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.

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.