TITLE:   GCN CIRCULAR
NUMBER:  25958
SUBJECT: GRB 191004B: COATLI Optical Observations
DATE:    19/10/06 03:15:25 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 191004B (Cenko et al., GCN Circ. 25947) 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
2019-10-05 06:54 to 12:11, obtaining a total of 1.65 hours of exposure in the w
filter.

At the position of the bright afterglow detected by Swift/UVOT (Cenko et al.,
GCN Circ. 25947), we detect a source with w = 21.47 +/- 0.17.

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.