TITLE:   GCN CIRCULAR
NUMBER:  21026
SUBJECT: Swift Trigger 748858: RATIR Optical and NIR Observations
DATE:    17/04/20 23:58:05 GMT
FROM:    Nat Butler at Az State U  <natbutler@asu.edu>

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier
Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora
Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jesús
González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley (GSFC), John
Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report:

We observed the field of Swift Trigger 748858 (Siegel, et al., GCN 21011)
with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico
Nacional on Sierra San Pedro Mártir from 2017/04 19.32 to 2017/04 19.49 UTC
(2.4 minutes to 4.10 hours after the BAT trigger), obtaining a total of
2.49 hours exposure in the r and i bands and 1.04 hours exposure in the Z,
Y, J, and H bands.  We observed the field for a second time for image
subtraction purposes from 2017/04 20.15 to 2017/04 20.48 UTC (19.84 to
27.89 hours after the BAT trigger), obtaining a total of 4.74 hours
exposure in the r and i bands and 2.09 hours exposure in the Z, Y, J, and H
bands.

We do not detect a fading source within either the GCN 21011 BAT error
circle or the BAT error circle from Sakamoto et al. (GCN 21017), although
our observations miss 4% of the southern portion of the GCN 21017 error
region.  For a source within the BAT error regions, in comparison with the
SDSS DR9 and 2MASS catalogs, we obtain the following (3-sigma) upper limits
for our observations on 4/19:

r > 24.41
i > 23.95
Z > 22.59
Y > 22.44
J > 22.03
H > 21.79

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We note that an optical source is cleanly detected at a position consistent
with the XRT candidate position (GCN 21017).  It has r~i~19.5 mag and is
also present in DSS and SDSS.   This source does not vary in flux between
our epochs and is thus not likely to be the GRB afterglow.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.