[Previous] | [Session 35] | [Next]
A. A. Esin (Caltech)
The low/hard state, characterized by a power-law spectrum with photon index 1.4-1.8 and extending to ~100 keV, is observed in soft X-ray transients in outburst (e.g. GRS 1124-68, GRO J0422+32), as well as in some persistent X-ray binaries (e.g. Cyg X-1). It is generally assumed that these spectra are produced by thermal Comptonization of seed photons in the vicinity of an accreting black hole or neutron star. However, the geometry of an accreting flow as well as the source of soft photons currently remains a point of debate. Recently two competing pictures have emerged. In one the hard power-law spectrum is produced by Comptonization in a hot corona located above a cool thin disk, which supplies seed photons. In another scenario, the thin disk is truncated at some large inner radius and the bulk of the emission forms in the hot quasi-spherical accretion flow through inverse Compton scattering of synchrotron photons. So far the attempts to distinguish between these two scenarios have been based on interpretation of the X-ray reflection features, complicated by uncertainties in the models.
We show that EUVE, RXTE (Hynes et al. 2000) and Chandra (McClintock et al. 2000) observations of the new X-ray transient XTE J1118+480 for the first time allow us to place direct limits on the accretion geometry in the vicinity of the central object. We argue that the combined data rules out the presence of the standard thin accretion disk within ~100 Schwarzschild radii, and therefore, presents a problem for disk+corona models. We also show that a model based on an advection-dominated accretion flow surrounded by a truncated thin disk gives an excellent fit to the combined optical, UV and X-ray data. We further speculate that the absence of any thermal emission component in the data is a signature of an accreting black hole. This work was supported by Chandra Postdoctoral Fellowship grant #PF8-10002 awarded by the Chandra X-Ray Center, which is operated by the SAO for NASA under contract NAS8-39073.