TITLE:   GCN CIRCULAR
NUMBER:  15650
SUBJECT: GRB 131231A: P60 observations
DATE:    14/01/02 05:09:18 GMT
FROM:    Daniel Perley at Caltech  <dperley@astro.caltech.edu>

D. A. Perley (Caltech) reports on behalf of a larger collaboration:

We imaged the location of GRB 131231A with the Palomar 60-inch (P60) 
robotic telescope on UT 2014-01-01 between 03:28:31 and 04:22:05, and 
again on 2014-01-02 between 01:56:28 and 02:37:12. Observations were 
taken in the g, r, i, and z filters during each epoch.

The afterglow (Xu et al., GCN 15641) is well-detected in individual 
exposures in all bands.  Both nights were photometric and we calibrated 
the field using an observation of the nearby standard field SA92.

Some photometry of the GRB afterglow in the gri bands (all on the 
SDSS/AB system) is:

r = 18.84 +/- 0.02  (t = 0.965 d)
i = 18.72 +/- 0.02  (t = 0.970 d)
g = 19.14 +/- 0.02  (t = 0.982 d)

r = 19.77 +/- 0.03  (t = 1.886 d)
i = 19.64 +/- 0.04  (t = 1.893 d)
g = 20.06 +/- 0.03  (t = 1.907 d)

The power-law temporal decay index in all bands is approximately alpha=1.3.


We also give gri magnitudes of the five nearby calibration stars 
presented by Halpern (GCN 15646) for reference:

star  RA          dec       R         g      r      i
  A    00:42:30.4 -01:36:13  16.07     16.86  16.28  16.11
  B    00:42:11.7 -01:37:29  17.95     19.50  18.24  17.66
  C    00:42:15.9 -01:39:08  17.92     19.16  18.20  17.85
  D    00:42:20.0 -01:41:53  17.22     17.62  17.48  17.58
  E    00:42:28.8 -01:40:16  17.96     18.76  18.18  18.02