Pi of the Sky observation of GRB160625B

The Pi of the Sky is a system of wide field of view robotic
telescopes, which search for short timescale astrophysical phenomena,
especially for prompt optical GRB emission. The system was designed
for autonomous operation, monitoring a large fraction of the sky to a
depth of 12m-13m and with time resolution of the order of 10
seconds. Custom designed CCD cameras are equipped with Canon lenses
f=85 mm, f/d = 1.2 and cover 20degx20deg of the sky each. The final
system with 16 cameras on 4 equatorial mounts was completed in 2014 at
the INTA El Arenosillo Test Centre in Spain.

GRB 160625B was an extremely bright GRB with three distinct emission
episodes. Emission features as measured in different spectral bands
indicate that the three episodes were dominated by distinct physics
process. Simultaneously observations in gamma-rays and optical
wavelengths support the hypothesis that this was the first observed
transition from thermal to non-thermal radiation in a single GRB.

Cameras of the Pi of the Sky observatory in Spain were not observing
the position of the GRB160625B prior to the first emission
episode. Observations started only after receiving Fermi GBM trigger,
about 140 seconds prior to the second emission. As the position
estimate taken from Fermi alert and used to position the telescope was
not very accurate, the actual position of the burst happened to be in
the overlap region of two cameras, resulting in two independent sets
of measurements. Light curves from both cameras were reconstructed
using the Luiza framework. No object brighter than 12.4m (3 sigma
limit) was observed prior to the second emission. An optical flash was
identified on an image starting -5.9 s before the time of the LAT
trigger, brightening to about 8m on the next image and then becoming
gradually dimmer, fading below our sensitivity after about 400 s.