{
  "bibcode": "2009GCN.10049....1C",
  "body": "S. B. Cenko, D. A. Perley, B. E. Cobb, J. S. Bloom, and N. R. Butler (UC\nBerkeley) report on behalf of a larger collaboration:\n\nWe have imaged the field of the Fermi GRB090926A (Bissaldi et al., GCN\n9933; Uehara et al., GCN 9934) with the Gemini Multi-Object Spectrograph\nmounted on the 8-m Gemini South telescope.  Observations were taken in\nthe Sloan g', r', and i' filters beginning at 2:28 UT on 19 October 2009\n(~ 22.9 d after the GBM trigger).\n\nWe detect a faint source in all filters at the location of the optical\nafterglow (Haislip et al., GCN 9937, Gronwall et al., GCN 9938). Using\nseveral unsaturated USNO-B objects in the field of view, along with the\nfilter transformations of Jordi, Grebel, and Ammon (2006 A&A 460, 339), we\nmeasure a magnitude of r' ~ 23.7 for this source.  Along with its\nrelatively blue color (g' - i' ~ 0), the object appears marginally\nextended, suggesting it is likely dominated by emission from the host\ngalaxy of GRB090926A.  We caution, however, that the host candidate is\npartially blended with a nearby object (~ 1.5\" in the SW direction, just\noutside the optical and X-ray afterglow error circle) of comparable\nmagnitude, which likely affects the photometry.\n\nComparing with the last reported R-band detection (R ~ 21.5 at t ~ 11.1 d;\nHaislip et al., GCN 10003), the optical decay must have significantly\nsteepened from previous measurements (power-law index alpha >~ 2.5).  A\nsimilar steepening is hinted at in the latest XRT observations of this\nsource (see http://astro.berkeley.edu/~nat/swift/00020113/bat_xrt.jpg),\nsuggesting a possible jet break.  Using the measured redshift of z = 2.1\n(Malesani et al., GCN 9942), the isotropic gamma-ray energy derived from\nthe Konus-Wind instrument (E_iso ~ 2e54 erg; Golenetskii et al., GCN\n9959), and assuming expansion into a constant density medium (n ~ 1\ncm^-3) with a gamma-ray efficiency ~ 20%, we infer an opening angle ~ 0.1\nrad.  The collimation-corrected prompt energy release would therefore be\nlarge, E ~ 1e52 erg.",
  "circularId": 10049,
  "createdOn": 1256076604000,
  "email": "cenko@srl.caltech.edu",
  "subject": "GRB 090926A: Late-time Gemini South Observations and Possible Jet Break",
  "submitter": "S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>",
  "eventId": "GRB 090926A"
}