GCN Circular 511

Dick Joyce (KPNO), James Rhoads (STScI), Babar Ali (IPAC),
Ian Dell'Antonio (KPNO/Brown), and Buell Jannuzi (NOAO) report:

Joyce and Ali obtained J band spectrophotometry of the afterglow of GRB
991216 at 1999 Dec 18.372 UT, using the Kitt Peak National Observatory
2.1m telescope + Cryogenic Spectrometer (CRSP) with a 3.9 arcsec slit.
The spectral resolution obtained was about 40 Angstroms.  We detect
a weak continuum (s/n approx. 4 per resolution element).  No clearly
significant emission or absorption features are detected.  The continuum
flux level rises from ~150 uJy at 1.12 um to ~220 uJy at 1.23 um,
and remains approximately constant at ~220 uJy out to 1.33 um. 
It is thus not particularly well described by a single power law,
suggesting either that not all of the near-IR flux seen is due to
synchrotron emission, or that a break in the synchrotron spectrum was
near 1.25 microns at the time of observation.  The most obvious
alternative flux source is a host galaxy, although this host would
have to be luminous and red for this explanation to work, given the
redshift constraint (z>1, Vreeswijk et al, GCN #496
Not found
 
 
) and faint
optical flux of the candidate host galaxy (Djorgovski et al, GCN #510
Not found
 
 
).
Wavelength and flux calibration were based on observations of a
J=5.5 A0 star.  The overall flux calibration is good to about 30%.