arXiv subjects

P. Soffitta

Publications and source records attributed to P. Soffitta.

122 records · Page 7Linked to original sources

arXiv2001

The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature

We report on the prompt X- and gamma-ray observations of GRB990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No. 2. Due to Sun constraints, we could not perform a follow-up observation with the BeppoSAX Narrow Field Instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40s in X-rays. In gamma-rays the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the E F(E) spectrum: Ep is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the possible evidence of a 2s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kTbb ~1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball, when it becomes optically thin.

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arXiv2001

SuperAGILE: the X-ray Monitor of the AGILE gamma-ray Mission

SuperAGILE is the X-ray stage of the AGILE gamma-ray mission. It is devoted to monitor X-ray (10-40 keV) sources with a sensitivity better than 10 mCrabs in 50 ks and to detect X-ray transients in a field of view of 1.8 sr, well matched to that of the gamma-ray tracker, with few arc-minutes position resolution and better than 5 us timing resolution. SuperAGILE is designed to exploit one additional layer of four Si microstrip detectors placed on top of the AGILE tracker, and a system of four mutually orthogonal one-dimensional coded masks to encode the X-ray sky. The total geometric area is 1444 cm^2. Low noise electronics based on ASIC technology composes the front-end read out. We present here the instrumental and astrophysical performances of SuperAGILE as derived by analytical calculations, Monte Carlo simulations and experimental tests on a prototype of the silicon microstrip detector and front-end electronics.

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arXiv2000

Discovery of a Transient Absorption Edge in the X-ray Spectrum of GRB 990705

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

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arXiv2000

Discovery of a redshifted Iron K-line in the X-ray afterglow of GRB 000214

We report the detection (3 sigma significance level) of a strong iron emission line in the X-ray spectrum of the afterglow of GRB 000214 (``Valentine's Day Burst'') observed by BeppoSAX. An emission line feature was observed with a centroid energy of 4.7+/-0.2 keV which, if interpreted as K-alpha emission from hydrogen-like iron, corresponds to a redshift of z=0.47. The observed intensity (EW ~2 keV) and duration (tens of hours) of the line give information on the distance, from the burst region, of the emitting material (R>3x10^15 cm) and its mass (M>1.4 solar mass). These results are not easily reconciled with the binary merger and hypernova models for gamma ray bursts.

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arXiv2000

Prompt and afterglow emission from the X-ray rich GRB981226 observed with BeppoSAX

We discuss observations of the prompt X and gamma-ray emission and X-ray afterglow from GRB981226. This event has the weakest gamma-ray peak flux detected with the Gamma-Ray Burst Monitor. It shows an isolated X-ray precursor and the highest X-ray to gamma-ray fluence ratio measured thus far with the Wide Field Cameras. The event was followed up with the Narrow Field Instruments, and the X-ray afterglow was detected up to 10 keV. The afterglow flux is observed to rise from a level below the sensitivity of the MECS/LECS telescopes up to a peak flux of (5 +/-1) * 10^(-13) erg cm-2 s-1, in the 2-10 keV energy band. This rise is followed by a decline according to a power law with index of 1.31 (+0.44,-0.39). We discuss these results in the light of the current GRB models.

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arXiv1999

Prompt and delayed emission properties of Gamma-Ray Bursts observed with BeppoSAX

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 time-resolved 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. 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.

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arXiv1999

The AGILE Gamma-ray Astronomy Mission

We describe the AGILE gamma-ray astronomy satellite which has recently been selected as the first Small Scientific Mission of the Italian Space Agency. With a launch in 2002, AGILE will provide a unique tool for high-energy astrophysics in the 30 MeV - 50 GeV range before GLAST. Despite the much smaller weight and dimensions, the scientific performances of AGILE are comparable to those of EGRET.

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arXiv1999

BeppoSAX Observations of GRB980425: Detection of the Prompt Event and Monitoring of the Error Box

We present BeppoSAX follow-up observations of GRB980425 obtained with the Narrow Field Instruments (NFI) in April, May, and November 1998. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova 1998bw, which exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of X-ray emission from a Type I supernova above 2 keV. The latter source exhibits only marginally significant variability. The X-ray spectra and variability of the supernova are compared with thermal and non-thermal models of supernova high energy emission. Based on the BeppoSAX data, it is not possible to firmly establish which of the two detected sources is the GRB X-ray counterpart, although probability considerations favor the supernova.

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arXiv1999

Optical and Radio Observations of the Afterglow from GRB990510: Evidence for a Jet

We present multi-color optical and two-frequency radio observations of the bright SAX event, GRB 990510. The well-sampled optical decay, together with the radio observations are inconsistent with simple spherical afterglow models. The achromatic optical steepening and the decay of the radio afterglow both occuring at $t \sim 1$ day are evidence for hydrodynamical evolution of the source, and can be most easily interpreted by models where the GRB ejecta are collimated in a jet. Employing a simple jet model to interpret the observations, we derive a jet opening angle of $θ_o = 0.08$, reducing the isotropic gamma-ray emission of $2.9 \times 10^{53}$ erg by a factor $\sim 300$. If the jet interpretation is correct, we conclude that GRB observations to-date are consistent with an energy for the central source of $E \lsim 10^{52}$ erg.

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arXiv1999

BeppoSAX Detection and Follow-up of GRB980425

We present BeppoSAX GRBM and WFC light curves of GRB980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart.

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arXiv1999

The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope

Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO (Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in optical is the first one to show good evidence for jet-like outflow (Stanek et al. 1999, Harrison et al. 1999). We report the detection of significant linear polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger, and after 1.81 days is consistent with that same value, but much more uncertain. The polarization angle is constant on a time scale of hours, and may be constant over one day. We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission, and discuss the random and ordered field geometries that may be responsible for it.

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arXiv1998

The absorption properties of gamma-ray burst afterglows measured by BeppoSAX

We present an analysis of the X-ray absorption properties of 6 gamma-ray burst (GRB) afterglows measured with BeppoSAX. Between 8 hrs and 20 hrs after the initial GRB trigger, individual spectra can be described by a power-law with a photon index of ~2 and absorption marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero absorption at the >99.999 confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic absorption in the direction of each of the bursts (chi^2 =9.6 for 6 degrees of freedom). The data are consistent with cosmological models in which GRB occur within host galaxies. By simultaneously fitting a power-law spectral model with absorption fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average absorption within the host objects is found to be (1.01 ^0.28_0.51) 10^22 atom cm^-2. This value is compatible with the host galaxy column densities inferred from optical data for GRB970508, GRB971214 and GRB980329, supporting the hypothesis that GRB occur within heavily absorbed star forming regions of their host galaxies.

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arXiv1998

Evidence for a late-time outburst of the X-ray afterglow of GB970508 from BeppoSAX

The gamma-ray burst GB970508 was observed simultaneously by the Gamma Ray Burst Monitor (GRBM) and one of the X-ray Wide Field Cameras (WFC) aboard BeppoSAX. The latter provided a position within 1.9' radius. A series of follow-up observations with the Narrow Field Intruments (NFI) was then performed in a period from 6 hours to 6 days after the main event. A previously unknown source, which we associate with the afterglow of the GRB, was discovered in the error box. We find that, after the initial burst, X-ray emission is still present and decays as t^(-1.1) up to 6 times 10^4 s. This is followed by a burst of activity with a duration 10^5 s. The energy produced in this event is a substantial fraction of the total energy of the GRB, which means that the afterglow is not a remnant of the initial burst (the GRB) that fades away smoothly. Our results support the idea that the processes generating the GRB and its afterglow are the same.

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arXiv1997

Identification of the Periodic Hard X-Ray Transient GRO J1849-03 with the X-Ray Pulsar GS 1843-02 = X1845-024 - a New Be/X-Ray Binary

We identify the periodic transient hard X-ray source GRO J1849-03 with the transient x-ray pulsar GS 1843-02 = X1845-024 based on the detection of x-ray outbursts from X1845-024 coincident with hard x-ray outbursts of GRO J1849--03. Based on its spin period of 94.8 s and its orbital period of 241 days, we classify the system as a Be/X-ray binary.

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