GCN Circular 8301
Subject
GRB 080928: VLT redshift
Date
2008-09-29T16:26:29Z (16 years ago)
From
Paul Vreeswijk at Dark Cosmology Centre,U.of Copenhagen <pmv@dark-cosmology.dk>
Paul Vreeswijk, Daniele Malesani, Johan Fynbo (DARK/NBI), Pall
Jakobsson (U. of Iceland), Christina Thoene, Jesper Sollerman, Darach
Watson, Bo Milvang-Jensen (DARK/NBI) report on behalf of a larger
collaboration:
The afterglow of GRB 080928 detected by Swift (Sakamoto et al., GCN
8292, see also Rykoff et al., GCN 8293) was observed with FORS2
mounted on the Antu telescope of ESO's VLT on Cerro Paranal, Chile.
Following an acquisition image, which was started at 6:31 UT on
September 29, 2008 (15.5 hr after the GRB trigger), a single 1800 s
spectrum was secured. It was taken with a 1" slit and the 300V grism,
resulting in an approximate resolution of 11 A, and a useful
wavelength coverage of 3800-9300 A. From the 60 s acquisition image,
we measure an afterglow magnitude of R = 20.4 based on USNO-B1
magnitudes.
The extracted spectrum shows a multitude of significant absorption
lines, which we identify with common resonance lines of mainly MgII
and FeII, originating in at least two absorption systems at z=0.735
and z=1.692. The latter redshift presumably corresponds to that of the
GRB host galaxy. We do not find evidence for a spectral break due to
the Lyman alpha forest down to 3800 A, setting a firm upper limit on
the GRB redshift of z < 2.1. These results are consistent with the
redshift limits set by GROND (Rossi et al., GCN 8296) and UVOT (Kuin
et al., GCN 8298), but not with the value of z=2.49 reported by
Cucchiara et al. (GCN 8300).
The equivalent width of the MgII 2796 line of the z=0.735 system
(EW_rest ~ 4.3 A) is much larger than that of the z=1.692 system. We
note the presence of an R ~ 22 object, not covered by the slit, at
2.6" NNE of the afterglow position, which could possibly be a bright
host galaxy, or the counterpart of the z=0.735 absorber. The slit
serendipitously covers a galaxy 30" away from the afterglow (at
R.A. 06:20:13.35, decl. -55:11:55.0), which shows an apparent emission
line at 6470 A; this can be identified as [OII] 3727 at a redshift of
z=0.736. At this redshift, 2.6 and 30" correspond to projected
distances of 19 and 219 kpc, respectively.
We are grateful for the excellent support from the Paranal observing
staff, in particular Stephane Brillant, Michelle Doherty and Patricia
Guajardo.