GCN Circular 18098

Alan M. Watson (UNAM), Nat Butler (ASU), 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), Owen Littlejohns (ASU), 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 150728A (Krimm et al., GCN 18087
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 2015/07 31.22 to 2015/07 31.46 UTC (64.53 to 
70.24 hours after the BAT trigger), obtaining a total of 3.60 hours 
exposure in the r, i, and z bands.

For source A of Watson et al. (GCN 18096
Not found
 
 
), we determine:

   r	21.26 � 0.04
   i	20.90 � 0.04
   z	19.43 � 0.13

For source B of Watson et al. (GCN 18096
Not found
 
 
), we determine:

   r	20.91 � 0.04
   i	20.70 � 0.04
   z	20.48 � 0.31

These magnitudes are in comparison with the USNO-B1 and 2MASS catalogs, 
are in the AB system, and are not corrected for Galactic extinction in the 
direction of the GRB.

Our source A appears to correspond to the source reported by Dichiara et 
al. (GCN 18090
Not found
 
 
). If this association is correct, then there is little 
evidence for fading between observations at about 1 hour, 16-23 hours, and 
65-70 hours. This is in contrast to the suggestion of Mazaeva et al. (GCN 
18097
Not found
 
 
), who suggested that source A might indeed be the fading afterglow 
on the basis of a 3-sigma non-detection at 30 hours.

Similarly, there is little evidence for fading of source B between 16-23 
hours and 65-70 hours.

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