August 4, 2015: Dr. Gregory Sivakoff (U. of Alberta) has requested optical monitoring of the galactic microquasar V4641 Sgr beginning immediately, and continuing for the next 120 days, or until it is no longer observable from your location. Evidence from recent X-ray observations suggests that it has entered an active state, and it is possible that one of this star's rare optical outbursts that include rapid flaring may occur within the next 120 days. Observers are encouraged to begin monitoring with the highest cadence possible for the remainder of this observing season. A coordinated multiwavelength campaign may be announced in the near future, but AAVSO observers are encouraged to begin monitoring this star at their earliest opportunity. Dr. Sivakoff has posted details about this campaign on the AAVSO website forums, available at the following link:
https://www.aavso.org/request-aavso-support-observing-black-hole-microquasar-v4641-sgr
V4641 Sgr is located at the following (J2000) coordinates:
RA: 18 19 21.63 , Dec: -25 24 25.8
Both visual and CCD observations are encouraged for monitoring. If possible, CCD observers are encouraged to make filtered observations, with V or I filter at least if available. While the source is in its active, but non flaring state, quick changes between filters are scientifically valuable. In the case of a brighter outburst (V brighter than 13 mag), same filter observations may be more scientifically valuable. The most recent visual and V-filtered observations put V4641 Sgr at V ~ 13.3 (JD 2457235.1842, V=13.32+/-0.03, P. Starr, NSW, Australia; JD 2457238.8972, m(vis)=13.3, M. Linnolt, HI, United States). CCD observers are encouraged to get S/N of at least 20, with higher S/N preferred if exposure times can be kept shorter than a few minutes.
Charts with comparison stars are available via AAVSO's VSP:
https://www.aavso.org/vsp
Please clearly indicate which comparison stars are used. Observers are asked to submit all observations promptly via the WebObs feature on the AAVSO website (www.aavso.org/WebObs), particularly if a brighter outburst is detected. Please submit all observations using the name "V4641 SGR".
This AAVSO Special Notice was prepared by M. Templeton.