{
  "bibcode": "2007GCN..6934....1P",
  "body": "D. A. Perley, A. V. Filippenko, J. M. Silverman, R. J. Foley, M. Modjaz, \nD. Kocevski, and J. S. Bloom report:\n\nWe acquired an additional series of imaging observations of the field of \nGRB 071010A (Moretti et al., GCN 6859) with Keck I + LRIS starting at \n5:01 UT, 2007-10-16 (6.05 days after the trigger), in g and R filters.\n\nThe optical afterglow (Klotz et al. GCN 6860) has faded substantially. \nAn object is observed at the GRB position, resolvable into two regions: \na brighter, redder source to the east and a fainter, bluer source to the \nwest.  Comparison with our previous Keck imaging shows the afterglow \nposition to be consistent only with the fainter, western source.  The \ntwo sources may be a bright elongated host galaxy, a compact host galaxy \nwith the afterglow offset from the center, or a foreground star with the \nafterglow coincidentally located very nearby.  An image of the field is \nposted at: http://lyra.berkeley.edu/~dperley/071010a/071010a_keck.png\n\nAperture photometry shows the combined complex of both sources to have a \nmagnitude of R=22.5, using the same calibration system in previous \ncirculars.  The contribution from the afterglow is limited to R>23.3, \ndepending on the uncertain contribution of a possible bright host galaxy.\n\nRefined photometry of our imaging starting at 2007-10-11 UT 04:47 (GCN \n6885) shows the afterglow magnitude at that time to be R = 19.82+/-0.02. \n  This indicates that the afterglow decay underwent a sharp break, from \nalpha < 0.5 between the first and second night to a minimum of alpha > \n1.7 over the following five days.\n\nComparison with the XRT light curve at \nhttp://astro.berkeley.edu/~nat/swift/00293707/bat_xrt.jpg shows the \nX-ray afterglow to have undergone a break at 10^5 seconds (roughly \ncoincident with our measurement on 2007-10-11) from approximately flat \nevolution before this point to a rapidly decaying power law of alpha ~ \n1.8 afterward.  This suggests that this sharp break may be achromatic, \nand possibly indicative of a jet break.\n\n[GCN OPS NOTE(18oct07):  The \"and jet break\" was added back on to the \nSubject-line of this circular.  It was chopped off during processing\nbecause the mail sending/delivery system chopps wrapped Subject lines.]",
  "circularId": 6934,
  "createdOn": 1192744778000,
  "email": "dperley@astro.berkeley.edu",
  "subject": "GRB 071010A: Late-time Keck/LRIS photometry - possible host galaxy and jet break",
  "submitter": "Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>",
  "eventId": "GRB 071010A"
}