Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI),
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara
(UVI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and
Vicki Toy (UMD) report:
We observed the field of GRB 180305A (Axelsson, et al., GCN Circ. 22457)
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio
Astron��mico Nacional on Sierra San Pedro M��rtir from 2018/03 7.11 to
2018/03 7.20 UTC (41.30 to 43.46 hours after the BAT trigger), obtaining
a total of 1.64 hours exposure in the r and i bands.
The source reported by Troja et al. (GCN Circ. 22460) is still detected,
but has faded to:
r = 23.15 +/- 0.21
i = 22.80 +/- 0.15
These magnitudes are in the AB system, are calibrated against the
USNO-B1 catalog, and are not corrected for Galactic extinction in the
direction of the GRB.
The combination of the optical fading reported here and the X-ray fading
reported by D���Avanzo et al. (GCN Circ. 22462) confirm that this is the
afterglow of the GRB.
We thank the staff of the Observatorio Astron��mico Nacional in San Pedro