GCN Circular 448

GRB991106: BeppoSAX/BATSE results

G. Gandolfi and P. Soffitta (IAS/CNR,Roma), J. Heise and J. in 't Zand
(SRON, Utrecht), L.Amati (ITESRE/CNR, Bologna) on behalf of BeppoSAX GRB
Team and R. M. Kippen and P. M. Woods (University of Alabama in
Huntsville) and C. A. Meegan (NASA/MSFC) on behalf of the BATSE GRB team
report:

The BeppoSAX 4-sigma detection ((9.5 +/- 2.4)E-8 erg/cm2/s in the
40-700 keV range) of a short gamma peak of about 1 s of duration in
correspondence with the X-ray transient reported in GCN 443
Not found
 
 
 (GRB991106)
was not confirmed by BATSE observations. The WFC source position was
observable to BATSE (i.e., not occulted by the earth). The on-board
trigger was disabled, but the continuous data with 1.024 and 4.096 second
resolution shows no evidence for significant excess flux above background.
Upper limits (2-sigma), based on these data are:

   F < 6.6e-9 erg/cm^2/s  (25- 50 keV)
   F < 1.7e-8 erg/cm^2/s  (50-300 keV)
   F < 2.7e-8 erg/cm^2/s  (40-700 keV; the BeppoSAX GRBM energy range)

We therefore conclude that the BeppoSAX-GRBM detection was apparently
either a statistical fluctuation or a spurious event of a different origin
(e.g. Cosmic Rays).
On the other side, the faint WFC X-ray transient, with a (2.0 +/- 0.4)E-8
erg/cm2/s peak flux in the 2-26 keV range, shows marginal evidence
of a high hardness ratio.
These BeppoSAX/BATSE combined results suggest a peculiarly short and
hard X-ray burst or an anomalous GRB. The lower limit to the ratio of
X-ray to gamma peak flux is about 0.75, a factor 5 higher
than average values found for BeppoSAX prompt GRB counterparts,
indicating in the latter case a X-ray rich event, i.e. a candidate member
of the X-ray-GRB class.