Suzaku Broadband Spectroscopy of Swift J1753.5-0127 in the Low-Hard State

Abstract

We present Suzaku observations of the Galactic black hole candidate Swift J1753.5-0127 in the low-hard state (LHS). The broadband coverage of Suzaku enables us to detect the source over the energy range 0.6-250 keV. The broadband spectrum (2-250 keV) is found to be consistent with a simple power-law (Γ ~ 1.63). In agreement with previous observations of this system, a significant excess of soft X-ray flux is detected consistent with the presence of a cool accretion disk. Estimates of the disk inner radius infer a value consistent with the innermost stable circular orbit (ISCO; R _in lsim 6R_g , for certain values of, e.g., N _H, i), although we cannot conclusively rule out the presence of an accretion disk truncated at larger radii (R _in ~ 10-50R_g ). A weak, relativistically broadened iron line is also detected, in addition to disk reflection at higher energy. However, the iron-K line profile favors an inner radius larger than the ISCO (R _in ~ 10-20R_g ). The implications of these observations for models of the accretion flow in the LHS are discussed.