TITLE:   GCN CIRCULAR
NUMBER:  26103
SUBJECT: GRB 191031A: COATLI Optical Observations 
DATE:    19/10/31 03:43:46 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 2019-10-31 02:14 UTC (1.63 hours after the
trigger) to 02:25, obtaining a total of 540 seconds of exposure in the w filter.
The observations were taken at airmasses between 6.4 and 8.3.

We do not detetect any uncataloged sources in the BAT error region to a 10-sigma
limit of w = 18.75.

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.