TITLE:   GCN CIRCULAR
NUMBER:  26122
SUBJECT: GRB 191031D: COATLI Optical Observations 
DATE:    19/11/01 05:13:02 GMT
FROM:    Alan M Watson at UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM),  Nat Butler (ASU), Rosa L. Becerra (UNAM), Diego González
(UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Margarita Pereyra
(UNAM), and Eleonora Troja (GSFC) report:

We observed the field of GRB 191031A (D'Ai et al., GCN Circ. 26101; Fermi GBM
Team, GCN Circ. 26102) with the COATLI 50-cm telescope and interim imager at the
Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir
(http://coatli.astroscu.unam.mx) from 2019-11-01 02:47 UTC to 04:49 UTC (5.4 to
7.4 hours after the trigger), obtaining a total of 4530 seconds of exposure in
the w filter.

We do not detetect any uncataloged sources in the enhanced XRT error region
(Osborne et al., GCN Circ. 26117) to a 3-sigma limit of w = 23.2.

Our w magnitudes are calibrated against the Pan-STARRS1 catalog, are on an
approximate AB system (Becerra et al., 2019, ApJ, 872, 118), and are not
corrected for Galactic extinction in the direction of the GRB.

We thank the COATLI technical team and the staff of the Observatorio Astronómico
Nacional.