MASTER-OAFA: Fermi GRB faded optical counterpart detection

ATel #10063; T. Pogrosheva, V. Lipunov (Lomonosov MSU), R. Podesta (OAFA), H. Levato (ICATE), D. Buckley (SAAO), E. Gorbovskoy, N. Tiurina, P. Balanutsa, A. Kuznetsov, O. Gress, V. Kornilov , V. Vladimirov, V. Chazov, I. Gorbunov, A. Krylov, V. Shumkov, D. Kuvshinov (Lomonosov Moscow State University, SAI)
on 9 Feb 2017; 21:08 UT
Credential Certification: Nataly Tyurina (tiurina@sai.msu.ru)

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MASTER OT J072307.30-521446.6 discovery - possible optical counterpart of Fermi 508295323 GBM trigger

During Fermi GBM 508295323 trigger ( GRB_TIME: 2017-02-09 01:08:38.08 UT https://gcn.gsfc.nasa.gov/other/508295323.fermi ) inspection
MASTER-OAFA auto-detection system ( Lipunov et al., "MASTER Global Robotic Net", Advances in Astronomy, 2010, 30L ) discovered new OT source (Podesta et al. GCN #20650) at
(RA, Dec) = 07h 23m 07.30s -52d 14m 46.6s
on 2017-02-09 02:07:07.478UT with unfiltered m_OT=17.4 (mlimit=18.1m).
The second image is on 02:14:07.83 UT with m_OT=17.4.
There are only 2 inspection images of this area, the GRB_ERROR was 3.22 deg radius, and we were observing the error-box since 2017-02-09 01:10:30 UT (112s after the trigger time).
There is no minor planet at this place. There is no any sources in VIZIER database inside 5".
We have reference image without OT on 2017-01-29.17249 UT with 19.6 unfiltered magnitude limit.

MASTER-SAAO reobserved error-bix on 2017-02-09 18:37:14UT with unfiltered mlim=18.7 (sunset in SAAO, Sun altitude is -12.0).
There is no OT brighter 18.7m.
Deep photometry and spectral observations are required.
The discovery and reference images are available at:
http://master.sai.msu.ru/static/OT/072307.30-521446.6.png

MASTER Global Robotic Net