GCN Circular 6398

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
Not found
 
 
). The 
UVOT optical afterglow candidate (Marshall & Grupe, GCN 6392
Not found
 
 
) 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
Not found
 
 
).

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.