TITLE:   GCN CIRCULAR
NUMBER:  22037
SUBJECT: GRB 171020A: RATIR Optical and NIR Observations
DATE:    17/10/21 15:28:40 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), Jes��s
Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Harvey Moseley (GSFC), John
Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report:

We observed the field of GRB 171020A (Page, et al., GCN 22028) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional on
Sierra San Pedro M��rtir from 2017/10 21.16 to 2017/10 21.50 UTC (4.60 to
12.98 hours after the BAT trigger), obtaining a total of 5.30 hours
exposure in the r and i bands and 2.20 hours exposure in the Z, Y, J, and H
bands.

We detect the NOT optical transient (Malesani, et al., GCN 22029), which
continues to fade (see also, Moskvitin, et al., GCN 22031).  In comparison
with the USNO-B1 and 2MASS catalogs, we obtain the following detections and
upper limit (3-sigma):

 r = 22.91 +/- 0.12
 i = 22.76 +/- 0.11
 Z > 21.44
 Y = 21.72 +/- 0.35
 J = 21.54 +/- 0.23
 H = 21.69 +/- 0.35

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

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.