TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3801
SUBJECT: GRB 050813: Gemini spectroscopy and redshifts of galaxies B and C
DATE:    05/08/14 20:07:21 GMT
FROM:    Edo Berger at Carnegie Obs  <eberger@ociw.edu>

E. Berger (Carnegie Observatories) reports:

"We reduced and analyzed spectra of galaxies B and C in the XRT error
circle of GRB 050813 (GCN 3788) obtained on Aug 14.35 UT with GMOS on the
Gemini north telescope under program GN-2005B-Q-6.  Two 1800-sec exposures
were obtained with a 0.75" slit using the R400 grating.  We find that both
galaxies exhibit clear Ca H&K, H-beta, and H-gamma absorption at a
redshift of z=0.722.  The identical redshifts and the nature of the
galaxies (early types dominated by old stellar populations) support the
conclusion of Gladders et al. (GCN 3798) from Magellan imaging that the
position of GRB 050813 is centered on a galaxy cluster at z~0.5-1.  At
this redshift, given K~19 mag, galaxies B and C are about 0.5 L* compared
to the 2MASS luminosity function.

Assuming that this is the redshift of GRB 050813, we find an isotropic
equivalent gamma-ray energy of 1.7e50 erg (fluence = 1.24e-7 erg/cm^2;  
GCN 3793), which is somewhat larger than that of GRB 050724."