TITLE:   GCN CIRCULAR
NUMBER:  21051
SUBJECT: GRB 170428A: RATIR Afterglow Confirmation
DATE:    17/04/29 18:40:36 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

Eleonora Troja (GSFC), 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), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz
(UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey
Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki
Toy (UMD) report:

We observed the field of GRB 170428A (Beardmore, et al., GCN 21042)
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 2017/04 29.41 to
2017/04 29.48 UTC (24.69 to 26.25 hours after the BAT trigger), obtaining
a total of 0.90 hours exposure in the r and i bands and 0.35 hours
exposure in the Z, Y, J, and H bands.

For a source at the candidate afterglow position (Bolmer, et al., GCN 
21050),
in comparison with the SDSS DR9 and 2MASS catalogs, we obtain the following
detection and upper limits (3-sigma):

   r    > 22.3
   i    22.1 +/- 0.4
   Z    > 20.9
   Y    > 20.6
   J    > 20.1
   H    > 19.9

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

Our observations show that the source reported by Bolmer, et al., (GCN 
21050)
significantly faded, and is therefore the afterglow of the short 
GRB170428A.

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