GCN Circular 9353
Subject
GRB090510: VLT/FORS2 spectroscopic redshift
Event
Date
2009-05-12T17:42:34Z (17 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.