TITLE:   GCN CIRCULAR
NUMBER:  22528
SUBJECT: GRB 180324A: RATIR Optical Observations and Possible Afterglow Detection
DATE:    18/03/24 07:16:36 GMT
FROM:    Nat Butler at Az State U  <natbutler@asu.edu>

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), Eleonora
Troja (GSFC), 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 observed the field of GRB 180324A (Sonbas, et al., GCN 22524) 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 2018/03 24.19 to 2018/03 24.23 UTC (2.4
minutes to 0.92 hours after the BAT trigger), obtaining a total of 0.66
hours exposure in the r and i bands.

We detect a weak source within the Swift-XRT error circle (Evans, et al.,
GCN 22525) in our first 80 s exposure, i-band image only.  In comparison
with the USNO-B1 catalog, we find i=20.15+/-0.37.

The source is not on the frame in the subsequent image and is not detected
in the next possible frame 3.5 minutes later.  In our full 0.66 hour
exposure time, the source is not detected with the following upper limits
(3-sigma):

  r > 22.08
  i > 22.04

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.  If the detection in the first
frame is due to the afterglow, then the afterglow faded rapidly.

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