X-Ray Spectral and Timing Properties of the Black Hole X-Ray Transient SWIFT J1753.5-0127

We have carried out a detailed analysis of the black hole candidate (BHC) and X-ray transient SWIFT J1753.5-0127 as observed during its outburst in 2005-2006 by the Rossi X-Ray Timing Explorer. The spectral analysis shows that the emission is dominated by hard X-rays, thus revealing the low/hard state of the source during the outburst. The peak luminosity is found to be lower than the typical value expected from balance between the mass flow and evaporation of the inner edge of the disk. As a result, the disk is unable to extend inward to produce strong soft X-rays, which would correspond to the so-called high/soft state. These are typical characteristics of a small subset of BHCs, those soft X-ray transients that stay in the low/hard state during outburst. For most of our observation time, the frequency of the quasi-periodic oscillation (QPO) in the spectrum of this source is found to vary roughly linearly with the flux and spectral index, while a deviation from this relationship at peak luminosity may provide the first observational evidence of a partially evaporated inner edge to the accretion disk. An anticorrelation between QPO frequency and spectral color suggests that the global disk oscillation model proposed by Titarchuk & Osherovich is not likely at work in this case.