A request for AAVSO support observing the Black Hole Microquasar V4641 Sgr

Dear AAVSO,

This seems to be the summer of microquasar (i.e., black hole X-ray binary) outbursts. Just when the very successful multi-wavelength campaign on V404 Cyg appears to be dying down, the microquasar V4641 Sgr decided it would like some attention too. On 2015 July 30, the MAXI team announced that V4641 Sgr is undergoing renewed X-ray activity (Yoshii, T. et al., 2015, ATel 7858, http://www.astronomerstelegram.org/?read=7858). While astronomers expect to confirm that V4641 Sgr is in at least a minor outburst state with a second X-ray telescope within the next 24 hours, we wanted to announce this potential to the AAVSO observers and request their assistance with a potential multiwavelength campaign, beginning as soon as possible.

V4641 Sgr is located at RA = 18:19:21.63, Dec=-25:24:25.8. Although the source contains a B9III companion star, the accretion disk around the black hole is fed by the outer envelope of the star over filling the equipotential gravitational surface known as a Roche lobe. Among X-ray binaries whose accretion disks are fed this way in the Milky Way, V4641 Sgr has the most massive, brightest, and bluest companion star. In the radio, this source is a microquasar that has shown ejecta moving at nearly the speed of light.

V4641 Sgr can show relatively rapid outbursts and extreme flares. In its passive state during quiescence, the V band magnitude can vary between V~13.85 and V~13.45, in line with the orbital phase of its B9III companion star (MacDonald, R. et al. 2014, ApJ, 784, 1). In its active state during quiescence, the V band magnitude can be up to 0.15 mag brighter than in the passive state. When V4641 Sgr goes into a strong outburst, the source can be two to three magnitudes brighter than either quiescent state, and rapid flares and dips can occur on ~ half-hour to two-hour timescales (e.g., Uemura, M. et al. 2004, PASJ, 56, 823). In 1999 the source became active in early August, and underwent a major outburst on 1999 September 15, reaching 8.8 magnitudes in the R_c filter (Uemura, M. et al. 2001, PASJ, 54, 1).

Data taken by AAVSO obserer Peter Starr since the announcement of this X-ray outburst indicate V magnitudes of about 13.65 to 13.2, which suggests the source is at least in its active state. There is potential for a major outburst like that seen in 1999, or more moderate outbursts like those seen in 2002 and 2003. While we can not predict whether this will occur, we note that the source showed no evidence of spending more than ~ 120 days in the active, but non-outbursting state between 2001 and 2010 (Macdonald et al. 2014). We feel that there is strong potential for a second exciting multiwavelength campaign this summer, regardless of whether the source undergoes a major outburst or a more moderate one. The professional and citizen astronomers of the AAVSO have the ability to play a key role. We therefore request your assistance in monitoring the source. First, this monitoring could provide indication of the source going into a major outburst. Second, this monitoring will allow professional astronomers to maximize the science potential of any campaign, even if the source does not go into a major outburst. We note that given the bright nature of the source, this source may still be accessible to Northern hemisphere observers soon after (or even before) astronomical twilight begins. We therefore request monitoring observations by AAVSO observers begin as soon as possible, taking as high cadence observations as their circumstances allow.

Clear Skies,

Dr. Gregory R Sivakoff
University of Alberta, Edmonton, AB
sivakoff@ualberta.ca