{
  "bibcode": "2010GCN.10339....1C",
  "body": "S. B. Cenko, D. A. Perley, J. S. Bloom, B. E. Cobb, and A. N. Morgan (UC\nBerkeley) report on behalf of a larger collaboration:\n\nWe have obtained pre-explosion images of the field of GRB100117A from the\nCanada-France-Hawaii MegaCam archive.  Images were obtained in the g', r',\nand z' filters on 26 December 2005, and in the i' filter on 12 November\n2007.  Exposure times ranged from 240 s in g'-band to 500 s in i'-band.\n\nInside the candidate XRT afterglow localization (de Pasquale; GCN 10336),\nwe find four sources, with the following positions and R- and I-band\nmagnitudes (calibrated relative to several USNO-B1 sources in the field;\nexpected systematic uncertainty ~ 0.3 mag):\n\nSource \tRA\t\tDec (J2000.0)\t\tR\tI\n----------------------------------------------------------------------\nS1\t00:45:04.82 \t-01:35:47.7 \t\t23.1\t21.0\nS2\t00:45:04.78 \t-01:35:45.5 \t\t23.6\t22.9\nS3\t00:45:04.70 \t-01:35:42.0 \t\t24.5\t22.9\nS4\t00:45:04.53 \t-01:35:49.2 \t\t> 24.7\t23.6\n\nWe have also obtained a single epoch of acquisition imaging in i'-band of\nthe field with GMOS on Gemini South beginning at 00:46 on 18 January 2010\n(~ 4.7 hours after the burst).  We detect only S1 in this image, at a\nmagnitude consistent with the value derived from the CFHT image.\n\nFurthermore, given the uncertainty in our astrometry (~ 0.5\" in each\ncoordinate), we believe it likely this is the same source identified by\nXu et al. (GCN 10337) at R = 22.5 +/- 0.5.  We therefore have no evidence\nthat this source has varied.\n\nFurther observations are planned.\n\nWe wish to thank the Gemini Observatory staff, in particular Peter Pessev,\nfor assistance with these observations.",
  "circularId": 10339,
  "createdOn": 1263780383000,
  "email": "cenko@srl.caltech.edu",
  "subject": "GRB 100117A: Gemini South Imaging and CFHT Pre-Explosion Images",
  "submitter": "S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>",
  "eventId": "GRB 100117A"
}