TITLE:   GCN CIRCULAR
NUMBER:  21837
SUBJECT: GRB 170906C: COATLI Observations
DATE:    17/09/06 19:47:21 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Carlos
Rom��n-Z����iga (UNAM), and William H. Lee (UNAM) report:

We observed the field of the GRB 170906C (Siegel et al., GCN Circ.
21826) with the COATLI 50-cm telescope and interim CCD imager (Watson et
al. 2016, Proc. SPIE, 9908, 50) at the Observatorio Astron��mico Nacional
on the Sierra de San Pedro M��rtir from 2017-09-06 08:17 to 12:11
obtaining a total of 2.93 hours of observation in the clear w filter.

We do not detect a source within the enhanced XRT error region reported
by Evans et al. (GCN Circ. 21831). Calibrating with respect to the
USNO-B1 catalog, we estimate a 3-sigma upper limit of:

w > 21.7

This magnitude is in the AB system and is not corrected for Galactic
extinction in the direction of the GRB.

We thank the COATLI technical team (Fernando ��ngeles, Oscar Chapa,
Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosal��a Langarica,
Fernando Quir��s, and Carlos Tejada) and the staff of the Observatorio
Astron��mico Nacional.