TITLE: GCN GRB OBSERVATION REPORT NUMBER: 832 SUBJECT: GRB000926: X-ray afterglow by BeppoSAX. Refined analysis DATE: 00/10/05 20:00:05 GMT FROM: 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 curve.