TITLE:   GCN CIRCULAR
NUMBER:  21187
SUBJECT: GRB 170531B: SMARTS optical/IR observations
DATE:    17/06/01 18:25:15 GMT
FROM:    Bethany Cobb at GWU  <bcobb@gwu.edu>

B. E. Cobb (GWU), reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 170531B
(GCN 21171, Evans et al.) at two epochs (with mid-exposure times
of 2017-06-01 04:22 UT & 08:19 UT). For each epoch, several
dithered images were obtained with total summed exposure times
amounting to 36 minutes in I and 30 minutes in J.

No source is detected at the position of the X-ray/optical
afterglow (GCN 21175, Osborne et al.; GCN 21174, Pozanenko et al.;
GCN 21177, de Ugarte Postigo et al.; GCN 21178, Schady & Kruehler;
GCN 21179, Tyurina et al.) to the approximate 3-sigma limiting
magnitudes listed below:

mid-exposure
time post-burst
(hours)                I-limit    J-limit
 6.33056             >20.8    >18.5
10.27333            >21.2    >18.7

(Optical photometry is calibrated against USNO-B1.0 stars
and IR photometry is calibrated against 2MASS stars in the
field.)