TITLE: GCN CIRCULAR NUMBER: 19615 SUBJECT: Pi of the Sky detection of an optical flash accompanying GRB 160625B DATE: 16/06/28 19:34:52 GMT FROM: Lech Wiktor Piotrowski at U Warsaw T. Batsch, A.J. Castro-Tirado, R. Cunniffe, H. Czyrkowski, A. Ćwiek, M. Ćwiok, R. Dąbrowski, M. Jelínek, G. Kasprowicz, A.Majcher, K. Małek, L. Mankiewicz, K. Nawrocki, Ł. Obara, R. Opiela, M. Siudek, M. Sokołowski, L. W. Piotrowski, R. Wawrzaszek, G. Wrochna, A Zadrożny, M. Zaremba, A.F. Żarnecki (Pi of the Sky Collaboration) Following GCN 19603 we present the reconstructed light curve of GRB 160625B. One of four Pi of the Sky North detectors located at INTA - El Arenosillo observatory in Mazagón near Huelva, Spain imaged the region of GRB 160625B (Swift-XRT error circle (Melandri, et al., GCN 19585), at a position RA, Dec = 20:34:23.50, +6:55:8.1) before, during and after the GRB with 10s exposures (the exposures were taken in white light, IR-cut and UV-cut filters only, to achieve deepest detection limit). Cameras of the Pi of the Sky North observatory were observing the position of the GRB160625B 48s after Fermi GBM trigger 488587220 time (Jun 25 22:40:16.28 UT) 140 seconds before the LAT 488587408 trigger (Jun 25 22:43:24.82 UT). We observed optical emission at the position given by Swift XRT recording a bright light curve starting -5.9 s before the LAT trigger. The first 10 s exposure shows initial magnitude of ~9.18 (unfiltered) brightening to ~8.04 on the second exposure, than becoming gradually dimmer. It is important to note that both cameras, 35 and 39, identified a new object on exposures starting just before the time of the trigger. Below we give the light curve of GRB 160625B separately for two cameras: For camera id = 35 t - t0 MagV ErrV -5.90351900 9.17858000 0.01979590 7.30034010 8.04185240 0.00692530 20.50411000 8.85256720 0.01464740 33.70891000 9.16411390 0.01958250 46.91391000 9.35221170 0.02310580 60.12562000 9.48356820 0.02605830 73.33060000 9.76273220 0.03348310 86.53484000 9.88869010 0.03764580 110.75906000 10.36167800 0.05831040 123.96549000 10.36315500 0.05835510 137.17366000 10.65792100 0.07717340 150.37855000 10.78365300 0.08672800 163.58296000 10.90103300 0.09643880 176.79191000 11.10503700 0.11532510 190.00141000 11.27645400 0.13539200 203.20498000 11.43088800 0.15646540 216.40969000 11.52125300 0.16944510 229.59719000 11.66219300 0.19535900 242.80196000 11.97388500 0.26215110 255.98556000 11.44272600 0.15893520 269.18835000 12.22780400 0.33061590 282.39288000 11.70644300 0.20138410 295.59737000 12.10355100 0.29637090 308.80171000 12.01954200 0.27279960 348.41412000 12.14943500 0.30407830 388.02095000 12.20940300 0.32526530 For camera id = 39 t-t0 MagV ErrV -0.21738000 8.08332450 0.00848860 15.13208000 8.39624730 0.01125160 30.48032000 9.30388780 0.02600590 45.83497000 9.38090950 0.02766380 61.69324000 9.60060770 0.03356240 77.04026000 9.76118030 0.03894990 92.38808000 10.00068600 0.04923380 107.71683000 10.23761200 0.05958340 123.06963000 10.43917000 0.07335490 138.42439000 10.63567700 0.08856020 153.77331000 10.78614800 0.10097800 169.12145000 10.99128200 0.12175100 184.46995000 11.23445500 0.15126550 199.82483000 11.04686000 0.12786780 215.17292000 11.27135000 0.15866670 232.56505000 11.47398300 0.19109480 247.89472000 11.99905600 0.31466480 263.75136000 11.78799000 0.25153010 279.09823000 12.08530200 0.33607390 294.44611000 11.95412600 0.30025650 325.14680000 12.14188100 0.35312590 432.59537000 12.18551500 0.36922680 Indicated errors are statistical only. Estimated systematic uncertainty of the image calibration is around 0.06-0.08 mag. T0 is the Fermi GBM trigger 488587408 time (Jun 25 22:43:24.82 UT) and t is the time of shutter oppening. Each image taken covers approximately 400 square degrees. The magnitudo limit was about 12.5 mag. Pi of the Sky observations are performed in wide visible band, with IR-cut and UV-cut filters only. We calibrate our observations to the reference stars from Tycho 2 using the transformation from Tycho to Pi of the Sky system given by: VPi = VT + 0.235313 - 0.292266*(BT - VT) Due to the proximity of the burst to the edge of our FoV, the transient was not detected automatically. However, this allowed us to record it with two cameras, with exposures slightly shifted in time. The light curve is available at: http://grb.fuw.edu.pl/results/gamma-ray-bursts/grb160625b/. More information will be published at http://grb.fuw.edu.pl and http://grb.fuw.edu.pl/results/gamma-ray-bursts/grb160625b/ [GCN OPS NOTE(04jul16): Per author's request, the GRB name in the 2nd paragraph was corrected, and RC was added to the author list.]