[vsnet-grb-info 4642] GRB 070508: OA fading and tentative redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6398
SUBJECT: GRB 070508: OA fading and tentative redshift
DATE:    07/05/09 15:11:02 GMT
FROM:    Pall Jakobsson at U Hertfordshire  <P.Jakobsson at herts.ac.uk>

Pall Jakobsson (U. Hertfordshire), Johan P. U. Fynbo (DARK, NBI), 
Michael I. Andersen (Potsdam), Andreas O. Jaunsen (U. Oslo),
Christina C. Thoene, Darach Watson, Jens Hjorth, Bo Milvang-Jensen
(DARK, NBI), Nial R. Tanvir (U. Leicester) and P. Moller (ESO) 
report on behalf of a larger collaboration:

Using FORS1 on the Very Large Telescope, we have obtained 3*30 min 
spectra (grism 300V) of GRB 070508 (Grupe et al., GCN 6383). The 
UVOT optical afterglow candidate (Marshall & Grupe, GCN 6392) has 
clearly faded in our acquisition image. Using photometric zeropoints 
from the ESO webpages we estimate the afterglow magnitude to be
R ~ 22.0 on May 8.337 (3.79 hours post-burst).

The combined spectrum does not display any obvious absorption 
features. However, a firm upper limit of z < 2.3 can be placed on 
the redshift of GRB 070508 from the lack of Ly-alpha forest lines in 
the spectrum of the afterglow. This is consistent with the UVOT 
observations (Marshall & Grupe, GCN 6392).

We do detect a tentative emission line at 6781 A. If it corresponds 
to H-alpha at z = 0.033, we would expect to detect H-beta and the 
[O III] doublet in our spectrum. However, no features are visible 
at their predicted locations. The most probable association is with 
[O II] 3728, corresponding to a redshift of z = 0.82. We caution
that the magnitude reported above is then most likely affected by
host galaxy emission.

We thank the Paranal staff for excellent support.