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 constraint is weak, and 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 do not find Compton reflection features in the spectra of any of these objects.
Thermal Conduction in Accretion Disk Coronae Maciołek-Niedźwiecki, Andrzej; Krolik, Julian H; Zdziarski, Andrzej A
The Astrophysical journal,
07/1997, Letnik:
483, Številka:
1
Journal Article
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.
We study the efficiency of e± pair production in compact γ-ray sources, as measured by the fraction of power supplied to a source that is converted into pair annihilation radiation emitted by the ...source (the pair yield). We focus mainly on thermal plasmas, in contrast to previous studies of the pair yield in nonthermal plasmas. We calculate the pair yield in thermal plasmas where bremsstrahlung is the only source of soft photons as well in plasmas in which there is a copious source of soft photons. We find that the pair yield reaches its maximum of ~0.3 in Comptonized bremsstrahlung plasmas. The corresponding maximum fraction of source power converted into pair rest mass is ~0.2. Addition of ambient soft photons to the source reduces the pair yield. The maximum yield is still close to ~0.3 in plasmas where the soft photons are Comptonized into hard power laws of energy spectral index α ≃ 0.1–0.2, but decreases with increasing α and becomes < 0.01 for α ≳ 0.7. Even when the pair yield is large, we never find an observable annihilation feature from a thermal plasma. To have a high pair yield, typically either the plasma temperature must be relativistic or photons must undergo many Compton scatterings before escaping the source. Hence, the emergent annihilation spectrum is either intrinsically broad or strongly smeared by repeated Compton scatterings and is difficult to distinguish from the underlying Compton scattering and bremsstrahlung continuum. This is in contrast to the nonthermal case where strong, narrow annihilation features can be produced by pairs which have cooled in the source. We summarize the conditions when such strong features are produced, and calculate the pair yield when primary electrons are injected with power-law energy distributions. While observable annihilation features are not produced directly in thermal sources, strong pair outflows, e.g., electron-positron jets, may be created in sources with hard spectra. The outflowing pairs may cool and annihilate outside the source region, in which case a visible annihilation feature is possible. However, our results applied to Nova Muscae show that neither thermal nor nonthermal pair models can explain the large strength of the annihilation feature observed from that source (unless a complex obscuration scenario is invoked). On the other hand, the annihilation feature of the 1979 March 5 γ-ray burst can be explained by our models provided the source is located at a distance much less than that to the LMC.
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.
Mon.Not.Roy.Astron.Soc. 325 (2001) 1253 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.
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.
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.
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