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GCN Circular 448

Subject
GRB991106: BeppoSAX/BATSE results
Date
1999-11-13T10:50:09Z (25 years ago)
From
Giangiacomo Gandolfi at IAS/CNR Frascati <gandolfi@ias.rm.cnr.it>
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 (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.
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