{
  "bibcode": "2005GCN..3420....1E",
  "body": "Charles W. Engelbracht and Daniel J. Eisenstein (Arizona) report:\n\nWe acquired B-band imaging of the field of GRB050509b using the 90-prime\n1-degree camera on the 2.3-m Bok telescope.  The time of the observation\nwas 0500 UT May 10 (25 hours after the burst).  The exposure time was 20\nminutes, and the seeing was 1.7\" due to wind shake.  We clearly detect\nthe source S1 (GCN 3388, 3401) at a B-band magnitude of 23.9.  We do\nnot find any sources in our B-band image that are not in the Hjorth et\nal. JPG image (GCN 3410).\n\nWe are making the calibrated data for a 30' square region centered on\nthe burst available at \n    http://cmb.as.arizona.edu/~eisenste/grb050509b/grb050509b_B.fits.gz\n\nThe header contains WCS astrometric information, good to 0.5\".\nWe estimate the photometric calibration at 5%.  If you are using the B\nband image, please send an email to deisenstein@as.arizona.edu so that\nwe can inform you of any revisions.\n\nWhile this image is surely not as deep as the nearly contemporaneous\nKeck LRIS imaging (GCN 3409), it may be useful for photometric redshifts.\nThe B band lies blueward of but close to the 4000A break for the cluster\nat z=0.225 (Gal et al., AJ, 125, 2064, 2003; Prochaska et al., GCN 3399).\nCombined with slightly redder filters (V or g), one should be able to\nefficiently distinguish cluster members from higher redshift galaxies.\n\nWe remark that if the GRB is at a much higher redshift than the cluster,\nthen mapping of the cluster may be important for assessing the impact of\ngravitational lensing on the interpretation the burst.  The velocity\ndispersion of 340 km/s (Prochaska et al., GCN 3399) for the large\ngalaxy near the XRT position (Rol et al., GCN 3395) would predict an\nEinstein radius of 3.3\" with a singular isothermal sphere model and an\ninfinitely distant source.  The embedding of the galaxy in the cluster\nmay increase this value.  Although multiple imaging remains unlikely,\nthere should be non-negligible magnification.\n\nThis note can be cited.",
  "circularId": 3420,
  "createdOn": 1115944473000,
  "email": "deisenstein@as.arizona.edu",
  "subject": "GRB050509b, Bok telescope B-band imaging",
  "submitter": "Daniel Eisenstein at U of Arizona  <deisenstein@as.arizona.edu>",
  "eventId": "GRB 050509B"
}