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 – t0MagVErrV
-5.903519009.178580000.01979590
7.300340108.041852400.00692530
20.504110008.852567200.01464740
33.708910009.164113900.01958250
46.913910009.352211700.02310580
60.125620009.483568200.02605830
73.330600009.762732200.03348310
86.534840009.888690100.03764580
110.7590600010.361678000.05831040
123.9654900010.363155000.05835510
137.1736600010.657921000.07717340
150.3785500010.783653000.08672800
163.5829600010.901033000.09643880
176.7919100011.105037000.11532510
190.0014100011.276454000.13539200
203.2049800011.430888000.15646540
216.4096900011.521253000.16944510
229.5971900011.662193000.19535900
242.8019600011.973885000.26215110
255.9855600011.442726000.15893520
269.1883500012.227804000.33061590
282.3928800011.706443000.20138410
295.5973700012.103551000.29637090
308.8017100012.019542000.27279960
348.4141200012.149435000.30407830
388.0209500012.209403000.32526530
t – t0MagVErrV

For camera id = 39

t – t0MagVErrV
-0.217380008.083324500.00848860
15.132080008.396247300.01125160
30.480320009.303887800.02600590
45.834970009.380909500.02766380
61.693240009.600607700.03356240
77.040260009.761180300.03894990
92.3880800010.000686000.04923380
107.7168300010.237612000.05958340
123.0696300010.439170000.07335490
138.4243900010.635677000.08856020
153.7733100010.786148000.10097800
169.1214500010.991282000.12175100
184.4699500011.234455000.15126550
199.8248300011.046860000.12786780
215.1729200011.271350000.15866670
232.5650500011.473983000.19109480
247.8947200011.999056000.31466480
263.7513600011.787990000.25153010
279.0982300012.085302000.33607390
294.4461100011.954126000.30025650
325.1468000012.141881000.35312590
432.5953700012.185515000.36922680
t – t0MagVErrV

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