We report the discovery of a transient equivalent hydrogen column density with an absorption edge at ∼3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of ∼0.86 and with an iron abundance of ∼75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event.

We have examined the complete set of X-ray afterglow observations of dark and optically bright GRBs performed by BeppoSAX until February 2001. Xray afterglows are detected in ∼ 90% of the cases. We do not find significant differences in the X-ray spectral shape, in particular no higher X-ray absorption in GRBs without optical transient ( dark GRBs ) compared to GRBs with optical transient ( OTGRBs ). Rather, we find that the 1.6-10 keV flux of OTGRBs is on average about 5 times larger than that of the dark GRBs. A K-S test shows that this difference is significant at 99.8% probability. Under the assumption that dark and OTGRB have similar spectra, this could suggest that the first are uncaught in the optical band because they are just faint sources. In order to test this hypothesis, we have determined the optical-to-X ray flux ratios of the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which corresponds to an average optical-to-x spectral index α OT OX = 0.794 ± 0.054. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OT GRBs, the remaining 25% are 4 -10 times weaker in optical than in X-rays. The significance of this result is ≥ 2.6σ. If this sub-population of dark GRBs were constituted by objects assimilable to OTGRBs, they should have shown optical fluxes higher than upper limits actually found. We discuss the

The gamma-ray burst GRB 000210 had the highest γ-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow, despite prompt and deep searches down to R lim ≈ 23.5. It is therefore one of the events recently classified as dark GRBs, whose origin is still unclear.

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the c-ray burst GRB 010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40È700 keV Ñuence of (9.3^0.3) ] 10~5 ergs cm~2 is only surpassed by GRB 990123. In terms of the isotropic 20È 2000 keV energy output of 7.8 ] 1053 ergs, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2È10 keV Ñux history shows clear signs of a break, which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the Ðreball is braked to the nonrelativistic regime quickly (within a fraction of day) by a dense (D106 cm~3) circumburst medium. This implies that, after a mild beaming correction, GRB 010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33^0.04, the spectral index 0.97^0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.

The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high‐intensity state are reported and compared with monitoring X‐ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1–100 keV spectrum of Mrk 421 shows continuous convex curvature that can be interpreted as the high‐energy end of the synchrotron emission. The source shows significant short‐term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3–5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states.