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1.
We solve for the structure of a hot accretion disc with unsaturated thermal Comptonization of soft photons and with advection, generalizing the classical model of Shapiro et al. The upper limit on the accretion rate due to advection constrains the luminosity to ≲ 0.15 y3/5 α7/5 of the Eddington limit, where y and α are the Compton and viscosity parameters, respectively. The characteristic electron temperature and Thomson optical depth of the inner flow at accretion rates within an order of magnitude of that upper limit are ∼ 109 K and ∼ 1, respectively. The resulting spectra are then in close agreement with the X-ray and soft γ-ray spectra from black hole binaries in the hard state and Seyferts. At low accretion rates, bremsstrahlung becomes the dominant radiative process.  相似文献   

2.
Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l ∗∼ r g c , where r g is the Schwarzschild radius. The standard disc model assumes l ≫ l ∗. We argue that, in many black hole sources, accretion flows have angular momenta just above the threshold for disc formation, l ≳ l ∗, and assess the accretion mechanism in this regime. In a range l ∗< l < l cr, a small-scale disc forms in which gas spirals fast into the black hole without any help from horizontal viscous stresses. Such an 'inviscid' disc, however, interacts inelastically with the feeding infall. The disc–infall interaction determines the dynamics and luminosity of the accretion flow. The inviscid disc radius can be as large as 14 r g, and the energy release peaks at 2 r g. The disc emits a Comptonized X-ray spectrum with a break at ∼100 keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.  相似文献   

3.
Magnetized accretion flows around black holes which include standing or oscillating shock waves can produce very realistic spectrum till a few MeV. These shocks accelerate hot electrons which produce power-law spectrum. The post-shock region intercepts soft-photons from an external source, namely, a Keplerian disk and also from distributed sources such as the synchrotron photons emitted from thermal and non-thermal electrons originated in the pre-shock and post-shock flow. These photons are inverse Comptonized by the thermal and the non-thermal electrons present in the CENBOL region. Computations show that the emitted radiation is extended till a few MeV. We include the bulk motion Comptonization as well and discuss its importance vis-a-vis the power-law spectrum produced by non-thermal electrons.   相似文献   

4.
We investigate the process of synchrotron radiation from thermal electrons at semirelativistic and relativistic temperatures. We find an analytic expression for the emission coefficient for random magnetic fields with an accuracy significantly higher than those derived previously. We also present analytic approximations to the synchrotron turnover frequency, treat Comptonization of self-absorbed synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung.
We then study the importance of Comptonization of thermal synchrotron radiation in compact X-ray sources. We first consider emission from hot accretion flows and active coronae above optically thick accretion discs in black hole binaries and active galactic nuclei (AGNs). We find that for plausible values of the magnetic field strength, this radiative process is negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black hole mass results in a further reduction of the maximum Eddington ratio from this process. Then, X-ray spectra of intermediate-luminosity sources, e.g. low-luminosity AGNs, can be explained by synchrotron Comptonization only if they come from hot accretion flows, and X-ray spectra of very weak sources are always dominated by bremsstrahlung. On the other hand, synchrotron Comptonization can account for power-law X-ray spectra observed in the low states of sources around weakly magnetized neutron stars.  相似文献   

5.
Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the 3 months of survey had an average γ-ray luminosity above 1048 erg s−1. For 17 out of the 23 sources we found and analysed X-ray and optical–ultraviolet data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high-energy states of single sources, like 0528+134 and 3C 454.3, for which multiple good sets of multiwavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disc emission. We can then constrain the black hole mass and the disc luminosity. Both are large (i.e. masses equal or greater than  109 M ⊙  and disc luminosities above 10 per cent of Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disc luminosity, and proportional to the mass accretion rate.  相似文献   

6.
We present X-ray/ γ -ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA , RXTE and CGRO /OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f ≳0.5 of total available power is dissipated in the corona.
We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA RXTE data yield the most probable black hole mass of M X≈10 M⊙ and an accretion rate,     , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.
The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT e∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲ℓh≲7, and a presence of a significant population of electron–positron pairs is ruled out.
We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.  相似文献   

7.
We present an exhaustive analysis of five broad-band observations of GRS 1915+105 in two variability states, χ and ω, observed simultaneously by the Proportional Counter Array (PCA) and High-Energy X-ray Timing Experiment (HEXTE) detectors aboard the Rossi X-ray Timing Explorer , and the Oriented Scintillation Spectrometer Experiment (OSSE) detector aboard the Compton Gamma-ray Observatory . We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a non-thermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the χ state are typical of the very high/intermediate state of black hole binaries. The spectrum of the ω state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak non-thermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the quasi-periodic oscillations present in the χ state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find   d ≃ 11 kpc  as the most likely value, contrary to a recent claim of a much lower value.  相似文献   

8.
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9.
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. As a result of 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.  相似文献   

10.
Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ∼5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission ('synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C 123, with L X(2–10 keV)=2×1044 erg s−1 and kT ∼5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7×1022 cm−2 and an intrinsic 2–10 keV luminosity of 1044 erg s−1.  相似文献   

11.
We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L  ∼ 1043 erg s−1) are obscured ( N  ∼ 1023 cm−2) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s−1 and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T  ∼ 106 − 107 K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 107 M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.  相似文献   

12.
We numerically examine centrifugally supported shock waves in 2D rotating accretion flows around a stellar mass  (10 M⊙)  and a supermassive  (106 M⊙)  black holes over a wide range of input accretion rates of     . The resultant 2D shocks are unstable with time and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of 2–3 and with periods of a tenth of a second to several hours, depending on the black hole masses. The shock oscillation model may explain the intermediate frequency QPOs with 1–10 Hz observed in the stellar mass black hole candidates and also suggest the existence of QPOs with the period of hours in active galactic nuclei. When the accretion rate     is low, the luminosity increases in proportion to the accretion rate. However, when     greatly exceeds the Eddington critical rate     , the luminosity is insensitive to the accretion rate and is kept constantly around  ∼3 L E  . On the other hand, the mass-outflow rate     increases in proportion to     and it amounts to about a few per cent of the input mass-flow rate.  相似文献   

13.
We calculate the structure of the accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of large viscosity parameter, α>0.03, the accretion flow deviates strongly from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches maximum, and then falls off. The maximum is achieved in the advection-dominated regime of accretion. The maximum temperature in the disc around a massive black hole of M =108 M⊙ with α=0.3 is of order 3×108 K. The discs with large accretion rates can emit X-rays in quasars as well as in galactic black hole candidates.  相似文献   

14.
The high-energy continuum in Seyfert galaxies and galactic black hole candidates is likely to be produced by a thermal plasma. There are difficulties in understanding what can keep the plasma thermal, especially during fast variations of the emitted flux. Particle–particle collisions are too inefficient in hot and rarefied plasmas, and a faster process is called for. We show that cyclo-synchrotron absorption can be such a process: mildly relativistic electrons thermalize in a few synchrotron cooling times by emitting and absorbing cyclo-synchrotron photons. The resulting equilibrium function is Maxwellian at low energies, with a high-energy tail when Compton cooling is important. Assuming that electrons emit completely self-absorbed synchrotron radiation and at the same time Compton scatter their own cyclo-synchrotron radiation and ambient UV photons, we calculate the time-dependent behaviour of the electron distribution function, and the final radiation spectra. In some cases, the 2–10 keV spectra are found to be dominated by the thermal synchrotron self-Compton process rather than by thermal Comptonization of UV disc radiation.  相似文献   

15.
We present a ROSAT and ASCA study of the Einstein source X-9 and its relation to a shock-heated shell-like optical nebula in a tidal arm of the M81 group of interacting galaxies. Our ASCA observation of the source shows a flat and featureless X-ray spectrum well described by a multicolour disc blackbody model. The source most likely represents an optically thick accretion disc around an intermediate-mass black hole  ( M ∼102 M⊙)  in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disc galaxies. Using constraints derived from both the innermost stable orbit around a black hole and the Eddington luminosity, we find that the black hole is fast-rotating and that its mass is between ∼80 M⊙–1.5×102 M⊙. The inferred bolometric luminosity of the accretion disc is ∼(1.1×1040 erg s−1)/(cos  i ). Furthermore, we find that the optical nebula is very energetic and may contain large amounts of hot gas, accounting for a soft X-ray component as indicated by archival ROSAT PSPC data. The nebula is apparently associated with X-9; the latter may be powering the former and/or they could be formed in the same event (e.g. a hypernova). Such a connection, if confirmed, could have strong implications for understanding both the birth of intermediate-mass black holes and the formation of energetic interstellar structures.  相似文献   

16.
We report BeppoSAX and optical observations of the black hole candidate GX 339–4 during its X-ray 'off' state in 1999. The broad-band (0.8–50 keV) X-ray emission can be fitted by a single power law with spectral index, α ∼1.6. The observed luminosity is 6.6×1033 erg s−1 in the 0.5–10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630–47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well ( B =20.1, V =19.2). By comparing with previously reported 'off' and low states (LS), we conclude that the 'off' state is actually an extension of the LS, i.e. an LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.  相似文献   

17.
We present a model for the short time-scale spectral variability of accreting black holes. It describes the time-averaged spectra well, and also temporal characteristics such as the power-density spectrum, time/phase lags, and coherence function of Cygnus X-1. We assume that X/ γ -rays are produced in compact magnetic flares at radii ≲100 GM c 2 from the central black hole. The tendency for magnetic loops to inflate and detach from the underlying accretion disc causes the spectrum of a flare to evolve from soft to hard because of the decrease of the feedback from the cold disc, so causing time delays between hard and soft photons. We identify the observed time lags with the evolution time-scales of the flares, which are of the order of the Keplerian time-scale. We model the overall temporal variability using a pulse avalanche model in which each flare has a certain probability of triggering a neighbouring flare, thus occasionally producing long avalanches. The duration of the avalanches determines the Fourier frequencies at which most of the power emerges.  相似文献   

18.
In wind-fed X-ray binaries the accreting matter is Compton-cooled and falls freely on to the compact object. The matter has a modest angular momentum l and accretion is quasi-spherical at large distances from the compact object. Initially small non-radial velocities grow in the converging supersonic flow and become substantial in the vicinity of the accretor. The streamlines with l >( GMR ∗)1/2 (where M and R ∗ are the mass and radius of the compact object) intersect outside R ∗ and form a two-dimensional caustic which emits X-rays. The streamlines with low angular momentum, l <( GMR ∗)1/2, run into the accretor. If the accretor is a neutron star, a large X-ray luminosity results. We show that the distribution of accretion rate/luminosity over the star surface is sensitive to the angular momentum distribution of the accreting matter. The apparent luminosity depends on the side from which the star is observed and can change periodically with the orbital phase of the binary. The accretor then appears as a 'Moon-like' X-ray source.  相似文献   

19.
We investigate the polarization properties of Comptonized X-rays from relativistic jets in active galactic nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering of blackbody photons emitted from an accretion disc; scattering of cosmic microwave background (CMB) photons and self-Comptonization of intrinsically polarized synchrotron photons emitted by jet electrons. Our simulations show that for Comptonization of disc and CMB photons, the degree of polarization of the scattered photons increases with the viewing inclination angle with respect to the jet axis. In both cases, the maximum linear polarization is  ≈20 per cent  . In the case of synchrotron self-Comptonization (SSC), we find that the resulting X-ray polarization depends strongly on the seed synchrotron photon injection site, with typical fractional polarizations   P ≈ 10–20 per cent  when synchrotron emission is localized near the jet base, while   P ≈ 20–70 per cent  for the case of uniform emission throughout the jet. These results indicate that X-ray polarimetry may be capable of providing unique clues to identify the location of particle acceleration sites in relativistic jets. In particular, if synchrotron photons are emitted quasi-uniformly throughout a jet, then the observed degree of X-ray polarization may be sufficiently different for each of the competing X-ray emission mechanisms (synchrotron, SSC or external Comptonization) to determine which is the dominant process. However, X-ray polarimetry alone is unlikely to be able to distinguish between disc and CMB Comptonization.  相似文献   

20.
I solve analytically the viscous evolution of an irradiated accretion disc, as seen during outbursts of soft X-ray transients. The solutions predict steep power-law X-ray decays L X ∼ (1 + t/tvisc)−4, changing to L X ∼ (1 − t/t'visc)4 at late times, where t visc, t 'visc are viscous time-scales. These forms closely resemble the approximate exponential and linear decays inferred by King and Ritter in these two regimes. The decays are much steeper than for unirradiated discs because the viscosity is a function of the central accretion rate rather than of local conditions in the disc.  相似文献   

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