TITLE:   GCN CIRCULAR
NUMBER:  17736
SUBJECT: GRB 150423A: RATIR optical afterglow monitoring
DATE:    15/04/23 17:47:09 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 150423A (Pagani, et al., GCN 17728) 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 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), 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), 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), 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 Astronómico Nacional in San Pedro
Mártir.