TITLE:   GCN CIRCULAR
NUMBER:  22664
SUBJECT: GRB 180418A: Continued RATIR Optical Observations
DATE:    18/04/19 13:35:19 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

Eleonora Troja (GSFC), Nat Butler (ASU), Alan M. Watson (UNAM),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM),
Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB),
Antonino Cucchiara (UVI), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and
Vicki Toy (UMD) report:

We re-observed the field of GRB 180418A (D'Elia, et al., GCN 22646) with
the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on
the 1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional
on Sierra San Pedro Mártir from 2018/04 19.13 to 2018/04 19.41 UTC
(20.47 to 27.09 hours after the BAT trigger), obtaining a total of
4.17 hours exposure in the r and i bands.

The optical afterglow (Zheng & Filippenko, GCN 22647) is detected in all
bands and significantly faded with respect to our first night of
observations (Troja et al., GCN 22652). In comparison with the
SDSS DR9 catalog, we obtain the following detections:

   r = 23.63 +/- 0.22
   i = 23.14 +/- 0.16

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.