TITLE:   GCN CIRCULAR
NUMBER:  17770
SUBJECT: GRB 150428A: RATIR optical observations
DATE:    15/04/28 06:46:58 GMT
FROM:    Owen Littlejohns at Az State U  <olittlej@asu.edu>

Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer
(UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska (UCSC),
Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC), Eleonora Troja
(GSFC), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid
Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM),
Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We observed the field of GRB 150428A (Page, et al., GCN 17765) 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 2015/04 28.14 to 2015/04
28.27 UTC (1.80 to 4.88 hours after the BAT trigger), obtaining a
total of 2.49 hours exposure in the r, i and z bands.

We find no uncatalogued source within the Swift-XRT error circle
(Osborne, et al., GCN 17768). In comparison with the SDSS DR9, we
obtain the following upper limits (3-sigma):

  r     > 23.94
  i     > 23.90
  z     > 19.56

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. We note that the source detected
by GROND (Knust, et al., GCN 17767) and P60 (Perley, GCN 17769) is not
consistent with the enhanced X-ray position. For the GROND source, in
comparison with the SDSS DR9, we obtain the following detections and
upper limit (3-sigma):

  r     = 23.41 +/- 0.22
  i     = 22.75 +/- 0.14
  z     > 19.56

The RATIR observations of the GROND source, therefore, do not show
evidence of fading between the GROND and RATIR epochs of observations.
Further RATIR observations are ongoing.

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