[vsnet-grb-info 16426] GRB 150413A: additional P60 observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17707
SUBJECT: GRB 150413A: additional P60 observations
DATE:    15/04/15 11:29:50 GMT
FROM:    Daniel Perley at Caltech  <dperley at astro.caltech.edu>

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

We have continued to follow the optical counterpart of GRB 150413A 
(Markwardt et al., GCN 17688; Ivanov et al.,  GCN 17689) with the 
Palomar 60-inch robotic telescope.  A second griz epoch was acquired 
between 10:08:52 and 10:49:31 UT on the night of 2015-04-14, followed by 
two deeper epochs (one gri, one r only) on the night of UT 2015-04-15.

Calibrating our r-band observations to a common set of USNO R-band 
reference stars, we derive the following photometry (statistical 
uncertainties only):

dt_mid = 0.5602 day :  R = 19.13 +/- 0.03
dt_mid = 0.8464 day :  R = 20.23 +/- 0.06
dt_mid = 1.6034 day :  R = 21.52 +/- 0.16
dt_mid = 1.8652 day :  R = 21.74 +/- 0.18

This suggests steep decay over this period (decay index ~ 2.2).