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GCN Circular 832

GRB000926: X-ray afterglow by BeppoSAX. Refined analysis
2000-10-05T20:00:05Z (24 years ago)
Luigi Piro at IAS/CNR Frascati <>
L. Piro (BeppoSAX Mission Scientist, IAS/CNR Roma) & L. A. Antonelli (OaR)
report on behalf of the BeppoSAX team:

A BeppoSAX TOO observation of GB000926  started on Sept. 29.00, i.e.
2 days after the GRB and lasted for 12 hrs.
Effective exposure times were MECS(1.6-10 keV): 20 ksec, LECS(0.1-10
keV):5 ksec.

We confirm the relatively bright unknown source identified as the X-ray
afterglow by Piro (GCN 812)

The revised position is:

RA(2000)=17h 04m 07s Decl.(2000)= +51 47' 18"

with an error radius of 50", consistent with the previous preliminary
position distributed in GCN 812 and with the optical counterpart
(Gorosabel et al. GCN 803, Dall et al. GCN 804, Halpern et al. GCN 806).

The average flux of the source in the MECS(1.6-10 keV) is (4.1+/-0.6)
10^-3 cts/s, corresponding to a F(1.6-10 keV)=(4.8+-0.7) 10^-13 erg/cm2/s.
The spectrum is consistent with a power law with photon index=(1.9+/-0.7)
and absorption by our Galaxy (NH=2.7 10^20 cm-2).

The BeppoSAX observation occured in coincidence with the break of the
optical light curve reported e.g by Fynbo (GCN820), Veillet (GCN 823) and
Halpern et al (GCN824). In the X-ray band the source exhibits a
substantial decay, decreasing by a factor of (1.7+/-0.5) in 6 hours. This
would correspond to a power law decay F= c(T-T0)^d_X (where T0=26.99)
with slope d_X=-4.3+/-1, i.e. steeper that that measured in the
optical after the break (d_O=-2.5+/-0.2,  Halpern et al., GCN 824).
This behaviour is not immediately consistent with a jet-expansion, that
predicts d_X=d_O. Chandra data, taken few hours after the end of the
BeppoSAX observation, shall be crucial in confirming the X-ray light
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