TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3332 SUBJECT: GRB 050502A: Redshift measurement based on Keck HIRES Observations DATE: 05/05/02 12:37:32 GMT FROM: Hsiao-Wen Chen at MIT/CSR Jason X. Prochaska (UCO/Lick Obs.), Sara Ellison (U. Victoria), R. J. Foley (UCB), J. S. Bloom (UCB), & H.-W. Chen (MIT) report on behalf of the GRAASP collaboration: "We observed the afterglow of GRB 050502a for 3600s with HIRES on the KeckI telescope starting at UT 05:53:46 on May 02, 2005, 3.5 hours after the initial burst. We obtained a low SNR, high resolution spectrum (R~40000). We report a tentative redshift of z=3.793 based on the following lines of evidence: 1. We detect a prominent flux discontinuity at 4400 Ang, blueward of which no flux is present. We interpret the flux decrement at 4400 A as the Lyman limit absorption produced by the host of the GRB. 2. The flux blueward of ~5870 Ang is 'patchy', i.e. indicative of the Lya forest. A broad absorption feature is present at 5820 Ang, which we interpret as the Lya absorption due to the ISM of the host galaxy. 3. We identify a strong absorption feature at 6040 Ang, which is consistent with SiII1260 at z=3.793. Further analysis is underway. This message can be cited."