(abridged) We review how the recent increase in X-ray and radio data from black hole and neutron star binaries can be merged together with theoretical advances to give a coherent picture of the physics of the accretion flow in strong gravity. Both long term X-ray light curves, X-ray spectra, the rapid X-ray variability and the radio jet behaviour are consistent with a model where a standard outer accretion disc is truncated at low luminosities, being replaced by a hot, inner flow which also acts as the launching site of the jet. Decreasing the disc truncation radius leads to softer spectra, as well as higher frequencies (including QPO's) in the power spectra, and a faster jet. The collapse of the hot flow when the disc reaches the last stable orbit triggers the dramatic decrease in radio flux, as well as giving a qualitative (and often quantitative) explanation for the major hard--soft spectral transition seen in black holes and neutron stars. After collapse of the hot inner flow, the spectrum in black hole systems can be dominated by the disc emission. Its behaviour is consistent with the existence of a last stable orbit, and such data can be used to estimate the black hole spin. These systems can also show very different spectra at these high luminosities, in which the disc spectrum is strongly distorted by Comptonization. The structure of the accretion flow becomes increasingly uncertain as the luminosity approaches (and exceeds) the Eddington luminosity, though there is growing evidence that winds play an important role. We stress that these high Eddington fraction flows are key to understanding many disparate and currently very active fields such as ULX, Narrow Line Seyfert 1's, and the growth of the first black holes in the Early Universe.Comment: 76 pages, accepted for publication in Astronomy and Astrophysics Review

Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5-10 keV luminosities in the range 10 39−40 ergs s −1 , they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ∼ 100 solar masses. However, the observed innermost disk temperatures of these objects, T in = 1.1 − 1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.

The present paper describes the analysis of multiple RXTE/PCA data of the black hole binary with superluminal jet, XTE J1550 − 564, acquired during its 1999-2000 outburst. The X-ray spectra show features typical of the high/soft spectral state, and can approximately be described by an optically thick disk spectrum plus a power-law tail. Three distinct spectral regimes, named standard regime, anomalous regime, and apparently standard regime, have been found from the entire set of the observed spectra. When the X-ray luminosity is well below ∼ 6 × 10 38 erg s −1 (assuming a distance of 5 kpc), XTE J1550 − 564 resides in the standard regime, where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant. When the luminosity exceeds the critical luminosity, the apparently standard regime is realized, where luminosity of the optically thick disk rises less steeply with the temperature, and the spectral shape is moderately distorted from that of the standard accretion disk. In this regime, radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk. The results of the apparently standard regime are suggestive of a slim disk (e.g., Abramowicz et al. 1988, Watarai et al. 2000 which is a solution predicted under high mass accretion rate. In the intermediate anomalous regime, the spectrum becomes much harder, and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time. These properties can

We report on the result of an X-ray observation of the X-ray Nova Velorum 1993 (GRS 1009–45), made with ASCA on 1993 November 10-11. The energy spectrum was extremely soft and accompanied by a hard tail, characteristics of the Galactic black-hole binaries in the soft state. It is well represented by a two-component model consisting of a multicolor disk model or a general relativistic accretion disk model, both describing emission from an optically thick accretion disk, and a power-law component with a photon index of ∼ 2.5. A spectral analysis of the soft component allows an estimation of the mass of the central object. For the optically-estimated distance (> 1 kpc) and inclination (> 37°), the mass is estimated to be $\gt 3.1 M_\odot$. These results strongly support that the compact object is a black hole.