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We study spectra produced by weakly accreting black hole (BH) systems using the semi-analytic advection dominated accretion flow (ADAF) model and the general-relativistic magentohydrodynamic (GRMHD) simulation. We find significant differences between these two approaches related to a wider spread of the flow parameters as well as a much steeper radial distribution of the magnetic field in the latter. We apply these spectral models to the broad-band spectral energy distribution (SED) of the nucleus of M87 galaxy. The standard (in particular, one-dimensional) formulation of the ADAF model does not allow to explain it; previous claims that this model reproduces the observed SED suffer from an inaccurate treatment of the Compton process. The spectra based on GRMHD simulation are in a much better agreement with the observed data. In our GRMHD model, in which we assumed the BH spin $a=0.9$, bulk of radiation observed between the millimeter and the X-ray range is produced in the disk area within 4 gravitational radii from the BH. In this solution, the synchrotron component easily reproduces the spectral data between the millimeter and the UV range, and the Compton component does not violate the X-rays constraints, for accretion rates not exceeding 0.01 Msun/year and a relatively strong magnetic field, with the plasma $\beta \sim 1$ in the region where radiation is produced. However, the Compton component cannot explain the observed X-ray spectrum. Instead, the X-ray spectrum can be reproduced by a high energy tail of the synchrotron spectrum if electrons have a hybrid energy distribution with a $\sim 5$ per cent nonthermal component., Comment: ApJ, in press

We perform a detailed study of the black hole spin of Cyg X-1, using accurate broad-band X-ray data obtained in the soft spectral state by simultaneous NICER and NuSTAR observations, supplemented at high energies by INTEGRAL data. We use the relativistic disk model kerrbb together with different models of the Comptonization high energy tail and the relativistically-broadened reflection features. Unlike most previous studies, we tie the spin parameters of the disk and relativistic broadening models, thus combining the continuum and reflection methods of spin determination. We also consider a likely increase of the disk color correction due to a partial support of the disk by large scale magnetic fields. We find that such models yield the spin parameter of $a_*= 0.87^{+0.04}_{-0.03}$ if the disk inclination is allowed to be free, with $i= 39^{+1}_{-1}$ degree. Assuming $i=27.5$ degree, as determined by optical studies of the binary, worsens the fit, but leads to similar values of the spin, $a_*= 0.90^{+0.01}_{-0.01}$. In addition, we consider the presence of a warm Comptonization layer on top of the disk, motivated by successful modeling of soft X-ray excesses in other sources with such a model. This dramatically lowers the spin, to $a_*\lesssim 0.1$, consistent with the spin measurements from black-hole mergers. On the other hand, if the natal spin of Cyg X-1 was low but now $a_*\approx 0.9$, a period of effective super-critical accretion had to take place in the past. Such accretion could be facilitated by photon advection, as proposed for ultraluminous X-ray sources., Comment: ApJL, in press

The black-hole spin parameter, $a_*$, was measured to be close to its maximum value of 1 in many accreting X-ray binaries. In particular, $a_*\gtrsim 0.9$ was found in a number of studies of LMC X-1. These measurements were claimed to take into account both statistical and systematic uncertainties. We perform new measurements using a recent simultaneous observation by NICER and NuSTAR, providing a data set of high quality. We use the disk continuum method together with improved models for coronal Comptonization. With the standard relativistic disk model and optically thin Comptonization, we obtain values of $a_*$ similar to those obtained before. We then consider modifications to the standard model. Using a color correction of 2, we find $a_*\approx 0.64$--0.84. We then consider disks with dissipation in surface layers. To account for that, we assume the standard disk is covered by a warm and optically thick Comptonizing layer. Our model with the lowest $\chi^2$ yields then $a_*\approx 0.40^{+0.41}_{-0.32}$. In order to test the presence of such effects in other sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft state. We confirm the previous findings of $a_*>0.99$ using the standard model, but then we find a weakly constrained $a_*\approx 0.82^{+0.16}_{-0.74}$ when including an optically thick Comptonizing layer. We conclude that determinations of the spin using the continuum method can be highly sensitive to the assumptions about the disk structure., Comment: ApJ, in press,13 pages

We study the high energy emission of two active galactic nuclei (AGN), NGC 4258 and NGC 7213. We directly apply the general-relativistic (GR) hot flow model, kerrflow, to the archival BeppoSAX, NuSTAR and Suzaku observations of these objects. Most of these data sets indicate that about 10-20 per cent of the accretion power is used for the direct heating of electrons, however, we find also indications for significant changes of the electron heating efficiency in some cases. Furthermore, all these X-ray data sets indicate rather strongly magnetized flows, with the magnetic field close to the equipartition with the gas pressure. Comparison of the model prediction with the Fermi/LAT data for NGC 7213 allows us to constrain the content of nonthermal protons to at most 10 per cent., Comment: Accepted (MNRAS)

Relativistic reflection observed in the hard states of accreting black holes often shows a weak amplitude relative to the main Comptonization component, which may result from either a disc truncation or a non-isotropy of the X-ray source, e.g. due to a motion away from the reflector. We investigate here the latter case, assuming that the X-ray source is located on the symmetry axis of the Kerr black hole. We discuss effects relevant to a proper computation of the reflected radiation and we implement them in the model for data analysis, reflkerrV. We apply it to the simultaneous Suzaku and NuSTAR observation of Cyg X-1 in the hard state and we find a good fit for an untruncated disc irradiated by the source moving away from it at 0.36c. However, we find a slightly better solution in a geometry closely approximating the truncated disc irradiated by an inner hot flow. In this solution we either still need a subrelativistic outflow or the source opposite to the observer must contribute to the directly observed radiation. We also discuss differences between the implementation of the outflow effect in reflkerrV and in relxilllpCp., Comment: MNRAS, in press

We have developed an improved model of X-ray emission from optically thin, two-temperature accretion flows, \texttt{kerrflow}, using an exact Monte Carlo treatment of global Comptonization as well as with a fully general relativistic description of both the radiative and hydrodynamic processes. It also includes pion-decay electrons, whose synchrotron emission dominates the seed photons yield at high accretion rates in flows around supermassive black holes. We consider in detail the dependence of the model spectra on the black hole spin, the electron heating efficiency, the plasma magnetization and the accretion rate, and we discuss feasibility of constraining these parameters by analyzing X-ray spectra of nearby low-luminosity active galactic nuclei. We note some degeneracies which hinder precise estimations of these parameters when individual X-ray spectra are analyzed. These degeneracies are eliminated when several spectra from a given source are fitted jointly, which then allows us to reliably measure the model parameters. We find significant differences with previous spectral models of hot-flow emission, related with the computational methods for Comptonization. Finally, we briefly consider and discuss the dependence on the viscosity parameter and on the outflow strength., Comment: ApJ, in press

We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform analysis of two joint spectra from NuSTAR and INTEGRAL, covering the range of 3--650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3--2200 keV range. The spectra are well modelled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modelled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron-positron pair production, 511 keV. This allows us to calculate the rate of the electron-positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of $>$20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum., Comment: ApJL, in press

We study X-ray spectra from the outburst rise of the accreting black-hole binary MAXI J1820+070. We find that models having the disk inclinations within those of either the binary or the jet imply significant changes of the accretion disk inner radius during the luminous part of the hard spectral state, with that radius changing from $>$100 to $\sim$10 gravitational radii. The main trend is a decrease with the decreasing spectral hardness. Our analysis requires the accretion flow to be structured, with at least two components with different spectral slopes. The harder component dominates the bolometric luminosity and produces strong, narrow, X-ray reflection features. The softer component is responsible for the underlying broader reflection features. The data are compatible with the harder component having a large scale height, located downstream the disk truncation radius, and reflecting mostly from remote parts of the disk. The softer component forms a corona above the disk up to some transition radius. Our findings can explain the changes of the characteristic variability time scales, found in other works, as being driven by the changes of the disk characteristic radii., Comment: ApJL, 909, L9 (2021)

Relativistic reflection features are commonly observed in the X-ray spectra of accreting black holes. In the presence of high quality data and with the correct astrophysical model, X-ray reflection spectroscopy can be quite a powerful tool to probe the strong gravity region, study the morphology of the accreting matter, measure black hole spins, and possibly test Einstein's theory of general relativity in the strong field regime. In the last decade, there has been significant progress in the development of the analysis of these features, thanks to more sophisticated astrophysical models and new observational facilities. Here we review the state-of-the-art in relativistic reflection modeling, listing assumptions and simplifications that may affect, at some level, the final measurements and may be investigated better in the future. We review black hole spin measurements and the most recent efforts to use X-ray reflection spectroscopy for testing fundamental physics., Comment: 64 pages, 23 figures. Prepared within the ISSI International Team project "Can We Use X-Ray Reflection Spectroscopy For Precision Measurements Of Accreting Black Holes?". v3: refereed version

We investigate the X-ray spectrum of the Seyfert galaxy NGC 4151 using the simultaneous Suzaku/NuSTAR observation and flux-resolved INTEGRAL spectra supplemented by Suzaku and XMM observations. Our best spectral solution indicates that the narrow Fe Kalpha line is produced in Compton-thin matter at the distance of several hundred gravitational radii. In such a model, we find a weak but significant relativistic reflection from a disk truncated at about ten gravitational radii when the source is in bright X-ray states. We do not find evidence either for or against the presence of relativistic reflection in the dim X-ray state. We also rule out models with X-ray emission dominated by a source located very close to the black hole horizon, which was proposed in previous works implementing the lamp-post geometry for this source. We point out that accurate computation of the thermal Comptonization spectrum and its distortion by strong gravity is crucial in applications of the lamp-post geometry to the NuSTAR data., Comment: ApJ, in press

A fraction of the electromagnetic radiation emitted from the surface of a geometrically thin and optically thick accretion disk of a black hole returns to the disk because of the strong light bending in the vicinity of the compact object (returning radiation). While such radiation clearly affects the observed spectrum of the source, it is often neglected in theoretical models. In the present paper, we study the impact of the returning radiation on relativistic reflection spectra. Assuming neutral material in the disk, we estimate the systematic uncertainties on the measurement of the properties of the system when we fit the data with a theoretical model that neglects the returning radiation. Our NICER simulations show that the inclination angle of the disk and the black hole spin parameter tend to be overestimated for low viewing angles, while no clear bias is observed for high viewing angles. The iron abundance of the disk is never overestimated. In the most extreme cases (in particular, for maximally rotating black holes) the returning radiation flattens the radial emissivity beyond a few gravitational radii. In such cases, it also produces residuals that cannot be compensated by adjusting the parameters of models neglecting the returning radiation. This may be an important issue for interpretation of data from future X-ray missions (e.g. Athena). When we simulate some observations with NuSTAR and we fit data above 10 keV, we find that some conclusions valid for the NICER simulations are not true any longer (e.g., we can get a high iron abundance)., Comment: 21 pages, 12 figures. v2: refereed version

We investigate constraints for the size and location of the X-ray source in 1H 0707-495 determined from the shape of the relativistically smeared reflection from the accretion disc. We develop a new code to model an extended X-ray source and we apply it to all archival XMM observations of 1H 0707-495. Contrary to Wilkins et al. we find that the relativistic reflection in this source is not consistent with an extended uniform corona. Instead, we find that the X-ray source must be very compact, with the size of at most a gravitational radius, and located at most at a few gravitational radii from the black hole horizon. A uniform extended corona does indeed produce an emissivity which is like a twice broken power law, but the inner emissivity is fixed by the source geometry rather than being a free parameter. In 1H0707-495, reflection from the inner disc is much stronger than expected for a uniformly extended source. Including the effect of ionised absorption from a wind does not change this conclusion, but including scattered emission (and more complex absorption) from the wind can dramatically change the reflection parameters., Comment: Accepted for publication in Astronomy & Astrophysics journal

The accreting black-hole binary XTE J1752--223 was observed in a stable hard state for 25 d by RXTE, yielding a 3--140 keV spectrum of unprecedented statistical quality. Its published model required a single Comptonization spectrum reflecting from a disk close to the innermost stable circular orbit. We studied that model as well as a number of other single-Comptonization models (yielding similarly low inner radii), but found they violate a number of basic physical constraints, e.g., their compactness is much above the maximum allowed by pair equilibrium. We also studied the contemporaneous 0.55--6 keV spectrum from the Swift/XRT and found it well fitted by an absorbed power law and a disk blackbody with the innermost temperature of 0.1 keV. The normalization of the disk blackbody corresponds to an inner radius of $\gtrsim$20 gravitational radii and its temperature, to irradiation of the truncated disk by a hot inner flow. We have also developed a Comptonization/reflection model including the disk irradiation and intrinsic dissipation, but found that it does not yield any satisfactory fits. On the other hand, we found that the $\leq$10 keV band from RXTE is much better fitted by a reflection from a disk with the inner radius $\gtrsim$100 gravitational radii, which model then underpredicts the spectrum at $>$10 keV by $<$10%. We argue that the most plausible explanation of the above results is inhomogeneity of the source, with the local spectra hardening with the decreasing radius. Our results support the presence of a complex Comptonization region and a large disk truncation radius in this source., Comment: ApJ, published

We probe models of disc truncation in the hard spectral state of an outburst of the well-known X-ray transient GX 339--4. We test a large number of different models of disc reflection and its relativistic broadening, using two independent sets of codes, and apply it to a \textit{Rossi X-ray Timing Explorer} spectrum in the rising part of the hard state of the 2010/11 outburst. We find our results to be significantly model-dependent. While all of the models tested show best-fits consistent with truncation, some models allow the disc to extend close to the innermost stable circular orbit (ISCO) and some require substantial disc truncation. The different models yield a wide range in best-fit values for the disc inclination. Our statistically best model has a physical thermal Comptonization primary continuum, requires the disc to be truncated at a radius larger than or equal to about two ISCO radii for the maximum dimensionless spin of 0.998, and predicts a disc inclination in agreement with that of the binary. Our preferred models have moderate Fe abundance, $\gtrsim$2 times solar. We have also tested the effect of increasing the density of the reflecting medium. We find it leads to an increase in the truncation radius, but also to an increase in the Fe abundance, opposite to a previous finding., Comment: 12 pages, accepted to MNRAS, comments welcome

We have developed improved spectral models of relativistic reflection in the lamppost and disc-corona geometries. The models calculate photon transfer in the Kerr metric and give the observed photon-energy spectra produced by either thermal Comptonization or an e-folded power law incident on a cold ionized disc. Radiative processes in the primary X-ray source and in the disc are described with the currently most precise available models. Our implementation of the lamppost geometry takes into account the presence of primary sources on both sides of the disc, which is important when the disc is truncated. We thoroughly discuss the differences between our models and the previous ones., Comment: MNRAS in press

We study the lamppost model, in which the primary X-ray sources in accreting black-hole systems are located symmetrically on the rotation axis on both sides of the black hole surrounded by an accretion disc. We show the importance of the emission of the source on the opposite side to the observer. Due to gravitational light bending, its emission can increase the direct (i.e., not re-emitted by the disc) flux by as much as an order of magnitude. This happens for near to face-on observers when the disc is even moderately truncated. For truncated discs, we also consider effects of emission of the top source gravitationally bent around the black hole. We also present results for the attenuation of the observed radiation with respect to that emitted by the lamppost as functions of the lamppost height, black-hole spin and the degree of disc truncation. This attenuation, which is due to the time dilation, gravitational redshift and the loss of photons crossing the black-hole horizon, can be as severe as by several orders of magnitude for low lamppost heights. We also consider the contribution to the observed flux due to re-emission by optically-thick matter within the innermost stable circular orbit., Comment: submitted to MNRAS

We report hints for the correlation between the X-ray and gamma-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low (L) and high (H) level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. Both datasets contain an equal amount of 3.8 years of LAT data and NGC 4945 is detected with a similar statistical significance of ~15 $\sigma$ in L and 14 $\sigma$ in H. However, the slope of the gamma-ray spectrum hardens with the increase of the X-ray flux, with the photon index $\Gamma$ = 2.47 +/- 0.07 in L and 2.11 +/- 0.08 in H. The change is confirmed by a systematic variation of the spectral energy distribution as well as a substantial reversal of the gamma-ray signal in significance maps for low and high gamma-ray energies. The X/gamma-ray correlation indicates that the gamma-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. We discuss possible locations of the gamma-ray source. We also compare NGC 4945 with other starburst galaxies detected by LAT and we note similarities between those with active nuclei, e.g. unlikely high efficiencies of gamma-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the gamma-ray emission., Comment: ApJ in press

We study the lamppost model, in which the X-ray source in accreting black-hole systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in the widely used lamppost model relxilllp, e.g., the neglect of the redshift of the photons emitted by the lamppost and directly observed. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, if those results were correct, most of the photons produced in the lamppost would be trapped by the black hole, and the luminosity generated in the source as measured at infinity would be much larger than that observed. This appears to be in conflict with the observed smooth state transitions between the hard and soft states of X-ray binaries. The required increase of the accretion rate and the associated efficiency reduction present also a problem for AGNs. Then, those models imply the luminosity measured in the local frame to be much higher than that produced in the source and measured at infinity, due to the additional effects of time dilation and redshift, and the electron temperature to be higher significantly than that observed. We show that these conditions imply that the fitted sources would be out of the pair equilibrium. On the other hand, the above issues pose relatively minor problems for sources at large distances from the black hole, where relxilllp can still be used., Comment: ApJL, in press

We check how the Fermi/LAT data constrain physics of hot accretion flows, most likely present in low-luminosity AGNs. Using a precise model of emission from hot flows, we examine the dependence of their gamma-ray emission, resulting from proton-proton interactions, on accretion rate, black hole spin, magnetic field strength, electron heating efficiency and particle distribution. Then, we compare the hadronic gamma-ray luminosities predicted by the model for several nearby Seyfert 1 galaxies with the results of our analysis of 6.4 years of the Fermi/LAT observations of these AGNs. In agreement with previous studies, we find a significant gamma-ray detection in NGC 6814 and we could only derive upper limits for the remaining objects, although we note marginally significant (~3 sigma) signals at the positions of NGC 4151 and NGC 4258. The derived upper limits for the flux above 1 GeV allow us to constrain the proton acceleration efficiency in flows with heating of electrons dominated by Coulomb interactions, which case is favored by X-ray spectral properties. In such flows, at most ~10% of the accretion power can be used for a relativistic acceleration of protons. Upper limits for the flux below 1 GeV can constrain the magnetic field strength and black hole spin value, we find such constraints for NGC 7213 and NGC 4151. We also note that the spectral component above ~4 GeV found in the Fermi/LAT data of Centaurus A by Sahakyan et al. may be due to hadronic emission from a flow within the above constraint. We rule out such an origin of the gamma-ray emission from NGC 6814. Finally, we note that the three Seyfert 2/starburst galaxies, NGC 4595, NGC 1068 and Circinus, show an interesting correlation of their gamma-ray luminosities with properties of their active nuclei, and we discuss it in the context of the hot flow model., Comment: A&A, in press

We present preliminary results of our analysis of the {\it Fermi}-LAT data from the direction of NGC 4151. We find a new gamma-ray source with a statistical significance sigma > 5, shifted by 0.5degr from the position of NGC 4151. Apparently, the source was bright only during a 1.5-year period between December 2011 and June 2013 and it strongly contaminated the signal from NGC 4151. Therefore, we neglect this period in our analysis. We find two additional, persistent gamma-ray sources with high sigma, shifted from NGC 4151 by ~1.5degr and 5degr, whose presence has been recently confirmed in the Third Fermi Catalog. After subtracting the above sources, we still see a weak residual, with sigma ~< 3, at the position of NGC 4151. We derive an upper limit (UL) for the gamma-ray flux from NGC 4151 and we compare it with predictions of the ADAF model which can explain the X-ray observations of this object. We find that the Fermi UL strongly constrains non-thermal acceleration processes in hot flows as well as the values of some crucial parameters. Here we present the comparison with the hot flow models in which heating of electrons is dominated by Coulomb interactions with hot protons. In such a version of the model, the gamma-ray UL, combined with the X-ray data, constrains the energy content in the non-thermal component of proton distribution to at most a few per cent, rules out a weak (sub-equipartition) magnetic field and favors a rapid rotation of the supermassive black hole., Comment: 2014 Fermi Symposium proceedings - eConf C141020.1

We study the X-ray spectra of tenuous, two-temperature accretion flows using a model involving an exact, Monte Carlo computation of the global Comptonization effect as well as general relativistic description of both the flow structure and radiative processes. In our previous work we found that in flows surrounding supermassive black holes, thermal synchrotron radiation is not capable of providing a sufficient seed photons flux to explain the X-ray spectral indices as well as the cut-off energies measured in several best-studied AGNs. In this work we complete the model by including seed photons provided by nonthermal synchrotron radiation and we find that it allows to reconcile the hot flow model with the AGN data. We take into account two possible sources of nonthermal electrons. First, we consider e+- produced by charged-pions decay, which should be always present in the innermost part of a two-temperature flow due to proton-proton interactions. We find that for a weak heating of thermal electrons (small delta) the synchrotron emission of pion-decay e+- is much stronger than the thermal synchrotron emission in the considered range of bolometric luminosities, L Ledd. The small-delta model including hadronic effects in general agrees with the AGN data, except for the case of a slowly rotating black hole and a thermal distribution of protons. For large-delta, the pion-decay e+- have a negligible effect and then in this model we consider nonthermal electrons produced by direct acceleration. We find an approximate agreement with the AGN data for the fraction of the heating power of electrons which is used for the nonthermal acceleration eta~0.1., Comment: 13 pages, 4 figures, accepted for publication in ApJ

We study radiative properties of hot accretion flows in a general relativistic model with an exact treatment of global Comptonization, developed in our recent works. We note a strong dependence of electron temperature on the strength of magnetic field and we clarify that the underlying mechanism involves the change of the flow structure, with more strongly magnetised flows approaching the slab geometry more closely. We find that the model with thermal synchrotron radiation being the main source of seed photons agrees with the spectral index vs Eddington ratio relation observed in black hole transients below 1 per cent of the Eddington luminosity, LEdd, and models with a weak direct heating of electrons (small delta) are more consistent with observations. Models with large delta predict slightly too soft spectra, furthermore, they strongly overpredict electron temperatures at ~0.01 LEdd. The low-luminosity spectra, at <0.001 LEdd, deviate from a power-law shape in the soft X-ray range and we note that the first-scattering bump often resembles a thermal like component, with the temperature of a few hundred eV, superimposed on a power-law spectrum. The model with thermal Comptonization of thermal synchrotron radiation does not agree with well studied AGNs observed below ~0.01 LEdd, for which there is a substantial evidence for the lack of an inner cold disc. This indicates that the model of hot flows powering AGNs should be revised, possibly by taking into account an additional (but internal to the flow) source of seed photons., Comment: 16 pages, 10 figures, accepted in MNRAS

Radiatively inefficient, hot accretion flows are widely considered as a relevant accretion mode in low-luminosity AGNs. We study spectral formation in such flows using a refined model with a fully general relativistic description of both the radiative (leptonic and hadronic) and hydrodynamic processes, as well as with an exact treatment of global Comptonization. We find that the X-ray spectral index--Eddington ratio anticorrelation as well as the cut-off energy measured in the best-studied objects favor accretion flows with rather strong magnetic field and with a weak direct heating of electrons. Furthermore, they require a much stronger source of seed photons than considered in previous studies. The nonthermal synchrotron radiation of relativistic electrons seems to be the most likely process capable of providing a sufficient flux of seed photons. Hadronic processes, which should occur due to basic properties of hot flows, provide an attractive explanation for the origin of such electrons., Comment: To be published in "Multiwavelength AGN surveys and studies", Proceedings of IAU Symposium No. 304, 2013, eds. A. Mickaelian, F. Aharonian, D. Sanders

We present a model of gamma-ray emission through neutral pion production and decay in two-temperature accretion flows around supermassive black holes. We refine previous studies of such a hadronic gamma-ray emission by taking into account (1) relativistic effects in the photon transfer and (2) absorption of gamma-ray photons in the radiation field of the flow. We use a fully general relativistic description of both the radiative and hydrodynamic processes, which allows us to study the dependence on the black hole spin. The spin value strongly affects the gamma-ray emissivity within ~10 gravitational radii. The central regions of flows with the total luminosities L < 0.001 of the Eddington luminosity are mostly transparent to photons with energies below 10 GeV, permitting investigation of the effects of space-time metric. For such L, an observational upper limit on the gamma-ray (0.1-10 GeV) to X-ray (2-10 keV) luminosity ratio of L_{0.1-10 GeV}/L_{2-10 keV} << 0.1 can rule out rapid rotation of the black hole; on the other hand, a measurement of L_{0.1-10 GeV}/L_{2-10 keV} ~ 0.1 cannot be regarded as the evidence of rapid rotation, as such a ratio can also result from a flat radial profile of gamma-ray emissivity (which would occur for nonthermal acceleration of protons in the whole body of the flow). At L > 0.01 of the Eddington luminosity, the gamma-ray emission from the innermost region is strongly absorbed and the observed gamma-rays do not carry information on the value of the black hole spin. We note that if the X-ray emission observed in Centaurus A comes from an accretion flow, the hadronic gamma-ray emission from the flow should contribute significantly to the MeV/GeV emission observed from the core of this object, unless it contains a slowly rotating black hole and protons in the flow are thermal., Comment: 11 pages, 8 figures, accepted in MNRAS

Hot accretion flows such as advection-dominated accretion flows are generally optically thin in the radial direction. Thus photons generated at some radii can cool or heat electrons at other radii via Compton scattering. Such global Compton scattering has previously been shown to be important for the dynamics of accretion flows. Here, we extend previous treatments of this problem by using accurate global general relativistic Monte Carlo simulations. We focus on an inner region of the accretion flow (R < 600R_g), for which we obtain a global self-consistent solution. As compared to the initial, not self-consistent solution, the final solution has both the cooling rate and the electron temperature significantly reduced at radii >=10 gravitational radii. On the other hand, the radiation spectrum of the self-consistent solution has the shape similar to that of the initial iteration, except for the high-energy cut-off being at an energy lower by a factor of ~2 and the bolometric luminosity decreased by a factor of ~2. We also compare the global Compton scattering model with local models in spherical and slab geometry. We find that the slab model approximates the global model significantly better than the spherical one. Still, neither local model gives a good approximation to the radial profile of the cooling rate, and the differences can be up to two orders of magnitude. The local slab model underestimates the cooling rate at outer regions whereas it overestimates that rate at inner regions. We compare our modelling results to observed hard-state spectra of black-hole binaries and find an overall good agreement provided any disc outflow is weak. We find that general-relativistic effects in flows which dynamics is modified by global Comptonization is crucial in approaching this agreement., Comment: 9 pages, 4 figures. Accepted to MNRAS. Add a new section to discuss on the impact of outflow and viscous electron heating

We study spectra generated by Comptonization of soft photons by cold electrons radially free-falling onto a black hole. We use a Monte Carlo method involving a fully relativistic description of Comptonization in the Kerr space-time. In agreement with previous studies, we find that Comptonization on the bulk motion of free fall gives rise to power-law spectra with the photon index of Gamma >~ 3. In contrast to some previous studies, we find that these power-law spectra extend only to energies << 511 keV. We indicate several effects resulting in generic cutoffs of such spectra at several tens of keV, regardless of any specific values of physical parameters in the model. This inefficiency of producing photons with energies > 100 keV rules out bulk motion Comptonization as a main radiative process in soft spectral states of black-hole binaries. The normalization of the power law (below the cutoff) with respect to the peak of the blackbody emission of the surrounding disc is typically very low, except for models with an overlap between the disc and the plasma, in which case the spectra are very soft, Gamma >~ 4., Comment: 10 pages, 4 figures, revised version, accepted for publication in MNRAS

Using a Monte Carlo method, we derive spectra arising from Comptonization taking place close to a Kerr black hole. We consider a model consisting of a hot thermal corona Comptonizing seed photons emitted by a cold accretion disc. We find that general relativistic effects are crucial for the emerging spectra in models which involve significant contribution of radiation produced in the black hole ergosphere. Due to this contribution, spectra of hard X-ray emission produced in the vicinity of a rapidly rotating black hole strongly depend on the inclination of the line of sight, with larger inclinations corresponding to harder spectra. Remarkably, such anisotropy could be responsible for properties of the X-ray spectra of Seyfert galaxies, which appear to be intrinsically harder in type 2 objects than in type 1, as reported recently., Comment: 13 pages, 15 figures. Accepted for publication in MNRAS

We present a general relativistic accretion disc model and its application to the soft-state X-ray spectra of black hole binaries. The model assumes a flat, optically thick disc around a rotating Kerr black hole. The disc locally radiates away the dissipated energy as a blackbody. Special and general relativistic effects influencing photons emitted by the disc are taken into account. The emerging spectrum, as seen by a distant observer, is parametrized by the black hole mass and spin, the accretion rate, the disc inclination angle and the inner disc radius. We fit the ASCA soft state X-ray spectra of LMC X-1 and GRO J1655-40 by this model. We find that having additional limits on the black hole mass and inclination angle from optical/UV observations, we can constrain the black hole spin from X-ray data. In LMC X-1 the constrain is weak, we can only rule out the maximally rotating black hole. In GRO J1655-40 we can limit the spin much better, and we find 0.68 < a < 0.88. Accretion discs in both sources are radiation pressure dominated. We don't find Compton reflection features in the spectra of any of these objects., Comment: 13 pages, 8 figures, accepted for publication in MNRAS

We study the effects of thermal conduction in a hot, active corona above an accretion disk. We assume that all of the dissipative heating takes place in the corona. We find that the importance of conduction decreases with increases in the local dissipative compactness of the corona, l_{diss,loc}, and increases with increasing abundance of electron-positron pairs. For l_{diss,loc} < 1, a significant fraction of the energy released in the corona may be carried away by the conductive flux, leading to formation of a relatively hot transition layer below the base of the corona. Comptonization of disk radiation in such a layer may account for the presence of soft X-ray excesses in the spectra emitted by disk-corona systems., Comment: 15 pages, 6 figures, accepted for publication in ApJ

We study possible models of two Galactic sources of transient pair annihilation radiation, 1E 1740.7-2942 and a source observed by High Energy Astronomy Observatory (HEAO) 1 A-4. We fit the observed spectral features by thermal annihilation spectra and find that the redshifts obtained by us are much larger than those obtained from fitting Caussian lines centered on 511 keV. This effect, which is due to the net blueshift (with respect to 511 keV) of the annihilation spectrum due to the thermal energies of pairs, puts strong constraints on models of sources. We consider those constraints first without considering the mechanism of positron production. From the shape of the observed spectra, we are able to rule out both spherical clouds and layers above cold matter as possible source geometries. The observed spectra are compatible with two source geometries: (1) a nearly face-on disk in the Kerr metric and (2) a jet close to a black hole. We consider, then, the origin of the pairs. Theories of both thermal and nonthermal pair equilibria predict that photon-pair production is unable to produce annihilation features that contain as much as half of the bolometric luminosity, which is observed. A possible solution to this problem is obscuration of a nonthermal source (in which pairs are produced by photon-photon collisions) and an outflow of pairs to an unobscured region. This makes annihilation in a jet the most likely model of the considered sources.

We show that the recent observations of the Seyfert galaxy NGC 4151 in hard X-rays and soft gamma rays by the OSSE and SIGMA detectors on board CGRO and GRANAT, respectively, are well explained by a nonthermal model with acceleration of relativistic electrons at an efficiency of less than 50 percent and with the remaining power dissipated thermally in the source (the standard nonthermal e(+/-) pair model assumed 100 percent efficiency). Such an acceleration efficiency is generally expected on physical grounds. The resulting model unifies previously proposed purely thermal and purely nonthermal models. The pure nonthermal model for NGC 4151 appears to be ruled out. The pure thermal model gives a worse fit to the data than our hybrid nonthermal/thermal model.