{
  "bibcode": "2007GCN..6296....1C",
  "body": "B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:\n\nUsing the ANDICAM instrument on the 1.3m telescope at CTIO, we\nobtained optical/IR imaging of the error region of GRB 070318\n(GCN 6210, Cummings et al.) with a mid-exposure time of\n2007-03-19 00:09 UT (~16.7 hrs post-burst) and again\nat 2007-03-21 00:07 UT (~64.6 hrs post-burst).  For each\nset of observations, total summed exposure times amounted\nto 36 minutes in I and 30 minutes in J.\n\nThe afterglow of GRB 070318 (GRB 6210, Cummings et al.) is detected\nin the first epoch images and is observed to fade between the\nfirst and second epochs.\n\ntime-post\nbursts (hrs)    I magnitude             J magnitude\n------------------------------------------------------\n16.7            19.82+/-0.07            18.29+/-0.11\n64.6            20.28+/-0.12            > 18.6 (3 sigma limit)\n\nMagnitudes are calibrated using Landolt standard stars in the\noptical and 2MASS stars in the IR.\n\nThe optical decay rate (afterglow flux proportional to t^-alpha)\nbetween 16.7 and 64.6 hours post-burst is alpha ~ 0.3 (and\nthe IR decay is consistent with this value).  This unusually\nshallow decay was also noted in the UVOT observations\nof this afterglow (GCN 6220, Page et al.).\n\nA third late-time epoch was obtained at 2007-04-06 23:46 UT\n(~20 days post-burst). No source or host galaxy is detected\nat the position of the optical afterglow to a limiting magnitude\nof I > 21.7 and J > 19.2.  This indicates that the decay\nrate of the afterglow must have steepened sometime after our\nsecond epoch observation.",
  "circularId": 6296,
  "createdOn": 1176581016000,
  "email": "cobb@astro.yale.edu",
  "subject": "GRB 070318,  SMARTS optical/IR afterglow observations",
  "submitter": "Bethany Cobb at Yale U  <cobb@astro.yale.edu>",
  "eventId": "GRB 070318"
}