TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2796
SUBJECT: GRB 040916:  Afterglow Color at Early Times
DATE:    04/10/11 15:18:33 GMT
FROM:    Daniel E. Reichart at U.North Carolina  <reichart@physics.unc.edu>

Christine Lamanna, Matt Bayliss, Ron Canterna, Dan Reichart, Arne Henden,
Don Lamb, and Melissa Nysewander report on behalf of the U. Wyoming, U.
North Carolina, USNO, and U. Chicago GRB teams of the FUN GRB
Collaboration:

We observed the afterglow location (Kosugi et al., GCN 2726) of XRF 040916
(Yamazaki et al., GCN 2712; Yamamoto et al., GCN 2713) in V with the
2.3-meter WIRO telescope beginning 4.78 hours (mean epoch = 4.86 hours) and
again 1.49 days after the burst.  For our first epoch, we find that V =
22.12 +/- 0.06.  For our second epoch, we find that V > 21.5 (2 sigma).

Using Subaru, Kosugi et al. (GCN 2726) find the afterglow to be R = 22.3
+/- 0.2 at 5.6 hours.  Using USNO's 1.0-meter telescope, Henden (GCN 2727)
finds the afterglow to be Ic = 20.51 +/- 0.23 at 4.6 hours.  As Henden
(2727) stated, this implies that the afterglow was very red or rapidly
fading at this time.

The WIRO and USNO observations are nearly simultaneous, and yield a
spectral index -2.8 +/- 0.6, which is very red.

If this is due to extinction, the observer-frame A_V is likely ~3 mag and R
- Ic ~ 0.6 mag and B - R ~ 1.4 mag.  Consequently, at the epochs of the
first three Subaru observations (t = 5.6, 8.8, and 60 hours after the
burst) the afterglow would be I ~ 21.7, 22.1, and 24.2 and B ~ 23.7, 24.1,
and 26.2.

If this turns out not to be the case, the color of the afterglow likely
changed, perhaps very rapidly, between the USNO/WIRO observations and the
Subaru observations.

One possible explanation for this would be an early-time dust echo.  Moran
& Reichart (astro-ph/0409390, posted two hours before the burst) predict
that light from the optical flash will scatter off of a circum-progenitor
dust shell, similar to the dust shells that are found around late-type WC
stars.  This results in excess light on a timescale of minutes to hours
after the burst, and this excess light quickly transitions from blue to red
before fading away.  For example, this ~1.6 mag difference between Ic and V
would not be difficult to explain with an optical flash of peak brightness
~12th mag and a dust shell of inner radius ~10^16 cm.

Finally, we note that GRB 980329 had a similar, unexplained two magnitude
difference between the I and R bands, in this case about 18 hours after the
burst (Reichart et al. 1999, ApJ, 517, 692).

Images and light curves will be posted at:

www.physics.unc.edu/~mbayliss/grb040916.html