TITLE:   GCN CIRCULAR
NUMBER:  18144
SUBJECT: GRB 150817A: RATIR Optical Afterglow Candidate
DATE:    15/08/17 04:02:40 GMT
FROM:    Nat Butler at UC berkeley  <natxbutler@gmail.com>

Nat Butler (ASU), Alan M. Watson (UNAM), 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), Jos� A. de Diego (UNAM), Leonid Georgiev (UNAM), Jes�s Gonz�lez (UNAM), Carlos Rom�n-Z��iga (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 (V. 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 Astron�mico Nacional on Sierra San Pedro M�rtir from 2015/08 17.14 to 2015/08 17.16 UTC (1.29 to 1.70 hours after the BAT trigger), obtaining a total of 0.36 hours exposure in the r, i, and z bands.

We find an uncatalogued source within the Swift-XRT error circle at RA, Dec = 16:38:31.98, -12:3:11.5 (J2000, +/-0.5").  In comparison with the USNO-B1 and 2MASS catalogs, we obtain the following detections and upper limit (3-sigma):

   r = 21.50 +/- 0.20
   i = 21.26 +/- 0.10
   z > 17.83

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

Further observations are planned.

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