GCN Circular 6651
Subject
GRB 070721B: optical observations and proposed redshift
Date
2007-07-22T19:37:46Z (17 years ago)
From
Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center <malesani@astro.ku.dk>
D. Malesani (DARK), P. Jakobsson (Univ. Hertfordshire), J.P.U. Fynbo, J.
Hjorth and P.M. Vreeswijk (DARK) report:
We observed the optical counterpart of GRB 070721B (Ziaeepour et al.,
GCN 6640; Schady, GCN 6641) with the ESO-VLT equipped with FORS2.
Close to the UVOT position (Schady, GCN 6641), we detect two objects:
1: RA = 02:12:32.97, Dec = -02:11:41.0
2: RA = 02:12:33.03, Dec = -02:11:41.6
We note that object #1 is consistent with the UVOT position, while #2 is
1.6" away. We estimate R~23.8 and R~24.3 on Jul 22.319 UT (21.6 hr after
the GRB) for objects #1 and #2, respectively.
We acquired spectra for both objects. Thanks to the detection of a DLA
and several metallic lines, we infer for object #2 a redshift z=3.626.
The spectrum is typical of a GRB afterglow, and we thus suggest that
object #2 is the optical counterpart of GRB 070721B. This redshift is
also consistent with the nondetection of the object in filters bluer
than V (De Pasquale & Ziaeepour, GCN 6650). Further observations are
planned to test the variability of both objects.
A finding chart and a plot of the 2D spectrum is available at:
http://www.astro.ku.dk/~malesani/GRB/070721B
We acknowledge excellent support from the ESO staff, in particular
Angela Cortes, Dominique Naef and Yuri Beletsky. We thank Massimiliano
De Pasquale for useful discussion.