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

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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 ouburst. The inner accretion disk is commonly assumed to be radially truncated in this state, and it has been suggested that this property may be tied to 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 = 0.05 L_Edd; our results from SWIFT J1753.5-0127 extend this finding down to L_X = 0.003 (d/8.5 kpc)^2 (M/10 Msun) L_Edd. We discuss our results within the context of low-luminosity accretion flow models and disk-jet connections.

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