[vsnet-grb-info 9858] GRB101008A, GROND detection of a fading afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11326
SUBJECT: GRB101008A, GROND detection of a fading afterglow
DATE:    10/10/09 10:58:06 GMT
FROM:    Marco Nardini at MPE  <nardini at mpe.mpg.de>

M. Nardini, P. Schady, J. Greiner (MPE Garching) report on behalf of the
GROND team:

We observed the field of GRB 101008A (Swift trigger 435903; Baumgartner et
al., GCN#11318) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla
Observatory (Chile).

Observations started at 00:26 UT 2010 October 9th, 7.8h time after the GRB
trigger, and lasted 2 hours. They were performed at an average seeing of
1.2  and at an average airmass of 2.5.

In all our g'r'i'z'JHK images we detect the 3 objects reported by
Moskvitin et al., GCN#11324.

While sources S1 and S3 do not show any clear fading with respect to that
reported by  Moskvitin et al., GCN#11324, our  preliminary  photometry
shows  that S2 has faded with respect to the value reported by Moskvitin
et al. (GCN#11324). Calibrating our images against the same USNO star as
Moskvitin et al., we calculate a fading of ~0.7mag between the GROND
preliminary photometry centred at 8.7h after the trigger and that reported
in Moskvitin et al.

We therefore conclude that this is the optical/NIR afterglow of GRB101008A.

Our photometry is not corrected for foreground extinction of E(B-V)=0.19.
The detection in g' band implies a redshift constraint z<3.5.