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

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