TITLE: GCN CIRCULAR NUMBER: 6451 SUBJECT: GRB 070521: Keck imaging DATE: 07/05/22 07:49:54 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley, J. S. Bloom (UC Berkeley), J. X. Prochaska (UC Santa Cruz), S. Stanford (LLNL), M. Brodwin (JPL), and N. R. Butler (UCB) report: Starting at 13:21:22 UT (2007-05-21), we imaged the field of GRB 070521 (GCN 6431) in V and R band with the Keck I telescope (+ LRIS), in a series of five exposures of 300 seconds each. The host galaxy candidate of Hattori et al. (GCN 6444, "S1") is clearly detected, as is a second source ("S2") at the edge of the XRT error circle (Guidorzi et al., GCN 6431). We also detect a third, extended source ("S3") further to the south, not consistent with the preliminary XRT position reported by Guidorzi et al. but consistent with our preliminary re-analysis of the XRT data. The positions of these sources (J2000) are: S1: 16:10:38.562 +30:15:27.50 S2: 16:10:38.734 +30:15:31.38 S3: 16:10:38.754 +30:15:21.57 The astrometric uncertainty is about 0.3" in each coordinate. Preliminary aperture magnitudes of these sources, calibrated with respect to the SDSS magnitudes from Cool et al. (GCN 6432) using the Lupton transform [1], are: S1: V = 25.46 +/- 0.12 , I = 23.88 +/- 0.12 S2: V = 25.98 +/- 0.20 , I = 24.26 +/- 0.16 S3: V = 26.63 +/- 0.39 , I = 24.39 +/- 0.17 These magnitudes are not corrected for Galactic extinction, which the NED extinction calculator [2] estimates to be E(B-V) = 0.027, or A_V = 0.09, A_I = 0.05, in this direction. An image of the field is located at: http://lyra.berkeley.edu/~dperley/070521/070521keckI.png http://lyra.berkeley.edu/~dperley/070521/070521keckI_clean.png --- [1] http://www.sdss.org/dr5/algorithms/sdssUBVRITransform.html#Lupton2005 [2] http://nedwww.ipac.caltech.edu/forms/calculator.html