{
  "bibcode": "2017GCN.21189....1M",
  "body": "E. Mazaeva (IKI),   A. Mokhnatkin (KIAM),  A. Pozanenko (IKI), A. \nVolnova (IKI),   I. Molotov (KIAM)  report on behalf of  larger GRB \nfollow-up collaboration:\n\nWe observed the field of GRB 170531B (Evans et al.,GCN  21171) with \nK-800 (0.8m) telescope  of ISON/Terskol   observatory starting  on May, \n31 (UT) 22:21:31.  We obtained several unfiltered images of 40 s \nexposure. The optical afterglow (Lipunov et al., GCN 21173 Pozanenko et \nal.  GCN 21174)  is clearly visible in combined images. Preliminary \nphotometry of the afterglow is following\n\n\nDate       UT start  t-T0   Filter   Exp.  OT   Err. UL\n                      (mid, days)     (s)\n\n2017-05-31 22:21:31 0.01810 CR     18*40  18.90 0.10 21.0\n2017-05-31 22:35:41 0.02949 CR     21*40  19.10 0.10 21.0\n\nPhotometry is based on nearby USNO-B1.0 stars\nUSNO-B.1_id R2\n0736-0718845 16.77\n0735-0729626 17.35\n0735-0729622 16.96\n0735-0729439 16.59\n0735-0729327 17.22",
  "circularId": 21189,
  "createdOn": 1496416962000,
  "email": "apozanen@iki.rssi.ru",
  "subject": "GRB 170531B: ISON/Terskol optical observations",
  "submitter": "Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>",
  "eventId": "GRB 170531B"
}