TITLE:   GCN CIRCULAR
NUMBER:  29864
SUBJECT: GRB 210420B: RATIR Optical and NIR Observations
DATE:    21/04/21 15:03:09 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 210420B (Moss, et al., GCN 29844) 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 21.39 to 2021/04 21.49 UTC (13.73 to 16.08 hours
after the BAT trigger), obtaining a total of 1.47 hours exposure in the r
and i bands and 0.55 hours exposure in the Z, Y, J, and H bands.

We detect the source reported by Oksanen (GCN 29846), Lipunov et al. (GCN
29845), Leonini et al. (GCN 29851), de Ugarte Postigo et al. (GCN 29852),
Watson et al. (GCN 29858), Zheng et al. (GCN 29861), and Breeveld et al.
(GCN 29862).  In comparison with the SDSS DR9 and 2MASS catalogs, we obtain:

  r = 19.98 +/- 0.02
  i = 19.77 +/- 0.02
  Z = 19.70 +/- 0.03
  Y = 19.48 +/- 0.05
  J = 19.31 +/- 0.05
  H = 19.26 +/- 0.08

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 Astronómico Nacional in San Pedro
Mártir.