GCN Circular 3520
Edo Berger (Carnegie Observatories) and George Becker (Caltech) report: "We obtained a 45-min spectrum of the afterglow of GRB 050603 (GCNs 3509, 3511) with IMACS on the Magellan/Baade telescope on 2005, June 5.40 UT (2.13 days after the burst). We find a single bright emission line at an observed wavelength of 4645.1A, which we interpret as Lyman-alpha at a redshift of z=2.821. At this redshift the isotropic-equivalent gamma-ray energy is 2.3e54 erg (using a fluence of 3.4e-5 erg/cm^2; GCN 3518). The steep fading observed by the UVOT (GCN 3517) and XRT (GCN 3519) suggests a jet break at t~12 hours after the burst, corresponding to a jet opening angle of about 1.7 deg, and hence a beaming-corrected energy of 1.1e51 erg."