[vsnet-grb-info 20748] GRB 180624A: RATIR Optical Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22846
SUBJECT: GRB 180624A: RATIR Optical Observations 
DATE:    18/06/25 22:16:14 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan at astro.unam.mx>

Alan M. Watson (UNAM), Eleonora Troja (GSFC), 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), 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 180624A (Gibson et al., GCN Circ. 22832)
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 2018/06 25.26 to
2018/06 25.47 UTC (16.42 to 21.45 hours after the BAT trigger),
obtaining a total of 3.91 hours exposure in the r and i bands.

We detect a source in the enhanced Swift-XRT error circle (Goad et al.,
GCN Circ. 22835) with the following magnitudes:

  r	= 21.03 +/- 0.02
  i	= 20.45 +/- 0.01

These magnitudes are calibrated against the SDSS DR9 catalog, are in the
AB system, and are not corrected for Galactic extinction in the
direction of the GRB.

We note that the source has faded significantly from the earlier
observations of Guidorzi et al. (GCN Circ. 22839) of r = 19.9 +/- 0.1 at
0.54 hours and Schady (GCN Circ. 22840) of r = 20.2 +/- 0.1 at 15.07
hours. We also see the source fade by about 0.5 mag in each our bands
during the course of our observations.

We confirm the observation of Rossi et al. (GCN Circ. 22845) that the
source is a couple of arcsec to the SE of the SDSS galaxy
J211223.25-022015.9.

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