optical monitoring of the galactic microquasar V4641 Sgr

PTS

Dr. Gregory Sivakoff (U. of Alberta) has requestedoptical monitoring of the galactic microquasar V4641 Sgr beginning immediately,and continuing for the next 120 days, or until it is no longer observable fromyour location. Evidence from recent X-ray observations suggests that it hasentered an active state, and it is possible that one of this star's rareoptical outbursts that include rapid flaring may occur within the next

120 days. Observers are encouraged to begin monitoring with the highest cadence possiblefor the remainder of this observing season. A coordinated multiwavelength campaign may be announced in the nearfuture, but AAVSO observers are encouraged to begin monitoring this star attheir earliest opportunity. Dr. Sivakoff has posted details about this campaignon 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 formonitoring.  If possible, CCD observersare encouraged to make filtered observations, with V or I filter at least ifavailable.  While the source is in itsactive, but non flaring state, quick changes between filters are scientificallyvaluable. In the case of a brighter outburst (V brighter than 13 mag), samefilter observations may be more scientifically valuable. The most recent visualand 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 fewminutes.

Charts with comparison stars are available via AAVSO'sVSP:

     https://www.aavso.org/vsp

Please clearly indicate which comparison stars are used.Observers are asked to submit all observations promptly via the WebObs featureon 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.