[vsnet-grb-info 17323] GRB 151212A: GROND Optical Afterglow Candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18688
SUBJECT: GRB 151212A: GROND Optical Afterglow Candidate
DATE:    15/12/13 09:07:28 GMT
FROM:    Philip Wiseman at MPE/Swift  <wiseman at mpe.mpg.de>

P. Wiseman (MPE Garching), S. Schmidl (TLS Tautenburg) and J. Greiner (MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 151212A (MAXI trigger; Nakahira et al., GCN
#18685) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started at 03:27 UT on December 13, 2015, 13.5 hrs after the
GRB trigger. They were performed at an average seeing of 1.7" and at an
average airmass of 1.1, and through a significant layer of clouds.

We detect a single source at the border of the enhanced Swift-XRT error
circle (http://www.swift.ac.uk/xrt_positions/00020570/), which we propose
is the afterglow, at RA, Dec 68.150, -3.9626, which is equivalent to:

RA (J2000.0) = 04:32:36.1
Dec (J2000.0) = -03:57:45.3

with an uncertainty of 0.5" in each coordinate.

Based on the 72 min of total exposures in g'r'i'z'J and H, at a mid-time of
14.1 hrs after the burst, we measure the following preliminary magnitudes
and upper limits (AB magnitude system):

g' = 24.3 +/- 0.2 mag,
r' = 23.6 +/- 0.2 mag,
i' = 23.1 +/- 0.2 mag,
z' = 23.3 +/- 0.3 mag,
J > 21.2 mag, and
H > 20.7 mag.

We cannot say anything about fading at this time, but the power-law-like
magnitudes lead us to suspect this is the afterglow.

Given magnitudes and upper limits are calibrated against SDSS as well as
2MASS field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V)=0.03 mag in
the direction of the burst (Schlegel et al. 1998).