[vsnet-grb-info 16875] GRB 150817A: Continued RATIR Optical Observations - Afterglow Confirmation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18156
SUBJECT: GRB 150817A: Continued RATIR Optical Observations - Afterglow Confirmation
DATE:    15/08/18 16:29:47 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan at astro.unam.mx>

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

We observed the field of GRB 150817A (D'Elia, et al., GCN 18143) 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 2015/08 18.16 to 2015/08 18.27 UTC (25.78 to 
28.42 hours after the BAT trigger), obtaining a total of 1.77 hours 
exposure in the r, i, and z bands.

The first source reported by Butler et al. (GCN 18144) has not faded since 
our first epoch of observations.

The second source, reported by Butler et al. (GCN 18145), has continued to 
fade and is now only marginally detected. In comparison with the USNO-B1 
and 2MASS catalogs, we obtain the following marginal detections and upper 
limit (3-sigma):

   r	= 23.8 ± 0.4
   i	= 23.8 ± 0.3
   z	> 20.2

These magnitudes are in the AB system and are not corrected for Galactic 
extinction in the direction of the GRB.

We note that the second source is consistent with the enhanced Swift-XRT 
error circle (Osborne et al. GCN 18146). This coincidence and the clear 
fading suggest that it is the afterglow of the GRB.

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