TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2468
SUBJECT: GRB 031203: Apparent Spectral Break
DATE:    03/12/04 22:06:09 GMT
FROM:    Josh Bloom at Harvard/CFA  <jbloom@cfa.harvard.edu>

GRB 031203: Apparent Spectral Break

C. Bailyn, P. van Dokkum, B. Cobb, M. Buxton (Yale) and J. S. Bloom
(Harvard/CfA) report further analysis of SMARTS data (see GCN #2463)
obtained between December 4 UT=3:00 and UT=6:11:

"The summed images reveal no source in I or J at the REM position (GCN
2466).  The I-band upper limit is I=23 mag.  In J, a faint source at
J=20.5 +/- 0.5 mag, undetectable in I, is found near the REM position at
RA = 8:02:29.78  dec = -39:51:07.1  (J2000)   
     +/- 0.4 arcseconds (1 sigma; found from astrometric tie to 2MASS) 

No other source is found within 4.5" of the REM position.   The J-band
data were obtained starting at UT=4:19, and are thus essentially
contemporaneous with the REM observations, while the I-band images span
the whole three hours of observations.  There are a number of sources
with I<22 within the XMM-Newton error circles (GCN #2464), but none of
them vary by more than 0.1 magnitudes over the three hour time series.

If the REM source and the faint J source are in fact the same, J-K = 7
mag, whereas H-K = 1.5 mag.  If the faint J source is not the
counterpart of the REM source, then J-K > 7.  This extreme color
requires either very unusual extinction, a very unusual spectral
energy distribution, or rapid time variability of many magnitudes
sampled differently by the SMARTS and REM observations.  One
interpretation is that this source is a J-band dropout, located
at a redshift >~ 9."

A website with finding charts, spectral energy distributions, and
other information will soon be available at

http://www.astro.yale.edu/dokkum/GRB031203.html

This message may be cited.