{
  "bibcode": "2004GCN..2796....1L",
  "body": "Christine Lamanna, Matt Bayliss, Ron Canterna, Dan Reichart, Arne Henden,\nDon Lamb, and Melissa Nysewander report on behalf of the U. Wyoming, U.\nNorth Carolina, USNO, and U. Chicago GRB teams of the FUN GRB\nCollaboration:\n\nWe observed the afterglow location (Kosugi et al., GCN 2726) of XRF 040916\n(Yamazaki et al., GCN 2712; Yamamoto et al., GCN 2713) in V with the\n2.3-meter WIRO telescope beginning 4.78 hours (mean epoch = 4.86 hours) and\nagain 1.49 days after the burst.  For our first epoch, we find that V =\n22.12 +/- 0.06.  For our second epoch, we find that V > 21.5 (2 sigma).\n\nUsing Subaru, Kosugi et al. (GCN 2726) find the afterglow to be R = 22.3\n+/- 0.2 at 5.6 hours.  Using USNO's 1.0-meter telescope, Henden (GCN 2727)\nfinds the afterglow to be Ic = 20.51 +/- 0.23 at 4.6 hours.  As Henden\n(2727) stated, this implies that the afterglow was very red or rapidly\nfading at this time.\n\nThe WIRO and USNO observations are nearly simultaneous, and yield a\nspectral index -2.8 +/- 0.6, which is very red.\n\nIf this is due to extinction, the observer-frame A_V is likely ~3 mag and R\n- Ic ~ 0.6 mag and B - R ~ 1.4 mag.  Consequently, at the epochs of the\nfirst three Subaru observations (t = 5.6, 8.8, and 60 hours after the\nburst) the afterglow would be I ~ 21.7, 22.1, and 24.2 and B ~ 23.7, 24.1,\nand 26.2.\n\nIf this turns out not to be the case, the color of the afterglow likely\nchanged, perhaps very rapidly, between the USNO/WIRO observations and the\nSubaru observations.\n\nOne possible explanation for this would be an early-time dust echo.  Moran\n& Reichart (astro-ph/0409390, posted two hours before the burst) predict\nthat light from the optical flash will scatter off of a circum-progenitor\ndust shell, similar to the dust shells that are found around late-type WC\nstars.  This results in excess light on a timescale of minutes to hours\nafter the burst, and this excess light quickly transitions from blue to red\nbefore fading away.  For example, this ~1.6 mag difference between Ic and V\nwould not be difficult to explain with an optical flash of peak brightness\n~12th mag and a dust shell of inner radius ~10^16 cm.\n\nFinally, we note that GRB 980329 had a similar, unexplained two magnitude\ndifference between the I and R bands, in this case about 18 hours after the\nburst (Reichart et al. 1999, ApJ, 517, 692).\n\nImages and light curves will be posted at:\n\nwww.physics.unc.edu/~mbayliss/grb040916.html",
  "circularId": 2796,
  "createdOn": 1097507913000,
  "email": "reichart@physics.unc.edu",
  "subject": "GRB 040916:  Afterglow Color at Early Times",
  "submitter": "Daniel E. Reichart at U.North Carolina  <reichart@physics.unc.edu>",
  "eventId": "GRB 040916"
}