A Prominent Accretion Disk in the Low-Hard State of the Black Hole Candidate SWIFT J1753.5-0127

Abstract

We report on simultaneous XMM-Newton and RXTE observations of the stellar-mass black hole candidate SWIFT J1753.5-0127. The source was observed in the low-hard state, during the decline of a hard outburst. The inner accretion disk is commonly assumed to be radially truncated in the low-hard state, and it has been suggested that this property may be tied the production of steady, compact jets. Fits to the X-ray spectra of SWIFT J1753.5-0127 with a number of simple models clearly reveal a cool (kT ~ 0.2 keV) accretion disk. The disk component is required at more than the 8 sigma level of confidence. Although estimates of inner disk radii based on continuum spectroscopy are subject to considerable uncertainty, fits with a number of models suggest that the disk is observed at or close to the innermost stable circular orbit. Recently, an observation of GX 339-4 revealed a disk extending to the innermost stable circular orbit at L_X/L_Edd ~ 0.05; our results from SWIFT J1753.5-0127 may extend this finding down to L_X/L_Edd ~ 0.003 (d/8.5 kpc)^2 (M/Msun). We discuss our results within the context of low-luminosity accretion flow models and disk-jet connections.