TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5238
SUBJECT: GRB 060502B: A Bright Early-type Galaxy as Putative Host of the Short Burst 
DATE:    06/06/07 10:36:49 GMT
FROM:    Josh Bloom at UC Berkeley  <jbloom@astron.berkeley.edu>

GRB 060502B: A Bright Early-type Galaxy as Putative Host of the Short  
Burst

J. S. Bloom, D. Perley, D. Kocevski, N. Butler (UC Berkeley), J. X.  
Prochaska (UCO/Lick), and H.-W. Chen report:

On 31 May 2006 UT, using Keck I (+LRIS), we obtained spectra of a  
bright red extended source ("G*"; RA, DEC J2000 = 18:35:45.7,  
+52:37:37) that is 11.2 arcsec South of the southern edge of the  
revised XRT error circle (Troja et al. GCN 5093). Strong absorption  
features, which we identify as Ca II H+K, indicate a redshift for  
this early type galaxy of z=0.287. Weak [OII] emission can be seen  
indicating some low level of active star formation.

We advance the hypothesis that this galaxy is the host of short burst  
GRB 060502B (Troja et al. GCN 5055) based on several points:

      1. The galaxy is a massive early-type similar to the putative  
hosts of 050509B [1] and 050724 [2].

      2. The redshift inferred of the hosts of 050509b (z=0.225) and  
050724 (z=0.258) are remarkably similar to that of G* indicating  
comparable energetics of the respective bursts.

      3. At the redshift of z=0.287, the burst location would be  
between 47 - 67 kpc in projection from the center of the putative  
host, similar to the offset (39 +/- 13 kpc) inferred for 050509b.  
There are viable progenitors scenarios (e.g. degenerate binary  
mergers) where bursts occur at such distances from their birthsite.

      4. Weak X-ray afterglow and no detected optical afterglow would  
seem to indicate a low density circumburst environment, as would be  
expected if the GRB originated far from the progenitor birthsite.

We recognize the difficulty of now confirming this hypothesis but  
note that in the striking similarity of the host, redshift,  
afterglow, and offset configuration to GRB 050509b, these  
observations and inferences have a priori precedent."

A false color image will be posted at:
   http://astro.berkeley.edu/~jbloom/grb060502B_color_keck1.gif

This message may be cited.

[1] Bloom et al. 2006ApJ...638..354B (astro-ph/0505480); Gehrels et  
al. 2005Natur.437..851G (astro-ph/0505630)
[2] Berger et al. Nature 438 (2005) 988-990 (astro-ph/0508115)