TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1428 SUBJECT: GRB 020531/Candidate Redshift DATE: 02/06/12 11:38:15 GMT FROM: Shri Kulkarni at Caltech S. R. Kulkarni, R. Goodrich, E. Berger, D. W. Fox, J. S. Bloom and C. A. Blake report, on behalf of the Caltech-NRAO-CARA collaboration: N. Butler et al. (GCN 1426) suggested that a fading X-ray source, CXOU J151455.8-192454 is the afterglow of the short hard burst GRB 020531 (Hurley et al. GCN 1407). Fox, Kulkarni and Weissman (GCN 1427) identified an object within 1 arcsecond of CXOU J151455.8-192454 and suggested that the object is the host galaxy of GRB 020531. On June 12, 2002 (UT) we undertook imaging and spectroscopic observations with the Echelle Spectrograph & Imager (ESI) on Keck II. With a seeing of 0.6 arcseconds in the R and I bands we confirm that the candidate object is indeed extended, with a size of about 1 arcsecond. Next, we obtained four 1800-s spectroscopic exposures (Echelle mode) and found two features: a broad feature (Gaussian full width at half maximum of 11.6 A) centered on 7455 Angstrom (A) and a fainter feature centered around 9725 A. We suggest that the broad feature is the [O II] 3728.8/3726.0 doublet with an intrinsic velocity dispersion of 330 km/s and the fainter feature is Hbeta. If these identifications are correct then the redshift of the candidate host galaxy is 1.00. The fluence and peak flux of GRB 020531 over the energy range 50 to 300 keV are 8E-7 erg cm^-2 and 6.4E-7 erg cm^-2 s^-1, respectively (Lamb et al. 2002; astro-ph/0206151). Assuming, H0=65 km/s/Mpc and flat universe with Omega-m=0.3, the isotropic energy release (without any k correction) is 2.4E51 erg and 3.9E51 erg/s. Lamb et al. (ibid) argue that GRB 020531 is a burst which belongs to the short duration group. The isotropic energy release of this short burst is not different from that of the true energy release (i.e. after accounting for the opening angles of the jets; see Frail et al. 2001, ApJ 562, L55) inferred for the long duration bursts.