The spectra of relativistic accretion disks: Application to A0620-00

The X-ray flux emitted from a geometrically thin, relativistic accretion disk in the steady state approximation is investigated in order to place limits on the quiescent state mass flow rate in the soft X-ray transient black hole candidate source A0620-00. Specific attention is focused on the effects associated with gravitational redshifts, Doppler shifts, and on the enhancement of the apparent accretion disk area due to gravitational light bending on the continuum spectrum. It is found that the upper limit to the mass flow rate within the inner regions of the disk, constrained by the lack of soft X-rays in the quiescent state (F_x ≲ 10^-12 ergs cm^-2 s^-1), is ∼2.8 × 10^-11 M_⊙ yr^-1 (2.8 times higher than that found previously) for black hole masses greater than ∼5.4 M_⊙. The optical data are consistent with these upper limits provided that the inclination angle of the binary system is ≲ 65°. These upper limits and the lack of a hard X-ray flux, together, suggest that the soft X-ray transient model based upon a mass transfer instability situated in the stellar envelope of the companion is inapplicable to A0620-00.