V4641Sgr – A super-Eddington source enshrouded by an extended envelope

¹ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow, Russia,

M. Revnivtsev

Received: 18 September 2001
Accepted: 18 January 2002

Abstract

Optical spectroscopy of an unusual fast transient V4641 Sgr constrains its mass to be 8.7–11.7  ( is the best fit value) and the distance to 7.4–12.3 kpc (Orosz et al. [CITE]). At this distance the peak flux as measured by ASM/RXTE in 2–12 keV energy band implies the X-ray luminosity exceeding 2– erg s^-1, i.e. near or above the Eddington limit for a black hole. Optical photometry shows that at the peak of the optical outburst the visual magnitude increased by relative to the quiescent level and reached . An assumption that this optical emission is due to the irradiated surface of an accretion disk or a companion star with a black body spectrum would mean that the bolometric luminosity of the system exceeds . We argue that the optical data strongly suggest the presence of an extended envelope surrounding the source which absorbs X–ray flux and reemits it in the optical and UV. The data also suggest that this envelope should be optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr at the peak of an optical flare are very similar to those of SS433. This envelope is likely the result of a near- or super-Eddington rate of mass accretion onto the black hole. The envelope vanishes during subsequent evolution of the source when the apparent luminosity drops well below the Eddington value. Thus this transient source provides us with direct proof of the dramatic change in the character of an accretion flow at the mass accretion rate near or above the critical Eddington value as predicted long ago by the theoretical models.