TITLE:   GCN CIRCULAR
NUMBER:  26957
SUBJECT: GRB 200131A: RATIR Optical Observations
DATE:    20/02/01 04:26:12 GMT
FROM:    Nat Butler at UC berkeley  <natxbutler@gmail.com>

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 200131A (Sbarufatti et al., GCN 26953) 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 2020/02 1.10 to 2020/02 1.14 UTC (3.79 to
4.61 hours after the BAT trigger), obtaining a total of 0.64 hours exposure
in the r and i bands.

The optical transient (see, also, Lipunov et al., GCN  26954) is
well-detected.  We find:

  r = 19.97 +/- 0.04
  i = 19.75 +/- 0.02

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.