[AAVSO-HEN] GRB 070721B: optical observations and proposed redshift

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TITLE:   GCN CIRCULAR
NUMBER:  6651
SUBJECT: GRB 070721B: optical observations and proposed redshift
DATE:    07/07/22 19:37:46 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani at 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.