The X-ray properties of the two Galactic superluminal sources GRS 1915+105 and GRO J1655-40 have been studied with the X-ray satellite ASCA and RXTE.
For GRO J1655-40, we mainly present the results of an ASCA observation from 1997 February 25 to February 28 covering a full orbital period (2.62 d). The averaged 2-10 keV flux was about 1.1 Crab. An absorption line feature centered at 6.8 keV was detected both in the GIS and SIS spectra. We interpret this as a blend of two resonance-absorption K α lines due to H-like and He-like iron ions. We can consistently explain both the ASCA spectra and the simultaneous RXTE/PCA spectrum by a combination of K-absorption lines and K-absorption edges of iron ions. The fact that the absorption line is stably present over the whole orbital phase implies that the distribution of the highly ionized plasma is not affected by the companion star, which is consistent with its presence around the black hole. The curve of growth analysis shows that the plasma contains a velocity dispersion along the line-of-sight larger than 300 km s-1 attributed to bulk motions. It is most probably a part of a geometrically-thick accretion flow in turbulent motions with velocities of 500-1600 km s-1 at an estimated radius of ~1010 cm.
For GRS 1915+105, we present the results of iron-K structures observed with ASCA as well as those of the long term observations of the continuum behavior with RXTE from January 1999 to May 2000. We detected no significant iron-K absorption line features in the ASCA/SIS spectra except for the 1994 and 1995 observations. This may suggest that the hard X-ray flux affects on the physical parameters of the plasma through photoionization. On the other hand, we detected iron-K emission line(s) around 6.5 keV. By investigating the time variability of the line on different time scales, we constrained the distance of the line emission region to be ~ 1011-1013 cm, being consistent with the same origin as the line-absorbing matter.
From the timing and spectral analysis of the continuum, we revealed that the variable hard component on a time scale of 1~0.1 sec is always represented by a broken power law with a photon index of about 1.8 below 5-10 keV. Utilizing the difference of variability, we modeled the continuum in the 1.2-100 keV range by a sum of a "fast varying" hard component and a "slowly varying" soft component. We found that all the data can be well explained by this unified model.
Consequently, we revealed that the soft component has two types of accretion-disk states. One is the high temperature branch (HTB), characterized by the innermost temperature Tin ~ 2 keV and innermost radius Rin ~ 20 km. The other is the low temperature branch (LTB), which has Tin of ~ 1 keV and Rin of ~ 50 km. In the HTB, the disk bolometric luminosity is almost constant at ~ 1 x 1039 erg s-1. The innermost temperature is too high to be explained by a standard disk extending down to 3 times Schwarzschild radius even assuming the Eddington luminosity. We suggest that the accretion disk in the HTB is an optically thick advection dominated accretion flow (optically thick ADAF). On the other hand, the LTB can be well explained by the standard disk model. Its innermost radius tends to increase with the mass accretion rate, probably due to a thermal instability of the accretion disk. The peculiar variability of this source would be caused by an intrinsic change of the disk states i.e. transition between an optically thick ADAF and a standard disk. Furthermore, we suggest that jet production is triggered by the transition between an optically thick ADAF and a standard disk from the detection of an infrared flare occured in the 2000 multiwavelength campaign.
The origin of the hard component was also investigated. The hard component in the LTB has a photon index of 2.0-3.3 below 20 keV while it is mostly constant in the energy band above 30 keV. We find a correlation between the photon index below 20 keV and the disk bolometric luminosity. This is consistent with the picture that electrons in the optically thin ADAF is cooled by inverse Compton scattering of the disk photons, being balanced by a supply of energy flow from ions.