TITLE:   GCN CIRCULAR
NUMBER:  14684
SUBJECT: GRB 130518A: Continued RATIR Optical Observations - Fading
DATE:    13/05/20 15:30:09 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

Eleonora Troja (GSFC), 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 (UCSC), 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 again observed the field of GRB 130518A (Xiong, et al., GCN 14674;
Omodei, et al., GCN 14675) 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
2013/05 20.43 to 2013/05 20.46 UTC (44.41 to 45.24 hours after the GBM
trigger), obtaining a total of 0.71 hours exposure in the r' and i' bands.

In comparison with SDSS DR8, we obtain the following detections of the source
reported earlier (Troja, et al., GCN 14678; Cenko, GCN 14680):

r'    20.47 � 0.04
i'    20.19 � 0.04

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

In comparison with our earlier observations (Troja, et al., GCN 14678), the source
has faded by slightly more than 1 magnitude in r' and i' between about 21 hours
and 45 hours after the trigger. This confirms that the source corresponds to the
afterglow of GRB 130518A.

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