GCN Circular 6572

J. S. Bloom (UC Berkeley), L. Pollack (UC Santa Cruz), D. A. Perley  
(UC Berkeley) report:

"We observed the field of GRB 070621 (GCN #6560) in the K-prime  
filter using
the Laser Guide Star (LGS) Adaptive Optics (AO) system + NIRC2 on the
Keck II telescope.  With integrations of 60 sec per frame, we stacked
45 frames with a mean time since GRB trigger of 13.8 hr.  The brighter
compact sources in the field have typical FWHM of 130 mas, and the
preliminary 5 sigma detection threshold is K-prime = 21.5 mag.  With
further refinements we expect this limit to improve.

A number of sources mentioned in previous circulars are detected (*)
as well as a few red and faint sources near the XRT position. In
particular, we note source l1 (ra=21:35:10.22, dec=-24:49:07.4, J2000)
which is 0.8 arcseconds East of apparent galaxy s1 (Bloom et al. GCN
#6568 = Malesani et al. source #1 GCN #6565?).  The source near the
Eastern edge of the XRT position (l2) is nearly coincident with the
position of s4 (Bloom et al. GCN #6568 = Tanvir et al. source #2 GCN
#6570) but appears only marginally extended at K-prime. Both l1 (being
quite red and close to a galaxy) and l2 (appearing nearly point-like
and close to the XRT position) are of prime interest in the search for
the afterglow of this GRB.  Given the large number of sources in the
field and the quality of the laser AO images, if either l1 or l2 are
confirmed as the GRB afterglow then the astrometric accuracy will be
comparable to that obtained previously with early and late HST
imaging of other afterglows."

(*) http://lyra.berkeley.edu/~jbloom/grb070621_bloom_kecklaser.pdf

We thank the WMKO staff for their assistance in obtaining these
observations, especially Randy Campbell and Al Conrad.