[vsnet-grb-info 6829] GRB 081028: GROND Detection of the Optical/NIR Afterglow Candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8424
SUBJECT: GRB 081028: GROND Detection of the Optical/NIR Afterglow Candidate
DATE:    08/10/28 07:44:21 GMT
FROM:    Christian Clemens at MPE  <cclemens at mpe.mpg.de>

C. Clemens, S. Loew and J. Greiner (all MPE Garching) report on behalf of the 
GROND team:

We observed the field of GRB 081028 (Swift trigger 332851; Guidorzi et al., 
GCN #8421) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, 
PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla Observatory 
(Chile).

Observations started at 06:19 UT on October 28th, 2008, 5.8 hr after the GRB 
trigger, and are continuing. They were performed at an average seeing of 1.4" 
and at an average airmass of 2.4.

We found a single point source within the 4.8'' Swift-XRT error circle 
reported by Guidorzi et al. (GCN #8421) at

RA (J2000.0) = 08h 07m 34.73s
DEC (J2000.0) = +02d 18' 29.1''

with an uncertainty of 0.5".

Based on the first 4.4 min of total exposures in g'r'i'z' and 4.0 min in JHK, 
we estimate preliminary magnitudes (all in AB system) of

g' ~ 19.9 mag,
r' ~ 19.3 mag,
i' ~ 19.2 mag,
z' ~ 19.1 mag,
J ~ 19.0 mag,
H ~ 18.7 mag and
K ~ 19.0 mag

with typical errors of +/- 0.1 in g'r'i'z' and +/- 0.15 in JHK. Given 
magnitudes are calibrated against SDSS as well as 2MASS field stars.

The object is seen in g' band, implying a redshift smaller than 3.5 and no 
statement about variability can be made at this point.

Please note, that no correction for the Galactic foreground extinction 
corresponding to a reddening of E_(B-V)= 0.03 mag in the direction of the 
burst (Schlegel et al. 1998) has been applied.