[vsnet-grb-info 9655] GRB 100816A: VLT and Gemini-N revised redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11123
SUBJECT: GRB 100816A: VLT and Gemini-N revised redshift
DATE:    10/08/17 12:56:20 GMT
FROM:    Nial Tanvir at U.Leicester  <nrt3 at star.le.ac.uk>

N. R. Tanvir (U. Leicester), S. Vergani (GEPI/Obs. Paris and APC/Univ. Paris 7),
J. Hjorth, J. P. U. Fynbo, B. Milvang-Jensen, D. Malesani (DARK),
K. Wiersema (U. Leicester),  P. Vreeswijk (Reykjavik), A. J. Levan (U. Warwick)
P. Goldoni (APC/Univ. Paris 7 and SAp/CEA) & S. Covino (INAF/Brera)
report on behalf of a larger collaboration:

We observed the location of possible short-hard GRB 100816A
(Oates et al. GCN 11102; Markwardt et al. GCN 11111; Norris et al. GCN 11116)
with VLT/X-Shooter on 2010 Aug 17 beginning 04:20 (UT).  We clearly
detect the putative host galaxy of the burst, and identify lines of Halpha,
Hbeta,  OII (3727), NII (6548/6583) and OIII (5007) in emission and CaII H&K
in absorption.  In fact, the system splits (spectrally and spatially) into two
components at redshifts z=0.8034 and z=0.8049 (based on a provisional
wavelength calibration), possibly indicating an interacting system or reflecting
internal velocity components within a single galaxy.  This redshift is inconsistent
with the tentative redshift for this galaxy suggested by Tanvir et al. (GCN 11116).

We have also re-analysed the Gemini-N GMOS afterglow spectroscopy initially
reported in GCN 11116.  The trace shows faint absorption features
consistent with MgII (2797/2803A) doublet and FeII (2600A) at a redshift of
z=0.8035. The agreement of this redshift with that obtained for the galaxy from
the X-shooter data, strongly suggests that it is the host.  We note that none of
the emission lines seen in the X-Shooter spectrum are within the spectral range
of the GMOS data.

We acknowledge the support of the VLT staff, in particular Giovanni Carraro.