A Relativistic Fe Kα Emission Line in the Intermediate-Luminosity BeppoSAX Spectrum of the Galactic Microquasar V4641 Sgr

Broad Fe Kα emission lines have recently been reported in a number of Galactic black holes. Such lines are useful accretion flow diagnostics because they may be produced at the inner accretion disk and shaped by relativistic effects, but in general they have been observed only at luminosities of L_X ~ 10³⁷-10³⁸ ergs s^-1 in soft X-rays. The Galactic microquasar V4641 Sgr—widely known for its 12.2 Crab (1.5-12 keV) outburst in 1999 September—displayed low-level activity in 1999 March. BeppoSAX observed the source in this state, and Fe Kα line emission was found by in 't Zand et al. In reanalyzing these data, we find strong evidence that the Fe Kα line profile is broadened. For the most likely values of the source distance and black hole mass measured by Orosz et al., our fits to the total spectrum indicate that the source was observed at a luminosity of L_X = 1.9 × 10³⁶ ergs s^-1 (2-10 keV), or L_X/L_Edd = 1.8 × 10^-3. Advection-dominated accretion flow models predict a radially recessed disk in this regime. In contrast, fits to the observed Fe Kα emission-line profile with a relativistic line model constrain the inner disk to be consistent with the marginally stable circular orbit of a Schwarzschild black hole.