{
  "bibcode": "2005GCN..3409....1C",
  "body": "S. Bradley Cenko, B. T. Soifer, Chao Bian, Vandana Desai, S. R. Kulkarni\n(Caltech), Brian P. Schmidt (ANU), Arjun Dey and Buell T. Jannuzi (NOAO)\nreport on behalf of the Caltech-NRAO-Carnegie GRB collaboration:\n\nWe have continued to image the field of GRB 050509b with the Low\nResolution Imaging Spectrometer mounted on the 10-m Keck I telescope.  Our\nlatest observations consisted of 3 x 300 s images simultaneously in the g'\nand R filters.  The mean epoch of these observations was approximately\n5:56 UT 10 May (25.8, 25.9, and 26.0 hours after the burst, respectively).\n\nWe stacked all the R-band images obtained on 9 May (GCNs 3391, 3401) and\ndifferenced this from the stack obtained on 10 May.  Adopting the value of\nR = 18.5 for the USNO-A2 star located at RA 12:36:19.970, Dec +29:00:36.31\n(J2000.0), we find no variation within the revised XRT error circle (GCN\n3395) to a limiting magnitude of R = 25.6.  Specifically for the extended\nsource in the XRT error circle (S1 in GCN 3401), we find this object has\nnot varied by more than +/- 0.18 magnitudes from 9 May to 10 May.\n\nIt is therefore unlikely that any of the 4 sources identified in GCN 3401\nis the optical afterglow of GRB 050509b.",
  "circularId": 3409,
  "createdOn": 1115793945000,
  "email": "cenko@srl.caltech.edu",
  "subject": "GRB 050509b: No Optical Variability in XRT Error Circle",
  "submitter": "S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>",
  "eventId": "GRB 050509B"
}