Pi of the Sky detection of an optical flash accompanying GRB160625B

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)

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:

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.

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
t – t0 MagV ErrV

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
t – t0 MagV ErrV

Indicated errors are statistical only. Estimated systematic uncertainty of the image calibration is around 0.06-0.08 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.

Animation of GRB160625B observed by camera 39 (click to run)

grb20160625b

Sequence of frames for camera 35

cam35

Sequence of frames for camera 39

cam39

Light Curve of GRB160625B observed by Pi of the Sky North

grb20160625b