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 [subscript in] [< over ~] 6R[subscript g] , for certain values of, e.g., N [subscript H], i), although we cannot conclusively rule out the presence of an accretion disk truncated at larger radii (R [subscript in] ~ 10-50R[subscript 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 [subscript in] ~ 10-20R[subscript g] ). The implications of these observations for models of the accretion flow in the LHS are discussed.