GCN Circular 17754

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 again 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 24.18 to 2015/04 24.48
UTC (21.74 to 28.97 hours after the BAT trigger), obtaining a total of
5.69 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 upper
limits (3-sigma):

  r     > 24.82
  i     > 24.79
  z     > 22.07

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. In comparison to the earlier
epoch of RATIR observations (Littlejohns, et al., GCN 17736
Not found
 
 
), these upper
limits indicate fading of at least 1 magnitude in the r and i bands. This
implies a continuing power-law decay temporal power-law index of t^-0.5
or steeper, which is consistent with the comparison between the first
epoch of RATIR observations and the earlier GROND measurements (Varela,
et al., GCN 17732
Not found
 
 
). The magnitude of source 2, as reported in the GROND
observations, is consistent with being constant in both epochs of RATIR
data.

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