TITLE:   GCN CIRCULAR
NUMBER:  30534
SUBJECT: GRB 210726A: RATIR Optical Upper Limits and Candidate Host Galaxy 
DATE:    21/07/27 16:14:25 GMT
FROM:    Alan M Watson at UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Eleonora Troja (GSFC), Nat Butler (ASU), Alexander
Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox
(STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Enrico Ramirez-Ruiz
(UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley
(GSFC), Rosa L. Becerra (UNAM), Simone Dichiara (UMD), and Océlotl López (UNAM)
report:

We observed the field of candidate short GRB 210726A (Bernardini et al., GCN
Circ., 30523) with the Reionization and Transients Infrared Camera (RATIR) on
the 1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on
Sierra San Pedro Mártir from 2021/07 27.16 to 2021/07 27.22 UTC (8.24 to 9.65
hours after the BAT trigger), obtaining a total of 1.09 hours exposure in the r
and i bands.

For a source within the enhanced Swift-XRT error circle (Osborne et al., GCN
Circ., 30524), in comparison with the SDSS DR9 catalog, we obtain the following
upper limits (3-sigma):

  r	> 23.0
  i	> 22.7

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

We note that the SDSS galaxy J125309.63+191127.2 is 3.1 arcsec from the enhanced
XRT position and just outside the error circle. In our observations, it has 
r = 22.2 +/- 0.2 and i = 22.2 +/- 0.2, in agreement with the cataloged values.
The cataloged photometric redshift is z = 0.35 +/- 0.15. The probability of a
chance alignment is approximately 3%. Given this low probability, we suggest
that this source might be the host galaxy.

Since this GRB appears to be potentially both short and low-redshift, we
encourage further observations.

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