TITLE:   GCN CIRCULAR
NUMBER:  16024
SUBJECT: GRB 140320B: P60 detection of a very red afterglow
DATE:    14/03/21 16:39:00 GMT
FROM:    Daniel Perley at Caltech  <dperley@astro.caltech.edu>

D. A. Perley (Caltech) and S. B. Cenko (NASA-GSFC) report:

The Palomar 60-inch telescope automatically responded to the INTEGRAL 
trigger for GRB 130420B (Mereghetti et al. GCN 16004) and began taking a 
series of 60-second exposures in the r, i, and z filters starting at 
2014-03-20 09:31:41.744 (7.12 minutes after the trigger).

The afterglow candidate reported by Guidorzi et al. (GCN 16003) is 
well-detected in the initial z-band exposure, weakly detected in the 
initial i-band exposure, and not detected in the initial r-band 
exposure.  However, a weak r-band detection is recovered by stacking 
several images.

The light curve (in the i- and z-band filters) shows a rise to a peak at 
around 15 minutes post-trigger, followed by a decline up until the end 
of our sequence at 1.3 hours after the GRB.

Stacking the first 7 exposures in each filter, we estimate magnitudes of:

r = 21.29 +/- 0.24
i = 20.07 +/- 0.09
z = 17.97 +/- 0.03

At a mid-UT time of approximately 09:46 in each filter (20 minutes after 
the trigger).

The redder apparent color in i-z compared to r-i may suggest a highly 
reddened afterglow with host contribution to the r-band, or a reddened 
afterglow with a very strong 2175 dust feature at z~2.5, or a Lyman 
break in the i-band with some Lyman-alpha forest blueward (z~5).  We 
encourage further observations, particularly in the NIR.