TITLE:   GCN CIRCULAR
NUMBER:  21389
SUBJECT: GRB 170728B: COATLI Observations And Confirmation of Fading
DATE:    17/07/30 02:20:58 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 short GRB 170728B (Cenko et al., GCN Circ.
21371) with the COATLI 50-cm telescope and interim imager (Watson et al.
2016, Proc. SPIE, 9908, 50) at the Observatorio Astron��mico Nacional on
the Sierra de San Pedro M��rtir from 2017-07-29 04:21 to 08:19 obtaining
a total of 2.76 hours of observation in the clear filter.

We detect the source reported by D���Avanzo et al. (GCN Circ. 21372).
Calibrating with respect to the USNO-B1 catalog, we estimate a
preliminary magnitude of

R = 22.40 +/- 0.16.

This magnitude is in the USNO-B1 Vega system and is corrected for
Galactic extinction in the direction of the GRB.

The source has faded by around two magnitudes between the observations
in the first hour or so (D���Avanzo et al., GCN Circ. 21372; Heintz et al,
GCN Circ. 21374; and Guidorzi et al., GCN Circ. 21375) and our
observations. This confirms that the source is the optical transient
associated with 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.