TITLE:   GCN CIRCULAR
NUMBER:  25655
SUBJECT: ZTF19abvizsw: COATLI Optical Observations
DATE:    19/09/04 14:39:06 GMT
FROM:    Rosa Leticia Becerra Godinez at Inst. de Astronoma,UNAM  <rbecerra@astro.unam.mx>

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

We observed the field of ZTF19abvizsw/AT2019pim (Kool et al., GCN Circ.
25616;
 Perley et al., GCN Circ. 25643) 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 2019-09-04 07:32 to 12:10, obtaining
a total
 of 3.04 hours of exposure in the w filter.

We detect the source at w = 21.60 +/- 0.10.

Our w magnitudes are calibrated against the Pan-STARRS1 catalog, are on
an approximate AB system, and are not corrected for Galactic extinction
in the direction of the GRB.

Applying the color transformation of Becerra et al. (2019, ApJ, 872, 118)
to the g-r color of Perley et al., we predict r = 21.50 +/- 0.10. Thus, we
do
not see fading compared to the measurement of r = 21.62 +/- 0.09 reported by
Perley et al. but do confirm their measurement of fading compared to the
discovery magnitude of r = 19.45 +/- 0.11 reported by Kool et al.

Further observations are planned.

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