TITLE: GCN CIRCULAR NUMBER: 6572 SUBJECT: Keck Laser Guide Star Adaptive Optics Imaging of GRB 070621 DATE: 07/06/22 17:01:48 GMT FROM: Josh Bloom at UC Berkeley 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.