TITLE:   GCN CIRCULAR
NUMBER:  31898
SUBJECT: GRB 220412A: RATIR Observations of the Fading Afterglow 
DATE:    22/04/13 20:05:34 GMT
FROM:    Alan M Watson at UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Nat Butler (ASU), Eleonora Troja (UTV), Alexander
Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox
(STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Enrico Ramirez-Ruiz
(UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley
(GSFC), Rosa L. Becerra (UNAM), Simone Dichiara (PSU), and Océlotl López (UNAM)
report:

We observed the field of GRB 220412A (Klingler et al. GCN 31881) with the
Reionization and Transients Infrared Camera (RATIR) on the 1.5m Harold Johnson
Telescope at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir
from 2022-04-12 06:39 to 06:56 UTC (0.04 to 0.33 hours after the trigger) and
2022-04-13 03:16 to 04:38 UTC (20.65 to 22.00 hours after the trigger) obtaining
totals of 0.21 and 1.07 hours of exposure in the i band.

In comparison with the SDSS DR9 catalog, the afterglow candidate reported by
Watson et al. (GCN Circ. 31882) has faded from i = 19.27 +/- 0.10 to i = 20.64
+/- 0.08. This photometry for the first night supersedes the preliminary
photometry reported by Watson et al. 

We note that the optical light curve does not seem to show the same steep
decline as the XRT light curve reported by the UKSSDC. From 0.33 to 21 hours,
the XRT light curve falls by a factor of about 50, whereas our photometry only
falls by a factor of about 3.5.

We have checked the SDSS DR9 i image, and there is no apparent host galaxy
present at the position of the afterglow bright enough to explain the
apparent slow decline.

Further observations are planned.

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