0.05c.

We report on recent polarimetric observations of the 18± 3 min quasi-periodicity present in near-infrared flares from Sagittarius A*. Observations in the K-band allow us a detailed investigation of the flares and their interpretation within the hot spot model. The interplay of relativistic effects plays a major role. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we give constraints to the parameters of the hot spot model, in particular, the dimensionless spin parameter of the black hole and its inclination. We consider all general relativistic effects that influence the polarization lightcurves. The synchrotron mechanism is most likely responsible for the intrinsic polarization. We consider two different magnetic field configurations as approximations to the complex structure of the magnetic field in the accretion flow. Considering the quality of the fit, we suggest that the spot model is a good description of the origin for the QPOs in NIR flares.

We discuss that in the extreme relativistic limit, i.e., when z ≫ 1, where z is the surface gravitational redshift, there could be radiation pressure supported and dominated stars with arbitrary gravitational mass, high or low. Such objects are called Eternally Collapsing Objects (ECOs). ECOs are practically as compact as Schwarzschild black holes (BH) and, observationally, are likely to be mistaken as BHs. Further since any object undergoing continued collapse must first become an ECO before becoming a true BH state characterized by M = 0, the observed BH Candidates are ECOs.

We perform Monte Carlo calculation to determine the radiation spectra from magnetized, low angular momentum accretion flow. Magneto-hydrodynamical simulation (with angular momentum parameter lambda of about 2 Rg c) was performed by Proga & Begelman 2003. Because simulation neglect radiative cooling, we compute electron temperature separately, including ohmic heating, parametrized with small δ coefficient, ion-electron coupling, radiative cooling and advection. Radiation spectra are obtained taking into account, thermal synchrotron and bremsstrahlung radiation, self absorption and Comptonization processes.

We show which parts of the flow are responsible for characteristic spectral features and how the spectrum changes in different accretion states. We compare our results with Galactic Center black hole radiation spectra, where low angular momentum accretion is suggested. Accretion state changes seems to be a promising model for the flaring behavior of Galactic Center black hole. We also show the radiation intensity maps in radio and X-rays energy band for viewing angle i = 90°.

We model the K-correction for emission lines formed on the irradiated face of companion stars in compact binaries. We compute this K-correction in a general approach as function of the mass ratio and the disc flaring angle. Our results, combined with the detection of high excitation emission lines arising from the donor star, can be used to set constraints to the masses of neutron stars and black holes in persistent Low Mass X-ray Binaries (LMXBs).

The application of our model to X1822-371 (Muñoz-Darias, Casares & Martínez-Pais 2005) lends strong support to the presence of a massive neutron star in this LMXB (i.e. MNS > 1.6M⊙). Here, we also present the K-correction for the Black Hole binary GX339-4, where we obtain a solid lower limit to the black hole mass of MBH > 5.8M⊙.

We present the proper motion study of the thin filaments observed in L'-band (3.8 μm) adaptive optics images of the central parsec of the Milky Way. Observed filaments are associated with the mini-spiral and, in some cases, with stars. They can be interpreted as shock fronts formed by the interaction of a central wind with the mini-spiral or extended dusty stellar envelopes.