GCN Circular 17931
Kosmas Gazeas and Konstantinos Sapountzis (National Univ. of Athens) report: Swift trigger 643949 occurred in June 15, 2015 and followed a few minutes afterwards with the 0.40 m f/8 robotic telescope at the University of Athens in R-band. Data collection has started on June 15 (UT) 18:58:37 and finishing on June 15 (UT) 20:28:09 (mid-exposure time). The bright GRB seems to be the X-ray binary V404 Cyg, consisting of a black hole and a late G-type companion. A sum of 30 exposures of 180 sec each was collected. Photometry with a 3 pixel (approximately 4 arcsec) radius aperture yields an R magnitude estimation of R = 12.65 +/- 0.01 mag in the beginning of data acquisition and R = 15.43 +/- 0.03 mag at the end. There is a luminosity fading trend of ~2.7 mag/hour as observed during the first hour of acquisition. Fading rate dramatically decreased afterwards, reaching the value of ~0.2 mag/hour. Differential photometry was performed utilizing the nearby stars, namely USNO 1200-15039207 (Rmag=12.9) and USNO 1200-15046396 (Rmag=12.7), located 140 arcsec west and 96 arcsec southeast of the source, respectively. No further absolute photometric calibration has been applied on these data. The extracted light curve can be found under: https://sites.google.com/site/astrofridaysmeetings/paratereseis/grb This message is quotable in publications.