No abstract

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.

Abstract. BeppoSAX/PDS experiment is one of four narrow field instruments of the BeppoSAX payload, that also includes two wide field cameras. The goal of PDS is to extend the energy range of BeppoSAX to hard X-rays. The operative energy range of PDS is 15 to 300 keV, where the experiment can perform sensitive spectral and temporal studies of celestial sources. The PDS detector is composed of 4 actively shielded NaI(Tl)/CsI(Na) phoswich scintillators with a total geometric area of 795 cm 2 and a field of view of 1• .3 (FWHM). In this paper we describe the experiment design and discuss its functional performance and calibration data analysis system.

We investigated the spectral evolution in the 2-700 keV energy band of Gamma-Ray Bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite before May 1998. Most of them have been followed-up with the Narrow Field Instruments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrotron shock model (SSM) provides an acceptable representation of most of the timeresolved GRB spectra extending down to 2 keV, except in the initial phases of several bursts and during the whole duration of the quite strong GRB970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. A strong and time variable low energy cut-off, consistent with absorption effect, is observed during the prompt emission of GRB980329. We find that the X-ray afterglow starts at about 50% of the GRB duration, and that its fluence, as computed from the WFC light curve, is consistent with the decay law found from the afterglow NFI observations. We also investigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index of the latest spectrum of the GRB prompt emission is correlated with the index of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball. We also find that for most of the GRBs in our sample the late emission is consistent with a slow cooling of the shock. Adiabatic shocks appear more likely than radiative shocks. Parameters of the shocks at earliest times have been derived.

We report on the 0.5È200 keV spectral properties of Cyg X-1 observed at di †erent epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the Ðrst observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Ka line and Compton reÑection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/ nonthermal plasma, probably forming a corona above the disk (also giving rise to the ComptonreÑection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of D60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we Ðnd the characteristic ratio of the bolometric Ñux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.