TITLE:   GCN CIRCULAR
NUMBER:  7998
SUBJECT: GRB 080721: OA fading and redshift
DATE:    08/07/22 08:45:39 GMT
FROM:    Pall Jakobsson at U Hertfordshire  <P.Jakobsson@herts.ac.uk>

P. Jakobsson (U. Hertfordshire), D. Malesani, P. M. Vreeswijk,
J. P. U. Fynbo, B. Milvang-Jensen (DARK/NBI), A. de Ugarte Postigo (ESO),
B. Nordstrom (NBI), E. Stonkute (ITPA, Vilnius) and P. Sorensen (NOT)
report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 080721 (Marshall et al., GCN 7988;
Chen et al., GCN 7990) with ALFOSC on the Nordic Optical Telescope at
La Palma. The afterglow is well detected in a single 300 s exposure
starting on 21.875 July 2008 (10.6 hr after the GRB). Comparison with
several USNO-B1 stars (R1 magnitudes) yields R = 20.0 +/- 0.2, where most
of the error comes from the calibration. Using the R-band magnitude
provided by Chen et al., the inferred decay index is alpha ~ 0.9.

Low resolution spectra (3 x 30 min) were also obtained. The combined
spectrum displays a strong absorption feature around 4350 A, with the flux
dropping substantially blueward of this feature. Associating it with
Ly-alpha gives a redshift of z ~ 2.6 for the burst. We find several other
absorption features, including O I, Si II, C II, Si IV, C IV, Fe II
and Al II, corresponding to a redshift of z = 2.591 +/- 0.001, consistent
with the redshift reported by D'Avanzo et al. (GCN 7997) and the UVOT
photometric redshift (Ward et al., GCN 7996).