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GRB 160625B: Mini-MegaTORTORA detection of bright optical flash simultaneous with LAT event

Russian version

    The on-sky position of GRB 160625B (Dirirsa et al., GCN Circ. 19580) has been observed by Mini-MegaTORTORA nine-channel wide-field monitoring system with high temporal resolution 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), 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 the Picture.
    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.
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) GRB 160625B: Mini-MegaTORTORA detection of bright optical flash simultaneous with LAT event, 2016, GRB Coordinates Network, Circular Service, N.19603

Contact - Beskin G.M.

Fig.1. First 20 images of the 20' x 20' region of the GRB160625B gamma-ray burst obtained with the Mini-MegaTORTORA system (a green circle indicates the position of the optical flare)

Fig.2. Light curve of the optical flare coinciding in time with the GRB160625B gamma-ray burst from observations in white light of the Mini-MegaTORTORA system (brightness measurements are given in the visible band)