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GCN Circular 9353

Subject
GRB090510: VLT/FORS2 spectroscopic redshift
Date
2009-05-12T17:42:34Z (15 years ago)
From
Arne Rau at MPE <arau@mpe.mpg.de>
Arne  Rau (MPE  Garching), Sheila  McBreen (UCD/MPE  Garching), Thomas
Kruehler, and Jochen Greiner (both MPE Garching) report:

We present follow-up optical spectroscopy of the short Swift/Fermi GRB
090510 (Hoversten et al., GCN  9331; Ohno & Pelassa, GCN 9334; Guiriec
et al.,  GCN 9336) using  VLT/FORS2 (PI McBreen). Observations  of the
optical source  consistent with position of the  afterglow (Marshall &
Hoversten, GCN 9332;  Olofsson et al., GCN 9338;  Olivares et al., GCN
9352) started  on May  12 08:17UT.  A  total of three  30min exposures
with the 300I grism  were obtained, covering an approximate wavelength
range of 6000-10000 Angstroms.

Preliminary analysis of the spectra reveals two emission lines at 7093
and 9250 Angstroms,  which we identify as [OII] and  Hbeta at a common
redshift of z=0.903 +/- 0.003. Furthermore, there are indications of a
spectral break  in the continuum  around 7600 Angstroms,  suggesting a
possible 4000A break at the same redshift.

The detection of  emission lines and the possible  4000A break suggest
the dominance of the underlying  host galaxy over the afterglow at the
time of our observations. It also confirms that host galaxies of short
GRBs are not  all old and dead ellipticals but  instead drawn from the
underlying  field galaxy  distribution (e.g.,  Berger 2009,  ApJ, 690,
231).

In the  acquisition image the center  of the host is  0.7" offset with
respect to the NOT refined afterglow position, which corresponds to 5.5 
kpc at a
z=0.903.

At a redshift of z=0.903, the isotropic equivalent energy of the burst
is  3.8E52 erg in the 1 keV - 10 MeV rest  frame (using  the Fermi/GBM
spectral parameters of Guiriec et  al., GCN 9336).  The peak energy of
the best fit  GBM Band function model  is 8.4 +/- 0.8 MeV  in the rest
frame.

We  acknowledge excellent support  from the  ESO staff,  in particular
F. Selman and I. Condor.
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