Does the Disk in the Hard State of XTE J1752–223 Extend to the Innermost Stable Circular Orbit?

Abstract

Abstract The accreting black hole binary XTE J1752–223 was observed in a stable hard state for 25 days by the Rossi X-ray Timing Explorer (RXTE), yielding a 3–140 keV spectrum of unprecedented statistical quality. Its published model required a single-Comptonization spectrum reflecting from a disk close to the innermost stable circular orbit. We studied that model as well as a number of other single-Comptonization models (yielding similarly low inner radii), but found they violate a number of basic physical constraints, e.g., their compactness is much above the maximum allowed by pair equilibrium. We also studied the contemporaneous 0.55–6 keV spectrum from the Swift/X-ray Telescope and found it well fitted by an absorbed power law and a disk blackbody with the innermost temperature of 0.1 keV. The normalization of the disk blackbody corresponds to an inner radius of ≳20 gravitational radii and its temperature, to irradiation of the truncated disk by a hot inner flow. We have also developed a Comptonization/reflection model including the disk irradiation and intrinsic dissipation, but found that it does not yield any satisfactory fits. On the other hand, we found that the ≤10 keV band from RXTE is much better fitted by a reflection from a disk with the inner radius ≳100 gravitational radii, which model then underpredicts the spectrum at >10 keV by <10%. We argue that the most plausible explanation of the above results is inhomogeneity of the source, with the local spectra hardening with the decreasing radius. Our results support the presence of a complex Comptonization region and a large disk truncation radius in this source.