GCN Circular 17736

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), Jos� A. de Diego (UNAM), Leonid
Georgiev (UNAM), Jes�s Gonz�lez (UNAM), Carlos Rom�n-Z��iga (UNAM),
Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We observed the field of GRB 150423A (Pagani, et al., GCN 17728
Not found
 
 
) with
the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astron�mico

Nacional on Sierra San Pedro M�rtir from 2015/04 23.41 to 2015/04
23.48 UTC (3.30 to 5.01 hours after the BAT trigger), obtaining a
total of 1.42 hours exposure in the r, i and z bands.

For a source within the Swift-XRT error circle (Evans, et al., GCN
17735
Not found
 
 
), in comparison with the SDSS DR9, we obtain the following
detections and upper limits (3-sigma):

  r     23.77 +/- 0.23
  i     23.62 +/- 0.26
  z     > 20.33

These magnitudes are in the AB system and are not corrected for
Galactic extinction in the direction of the GRB. Of the two sources
reported by GROND (Varela, et al., GCN 17732
Not found
 
 
), this source is at the
position of source 1. Our observations show a fading of 0.8 magnitudes
in the i band, implying a power-law decay with a temporal index,
t^-0.5. Similarly to the Keck observations (Perley, GCN 17733
Not found
 
 
), we
also find source 2 reported by GROND to have remained at approximately
constant brightness.

Continued observations are planned.

We thank the staff of the Observatorio Astron�mico Nacional in San Pedro
M�rtir.