GCN Circular 15437

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 (ORAU/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 131018B (Vianello, et al., GCN 15357
Not found
 
 
; 
Zhang, et
al., GCN 15360
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 2013/10 20.09 to
2013/10 20.27 UTC (33.94 to 38.36 hours after the GBM trigger),  and again
the next night from 2013/10 21.09 to 2013/10 21.27 UTC (57.93 to 62.34
hours after the GBM trigger). We obtained a total of 2.8 hours exposure 
in the
r and i bands and 1.3 hours exposure in the Z, Y, J, and H bands each 
night.

During the 1st night of observations, the position of the X-ray afterglow
reported by D'Avanzo et al. (GCN 15361
Not found
 
 
, 15431
Not found
 
 
) falls at the edge of our 
field
of view in the Z and J bands. For a source within the Swift-XRT error 
circle, in
comparison with 2MASS, we obtain the following upper limits (3-sigma):

   Z   > 22.6
   J    > 21.6

During the 2nd night of observations, the XRT position was observed in all
filters. For a source within the Swift-XRT error circle, in comparison 
with 2MASS
and SDSS DR9, we obtain the following upper limits (3-sigma):

   r   > 23.8
   i   > 23.5
   Z  > 22.8
   Y  > 22.2
   J   > 21.9
   H  > 21.4

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

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