Galactic Black-Hole Candidates Shining at the Eddington Luminosity

Abstract

We discuss distinctive features of luminous accretion disks shining at the Eddington luminosity in the context of galactic black-hole candidates (GBCs). We first note that the standard-disk picture is not applicable, although it is often postulated. Rather, the disk becomes advection-dominated while remaining optically thick (the so-called slim disk). The slim disk exhibits several noteworthy signatures: (1) The disk luminosity is insensitive to the mass-flow rates, Ṁ, and is always kept around the Eddington luminosity, LE, even if Ṁ greatly exceeds LE/c2. This reflects the fact that radiative cooling is no longer balanced by viscous heating and excess energy is carried by accreting matter to black holes. (2) The spectra of the slim disks are multi-color blackbody characterized by (i) a high maximum temperature, kTin ∼ a few keV, (ii) a small size of an emitting region, rin < 3 rg (with rg being Schwarzschild radius), due to substantial radiation coming out from inside 3 rg, and (iii) flatter spectra in the soft-X bands, vSv ∼ v0, because of a flatter effective temperature profile of the slim disk, Teff ∝ r−1/2 (in contrast with Teff ∝ r−3/4 in the standard disk). Thus, a small rin (≪ 3rg) does not necessarily mean the presence of a Kerr hole. Furthermore, (3) as Ṁ increases, Tin increases, while rin decreases as rin ∝ (Tin)−1 approximately. That is, the changes in rin derived from the fitting do not necessarily mean the changes in the physical boundary of the optically thick portions of the disk. Observational implications are discussed in relation to binary jet sources.

GRS 1915+105 in “Soft State”: Nature of Accretion Disk Wind and Origin of X-Ray Emission

“…Instead of the MCD model, the p-free disk model, expected from a onedimensional slim disk (Abramowicz et al 1988), gives a statistically acceptable fit to the spectrum in Epoch II . We consider it physically unlikely, however, because the inferred innermost radius is too small ( 0.3r g ) than a theoretical prediction (Watarai et al 2000). Also, we cannot consistently explain the spectra of two epochs with the same model.…”

The complex behaviour of the microquasar GRS 1915+105 in theρclass observed withBeppoSAX

“…With this value, a minimum distance of ∼36 km is acceptable, and considering the large uncertainty in the mass, which might be slightly overestimated, our results are not in conflict with the general scenario adopted for this peculiar source. On the other hand, the calculations of Watarai et al (2000) for the emission from a slim disk, demonstrated that radiation can be produced at distances much smaller than the last stable orbit, i.e. in the region between R S and 3R S .…”

“…Fitting points from all segments, except for that containing the pulse, with a power-law, the resulting values for the temperature exponent are 2.7 ± 0.4 for SLT-1, 2.0 ± 0.5 for SLT-2, and 1.9 ± 0.2 for FDT, respectively. Several models have been proposed to explain the relation L ∝ T 2 for a slim disk where advection dominates the energy transfer in the inner region and reduces the radiation efficiency (Watarai et al 2000). Moreover, the effects of the large inclination angle cannot be neglected since deviations from the L ∝ T 4 relation are mainly observed in highly inclined sources (Done et al 2007).…”

Discovery of Spectral Transitions from Two Ultraluminous Compact X-Ray Sources in IC 342

“…Although the scenario may be salvaged by invoking rapid BH rotation (Mizuno et al 1999;Paper I;also Zhang, Cui, & Chen 1997a) and/or a "slim disk" concept (Abramowicz et al 1988;Watarai et al 2000), it is important to assemble further observational clues.…”