[vsnet-grb-info 19332] Swift Trigger 748858: AbAO and TSHAO optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21029
SUBJECT: Swift Trigger 748858: AbAO and TSHAO optical observations
DATE:    17/04/21 20:16:17 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen at iki.rssi.ru>

E. Mazaeva (IKI), A. Pozanenko (IKI), R. Inasaridze (AbAO), I. Reva 
(Fesenkov Astrophysical Institute), A. Kusakin (Fesenkov Astrophysical 
Institute), A. Volnova (IKI),  V. Ayvazian  (AbAO),  O. Kvaratskhelia 
(AbAO), G. Inasaridze (AbAO), I. Molotov (KIAM), report on behalf of 
larger GRB follow-up collaboration:

We observed the field of the SWIFT trigger 748858 (Siegel, et al., GCN 
21011) with AS-32 (0.7m) telescope of Abastumani Observatory and with 
Zeiss-1000 1-m telescope of Tien Shan Astronomical Observatory. In each 
observation we fully cover both BAT error circles, initial (Siegel  et 
al., GCN 21011) and ground-calculated error circle (Sakamoto  et al., 
GCN 21017). Within  ground-calculated error circle we do not find any 
new source in comparison with SDSS catalog.  Within  XRT error circle of 
the source reported in GCN 21017 (Sakamoto  et al.) we find the source 
which is already mentioned by (Butler et al., GCN 21026). The source 
coincides with the star  of J131924.35+153001.6 and has the brightness 
of R =19.53 +/- 0.04 according to  SDSS DR9 and Lupton gri -> R 
transformations.

Limiting magnitudes as well as the brightness of the star 
J131924.35+153001.6 for each observation are presented below.

Date        UT start   t-T0      Filter Exp.   XRT    Err UpLim (3 sigma)
                       (mid, days)       (s)

2017-04-19 17:03:29    0.39918   CR     33*60  19.35  0.06 22.2
2017-04-19 17:37:14    0.42414   R      21*180 19.47  0.05 22.0

Photometry is based on nearby SDSS DR9 stars (Lupton transformations)
J131940.43+152853.2    15.63
J131934.46+152944.3    15.44
J131930.90+152831.0    16.02
J131927.38+153041.9    16.17
J131911.74+152820.9    15.72
J131859.43+153017.2    16.72
J131859.61+152435.2    16.63