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A. A. Esin (Caltech), J. A. Tomsick (UCSD)
Many transient and persistent Black Hole X-ray Binaries (BHXBs) show characteristic state transitions between low/hard and high/soft spectral states. A typical low/hard state X-ray spectrum, as seen for example in Cyg~X-1 and GRO~J0422+32, is a power-law with a photon index of order 1.4-1.6 and an exponential cutoff with a characteristic e-folding energy ~100\,{\rm keV}. These spectra are generally interpreted as emission from hot thermal plasma modeled either as an accretion disk corona, or as a hot accretion flow inside a truncated thin disk. We show that low state spectra of three transient BHXBs, XTE~J1118+480, XTE~J1550-564, and 4U1630-47, do not show the characteristic hard X-ray turnover. The absence of the turnover is difficult to understand in the context of the thermal Comptonization models, and argues for the presence of non-thermal emission in these systems. We propose that a typical hard power-law spectrum can be produced both in the (thermal) accretion flow or in the (non-thermal) jet (as suggested by Fender et al. 2000). The strength of the jet emission relative to that from the accretion flow (and therefore whether we observe the thermal cutoff in the spectrum or not) can depend on several parameters, e.g. inclination of the jet with respect to the binary and the observer, mass accretion rate, accretion flow configuration. 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, and by a CASS Postdoctoral Fellowship.