GCN Circular 19603

S.Karpov, G.Beskin (SAO RAS and Kazan Federal University, Russia), S.Bondar,
E.Ivanov, E.Katkova, N.Orekhova, A.Perkov (OJS RPC PSI, Russia), A.Biryukov
(SAI MSU and Kazan Federal University, Russia), V.Sasyuk (Kazan Federal
University, Russia)

The on-sky position of GRB 160625B (Dirirsa et al., GCN Circ. 19580
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 ) has
been observed by Mini-MegaTORTORA nine-channel wide-field monitoring system
 with high temporal resolution (http://mmt.favor2.info) before, during and
just after the LAT trigger time (T0 = 2016-06-25 22:43:24).

Mini-MegaTORTORA reacted to Fermi GBM trigger 488587220 and started
observing its error box at 2016-06-25 22:41:07, 52 seconds after GBM
trigger and 136 seconds before LAT trigger. Due to large size of error box,
the observations have been performed in "widefield+deep" regime, with
channels simultaneously covering ~30x30 deg field of view with 30 s
exposures in white light to achieve deepest detection limit. The system
acquired 20 frames in such regime, covering time interval from T0-136 to
T0+466 s.

The system detected a bright optical flash on a frame coincident with LAT
trigger time (T0-15.9 - T0+14.1 s) at position reported by Troja et al.
(GCN Circ. 19588
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 ), with a magnitude of about V=8.8 mag. Next frame
displayed a slight rebrightening of about 0.1 mag, and then the light curve
followed nearly  smooth power-law decay  with slope of about -1.6, down to
V=12.2 at last acquired frame. The images acquired prior to LAT trigger do
not display any object at that position down to about V=13.5 mag.

The system also reacted to second GBM trigger 488587880 with slightly
different coordinates, and due to large field of view again covered the
position of a transient, acquiring 20 more frames from T0+1691 to T0+2264
s. No object is seen in these frames down to about V=13.5 mag.

Snapshots of a frames from first follow-up (20 arcmin fields around the
transient) is available at
http://vo.astronet.ru/~karpov/grb160625b_mosaic.png

Further analysis and calibration of the data is ongoing.

Mini-MegaTORTORA belongs to Kazan Federal University and is located at
Special Astrophysical Observatory near Russian 6-m telescope.