TITLE:   GCN CIRCULAR
NUMBER:  22509
SUBJECT: GRB 180316A: RATIR Optical Observations
DATE:    18/03/16 14:37:38 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 180316A (Melandri, et al., GCN 22500) 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 2018/03 16.39 to 2018/03 16.52 UTC (4.34 to
7.49 hours after the BAT trigger), obtaining a total of 2.29 hours exposure
in the r and i bands.

The afterglow is well-detected within the Swift-XRT error circle (Evans, et
al., GCN 22501).   In comparison with the USNO-B1 catalog, we obtain:

  r = 20.07 +/- 0.02
  i = 19.87 +/- 0.02

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.  The source appears so fade in flux
with time as t^(-1.4+/-0.1).

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