TITLE:   GCN CIRCULAR
NUMBER:  29784
SUBJECT: GRB 210410A: RATIR Optical and NIR Observations
DATE:    21/04/10 13:41:41 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 (U. Wash.), and Vicki Toy (UMD) report:

We observed the field of GRB 210410A (Fermi GBM team, et al., GCN 29777)
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 2021/04 10.30 to 2021/04 10.51 UTC (6.26 to 11.25
hours after the BAT trigger), obtaining a total of 2.16 hours exposure in
the r and i bands and 0.87 hours exposure in the Z, Y, J, and H bands.

We detect one source within the Swift-XRT error circle (Melandri, et al.,
GCN 29778).  In comparison with the USNO-B1 and 2MASS catalogs, we find
these magnitudes (in the AB system and not corrected for Galactic
extinction in the direction of the GRB):

  r     = 23.79 +/- 0.30
  i     = 22.49 +/- 0.14
  Z     = 22.07 +/- 0.27
  Y     = 21.14 +/- 0.18
  J     = 20.24 +/- 0.11
  H     = 20.17 +/- 0.15

The source is located at RA, Dec = 17:59:1.00, +45:21:44.6 (J2000,
+/-0.5"), which is 2 arcsec away from the candidate optical afterglow
position reported by Jelinek et al. (GCN 29780).  It is possible that an
astrometric cross-comparison could reveal our sources to be the same, in
which case the optical afterglow continues to fade strongly.  It is also
possible that we are detecting the GRB host galaxy.  A photo-z analysis
suggests the redshift is z<3.6 (90% confidence).

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