TITLE:   GCN CIRCULAR
NUMBER:  11143
SUBJECT: GRB 100823A: VLT/X-Shooter spectroscopy
DATE:    10/08/24 13:33:09 GMT
FROM:    Nial Tanvir at U.Leicester  <nrt3@star.le.ac.uk>

P. D'Avanzo (INAF/Brera), P. Jakobsson (Reykjavik), H. Flores (Paris
Obs.), P. Goldoni (APC/Univ. Paris 7 and SAp/CEA), N. R. Tanvir, K.
Wiersema (U. Leicester), A. J. Levan (U. Warwick), D. Malesani, J. P. U.
Fynbo, and B. Milvang-Jensen (DARK), report on behalf of a larger
collaboration:

We observed the location of GRB 100823A (Mangano et al. GCN 11135) with
the ESO VLT equipped with the X-shooter spectrograph on 2010 Aug 24,
beginning 08:17 UT (15.5 hr after the GRB). The slit was aligned so to
include both objects A and B first mentioned by Levan et al. (GCN 11138;
see also Guziy et al., GCN 11140; D'Avanzo et al., GCN 11141). We detect
both objects in the acquisition image, and confirm that object B is
fainter than the DSS limit.

In the spectra, we detect continuum from source A, and identify its
spectrum as being that of an M-dwarf based on evident TiO bands. Given
that object B is also likely unrelated to the GRB (D'Avanzo et al., GCN
11141; Levan et al., GCN 11142), we conclude that either the GRB event
is itself associated with a flare from this star, or that the GRB
afterglow is a fainter source not reported so far.

We acknowledge the support of the VLT staff, in particular Claudia Cid,
Stephane Brillant, Jonathan Smoker, and Christophe Martayan.