M. Bursa

Monitoring the Dusty S-cluster Object (DSO/G2) on its Orbit toward the Galactic Center Black Hole

We analyze and report in detail new near-infrared (1.45-2.45 ?m) observations of the Dusty S-cluster Object (DSO/G2) during its approach to the black hole at the center of the Galaxy that were carried out with the ESO Very Large Telescope/SINFONI between 2014 February and September. Before 2014 May we detect spatially compact Br? and Pa? line emission from the DSO at about 40 mas east of Sgr A*. The velocity of the source, measured from the redshifted emission, is 2700???60 km s?1. No blueshifted emission above the noise level is detected at the position of Sgr A* or upstream of the presumed orbit. After May we find spatially compact Br? blueshifted line emission from the DSO at about 30 mas west of Sgr A* at a velocity of ?3320???60 km s?1 and no indication for significant redshifted emission. We do not detect any significant extension of the velocity gradient across the source. We find a Br? line FWHM of 50???10 ? before and 15???10 ? after the peribothron transit, i.e., no significant line broadening with respect to last year is observed. Br? line maps show that the bulk of the line emission originates from a region of less than 20 mas diameter. This is consistent with a very compact source on an elliptical orbit with a peribothron time passage in 2014.39???0.14. For the moment, the flaring activity of the black hole in the near-infrared regime has not shown any statistically significant increment. Increased accretion activity of Sgr A* may still be upcoming. We discuss details of a source model according to which the DSO is a young accreting star rather than a coreless gas and dust cloud.

Testing wind as an explanation for the spin problem in the continuum-fitting method

The continuum-fitting method is one of the two most advanced methods of determining the black hole spin in accreting X-ray binary systems. There are, however, still some unresolved issues with the underlying disk models. One of them manifests as an apparent decrease in spin for increasing source luminosity. Here, we perform a few simple tests to establish whether outflows from the disk close to the inner radius can address this problem. We employ four different parametric models to describe the wind and compare these to the apparent decrease in spin with luminosity measured in the sources LMC~X-3 and GRS~1915+105. Wind models in which parameters do not explicitly depend on the accretion rate cannot reproduce the spin measurements. Models with mass accretion rate dependent outflows, however, have spectra that emulate the observed ones. The assumption of a wind thus effectively removes the artifact of spin decrease. This solution is not unique; the same conclusion can be obtained with a truncated inner disk model. To distinguish among valid models, high resolution X-ray data and a realistic description of the Comptonization in the wind will be needed.

Inner disc obscuration in GRS 1915+105 based on relativistic slim disc model

We study the observational signatures of the relativistic slim disc of 10 M_sun black hole, in a wide range of mass accretion rate, mdot, dimensionless spin parameter, a_ast, and viewing angle, i. In general, the innermost temperature, T_in increases with the increase of i for a fixed value of mdot and a_ast, due to the Doppler effect. However, for i > 50 and mdot > mdot_turn, T_in starts to decrease with the increase of mdot. This is a result of self-obscuration -- the radiation from the innermost hot part of the disc is blocked by the surrounding cooler part. The value of mdot_turn and the corresponding luminosities depend on a_ast and i. Such obscuration effects cause an interesting behavior on the disc luminosity (L_disc) -- T_in plane for high inclinations. In addition to the standard-disc branch which appears below mdot_turn and which obeys L_disc propto T_in^4 -relation, another branch above mdot_turn, which is nearly horizontal, may be observed at luminosities close to the Eddington luminosity. We show that these features are likely observed in a Galactic X-ray source, GRS 1915+105. We support a high spin parameter (a_ast > 0.9) for GRS 1915+105 since otherwise the high value of T_in and small size of the emitting region (r_in < 1r_S) cannot be explained.

Bright X-ray flares from Sgr A*

We address a question whether the observed light curves of X-ray flares originating deep in galactic cores can give us independent constraints on the mass of the central supermassive black hole. To this end we study four brightest flares that have been recorded from Sagittarius A*. They all exhibit an asymmetric shape consistent with a combination of two intrinsically separate peaks that occur at a certain time-delay with respect to each other, and are characterized by their mutual flux ratio and the profile of raising/declining parts. Such asymmetric shapes arise naturally in the scenario of a temporary flash from a source orbiting near a super- massive black hole, at radius of only 10-20 gravitational radii. An interplay of relativistic effects is responsible for the modulation of the observed light curves: Doppler boosting, gravitational redshift, light focusing, and light-travel time delays. We find the flare properties to be in agreement with the simulations (our ray-tracing code sim5lib). The inferred mass for each of the flares comes out in agreement with previous estimates based on orbits of stars; the latter have been observed at radii and over time-scales two orders of magnitude larger than those typical for the X-ray flares, so the two methods are genuinely different. We test the reliability of the method by applying it to another object, namely, the Seyfert I galaxy RE J1034+396.

Modeling polarization signatures of NIR radiation from the Sgr A* black hole environs

The near-infrared (NIR) emission of Sagittarius A* (Sgr A*), the source associated with the supermassive black hole (4.4 × 106M⊙) at the center of our galaxy, is polarized and highly variable. Correlations between intensity and polarimetric parameters of the observed light curves compared with the predicted ones for different configurations, allow us to extract information about the geometry of the radiating region. Here we present the theoretical polarimetric light curves expected in the case of optically thin NIR emission from overdense regions close to the marginal stable orbit. Using a numerical code we track the time evolution of detectable polarization properties produced by synchrotron emission of compact sources in the vicinity of the black hole. We show that the different setups lead to very distinctive patterns in the time profiles of polarized flux and the orientation of the polarization vector and as such may be used for determining the geometry of the accretion flow around Sgr A*.

X-Ray Polarimetry: Polarization from the oscillating magnetized accretion torus

We study oscillations of accretion torus with azimuthal magnetic field. For several lowest-order modes we calculate eigenfrequencies and eigenfunctions and calculate corresponding intensity and polarization light curves using advanced ray-tracing methods.

X-Ray Quasi-periodic Oscillations in the Lense–Thirring Precession Model. I. Variability of Relativistic Continuum

We develop a Monte Carlo code to compute the Compton-scattered X-ray flux arising from a hot inner flow that undergoes Lense–Thirring precession. The hot flow intercepts seed photons from an outer truncated thin disk. A fraction of the Comptonized photons will illuminate the disk, and the reflected/reprocessed photons will contribute to the observed spectrum. The total spectrum, including disk thermal emission, hot flow Comptonization, and disk reflection, is modeled within the framework of general relativity, taking light bending and gravitational redshift into account. The simulations are performed in the context of the Lense–Thirring precession model for the low-frequency quasi-periodic oscillations, so the inner flow is assumed to precess, leading to periodic modulation of the emitted radiation. In this work, we concentrate on the energy-dependent X-ray variability of the model and, in particular, on the evolution of the variability during the spectral transition from hard to soft state, which is implemented by the decrease of the truncation radius of the outer disk toward the innermost stable circular orbit. In the hard state, where the Comptonizing flow is geometrically thick, the Comptonization is weakly variable with a fractional variability amplitude of ≤10%; in the soft state, where the Comptonizing flow is cooled down and thus becomes geometrically thin, the fractional variability of the Comptonization is highly variable, increasing with photon energy. The fractional variability of the reflection increases with energy, and the reflection emission for low spin is counterintuitively more variable than the one for high spin.

Testing the X-ray reverberation model KYNREFREV in a sample of Seyfert 1 Active Galactic Nuclei

We present the first results obtained by the application of the KYNREFREV-reverberation model, which is ready for its use in XSPEC. This model computes the time dependent reflection spectra of the disc as a response to a flash of primary power-law radiation from a point source corona located on the axis of the black hole accretion disc (lamp-post geometry). Full relativistic effects are taken into account. The ionisation of the disc is set for each radius according to the amount of the incident primary flux and the density of the accretion disc. We tested the model with a sample of Narrow-Line Seyfert 1 galaxies (ARK 564, MCG-6-30-15 and 1H 0707-495), which constitute part of the best available dataset from the XMM-Newton satellite so far. We tested the model by assuming either rapidly (a=0.99) or zero spinning black hole configurations. We found that the observed time-lags in these Seyferts strongly suggest a compact X-ray source, located close to the BH, at a height smaller than 4rg and that this result does not depend on the BH spin and/or the disc ionization. There is no significant statistical difference between the quality of the best-fits in the rapidly and zero spinning BH scenarios in Ark 564 and MCG-6-30-15. However, this is not the case in 1H 0707-495. From its best-fit residuals we conclude that the hypothesis of a non-rotating BH in 1H 0707-495 is not supported by our results. Also, the best-fits to the Ark 564 and 1H 0707-495 data are of rather low quality. We detect wavy-residuals around the best-fit reverberation model time-lags at high frequencies. This result suggests that the simple lamp-post geometry does not fully explain the X-ray source/disc configuration in Active Galactic Nuclei and that further developments of the modelling are needed.