TITLE:   GCN CIRCULAR
NUMBER:  12193
SUBJECT: GRB 110721A: Gemini-South spectroscopy of the potential optical counterpart
DATE:    11/07/24 15:11:38 GMT
FROM:    Edo Berger at Harvard  <eberger@cfa.harvard.edu>

E. Berger (Harvard) reports on behalf of a larger collaboration:

"We obtained spectroscopic observations of the candidate optical counterpart
(GCN 12192) of the LAT burst GRB110721A (GCN 12188) with GMOS on the
Gemini-South 8-m telescope starting on 2011 July 24.20 UT.  A total of 3600
sec was obtained on source with a wavelength coverage of about 4000-8100
ang.  We find two clear absorption features at 5487 and 5436 ang, with a
significant decline in flux at shorter wavelengths, but to a non-zero
level.  The two features match CaII H&K at a redshift of z=0.382, or could
alternatively represent Ly-alpha absorption with the highest redshift being
z=3.512.  No other features are detected at wavelengths corresponding to
other typical lines at either redshift.  Given the detectable flux blueward
of 5430 ang (down to at least 4600 ang), and the fact that the source
appears to be extended in our i-band acquisition image (with seeing of about
0.7"), we consider the possibility that the object is an early-type galaxy
at z=0.382 to be more likely.

We thank Jochen Greiner for sharing a finding chart of GRB110721A from GROND
data."