Bulk motion Comptonization in black-hole accretion flows

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 MNRA

X-ray spectra of hot accretion flows

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 MNRA

Gamma-ray activity of Seyfert galaxies and constraints on hot accretion flows

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 pres

Lamp-post with an outflow and the hard state of Cyg X-1

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 pres

Hot accretion flows in low-luminosity active galactic nuclei in NGC 4258 and NGC 7213

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

On the role and origin of nonthermal electrons in hot accretion flows

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 Ap