arXiv subjects

P. Soffitta

Publications and source records attributed to P. Soffitta.

At least 109 records · Page 6Linked to original sources

arXiv2006

Opening a New Window to Fundamental Physics and Astrophysics: X-ray Polarimetry

An extensive theoretical literature predicts that X-ray Polarimetry can directly determine relevant physical and geometrical parameters of astrophysical sources, and discriminate between models further than allowed by spectral and timing data only. X-ray Polarimetry can also provide tests of Fundamental Physics. A high sensitivity polarimeter in the focal plane of a New Generation X-ray telescope could open this new window in the High Energy Sky.

astro-ph↗

arXiv2006

Gas Pixel Detectors for X-ray Polarimetry applications

We discuss a new class of Micro Pattern Gas Detectors, the Gas Pixel Detector (GPD), in which a complete integration between the gas amplification structure and the read-out electronics has been reached. An Application-Specific Integrated Circuit (ASIC) built in deep sub-micron technology has been developed to realize a monolithic device that is, at the same time, the pixelized charge collecting electrode and the amplifying, shaping and charge measuring front-end electronics. The CMOS chip has the top metal layer patterned in a matrix of 80 micron pitch hexagonal pixels, each of them directly connected to the underneath electronics chain which has been realized in the remaining five layers of the 0.35 micron VLSI technology. Results from tests of a first prototype of such detector with 2k pixels and a full scale version with 22k pixels are presented. The application of this device for Astronomical X-Ray Polarimetry is discussed. The experimental detector response to polarized and unpolarized X-ray radiation is shown. Results from a full MonteCarlo simulation for two astronomical sources, the Crab Nebula and the Hercules X1, are also reported.

astro-ph↗

arXiv2005

Slow and fast components in the X-ray light curves of Gamma-Ray Bursts

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. We present a temporal and spectral study of the light curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the Wide Field Cameras on board BeppoSAX. For some events the time profiles are characterized by peaks superposed on a slowly evolving pedestal, which in some cases becomes less apparent at higher energies. We describe this behaviour with the presence of two components (slow and fast) having different variability time scales. We modelled the time evolution of slow components by means of an analytical function able to describe asymmetric rising and decaying profiles. The residual light curves, after the subtraction of the slow components, generally show structures more similar to the original curves in the highest energy band. Spectral study of these two components was performed evaluating their hardness ratios, used also to derive photon indices. Slow components are found generally softer than the fast ones suggesting that their origin is likely different. Being typical photon indices lower than those of the afterglows there is no evidence that the emission processes are similar. Another interesting possibility is that slow components can be related to the presence of a hot photosphere having a thermal spectrum with kT around a few keV superposed to a rapid variable non-thermal emission of the fast component.

astro-ph↗

arXiv2005

The BeppoSAX catalog of GRB X-ray afterglow observations

We present the X-ray afterglow catalog of BeppoSAX from the launch of the satellite to the end of the mission. Thirty-three X-ray afterglows were securely identified based on their fading behavior out of 39 observations. We have extracted the continuum parameters (decay index, spectral index, flux, absorption) for all available afterglows. We point out a possible correlation between the X-ray afterglow luminosity and the energy emitted during the prompt $γ$-ray event. We do not detect a significant jet signature within the afterglows, implying a lower limit on the beaming angle, neither a standard energy release when X-ray fluxes are corrected for beaming. Our data support the hypothesis that the burst should be surrounded by an interstellar medium rather than a wind environment, and that this environment should be dense. This may be explained by a termination shock located near the burst progenitor. We finally point out that some dark bursts may be explained by an intrinsic faintness of the event, while others may be strongly absorbed.

astro-ph↗

arXiv2005

The puzzling case of GRB 990123: prompt emission and broad-band afterglow modeling

We report on BeppoSAX simultaneous X- and gamma-ray observations of the bright GRB 990123. We present the broad-band spectrum of the prompt emission, including optical, X- and gamma-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. In the framework of the standard fireball model, we discuss the X-ray afterglow observed by the NFIs and its hard X-ray emission up to 60 keV several hours after the burst, detected for about 20 ks by the PDS. Considering the 2-10 keV and optical light curves, the 0.1-60 keV spectrum during the 20 ks in which the PDS signal was present and the 8.46 GHz upper limits, we find that the multi-wavelength observations cannot be readily accommodated by basic afterglow models. While the temporal and spectral behavior of the optical afterglow is possibly explained by a synchrotron cooling frequency between the optical and the X-ray energy band during the NFIs observations, in X-rays this assumption only accounts for the slope of the 2-10 keV light curve, but not for the flatness of the 0.1-60 keV spectrum. Including the contribution of Inverse Compton (IC) scattering, we solve the problem of the flat X-ray spectrum and justify the hard X-ray emission; we suggest that the lack of a significant detection of 15-60 keV emission in the following 75 ks and last 70 ks spectra, should be related to poorer statistics rather than to an important suppression of IC contribution. However, considering also the radio band data, we find the 8.46 GHz upper limits violated. On the other hand, leaving unchanged the emission mechanism requires modifying the hydrodynamics by invoking an ambient medium whose density rises rapidly with radius and by having the shock losing energy. Thus we are left with an open puzzle which requires further inspection.

astro-ph↗

arXiv2005

The GRB of 1999 January 23: prompt emission and broad-band afterglow modeling

We report on BeppoSAX simultaneous X- and gamma-ray observations of the bright gamma-ray burst (GRB) 990123. We present the broad-band spectrum of the prompt emission, including optical, X- and gamma-rays, confirming the suggestion that the emission mechanisms at low and high frequencies must have different physical origins. We discuss the X-ray afterglow observed by the Narrow Field Instruments (NFIs) on board BeppoSAX and its hard X-ray emission up to 60 keV several hours after the burst, in the framework of the standard fireball model. The effects of including an important contribution of Inverse Compton scattering or modifying the hydrodynamics are studied.

astro-ph↗

arXiv2005

GRB990123: multiwavelength afterglow study

We report on the BeppoSAX and multiwavelength data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. The backwards extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus indicating that both are consistent with being produced by the same phenomenon. For the X-ray afterglow we found a power-law decay with index alpha_X = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 2. An extensive set of multiwavelength observations on the afterglow, collected from the literature and made during the BeppoSAX pointing, allowed constructing a Spectral Flux Distribution. We performed an analysis of the GRB afterglow emission in the context of the "fireball" model.

astro-ph↗

arXiv2005

The puzzling case of GRB 990123: multiwavelength afterglow study

We report on the BeppoSAX data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123, one of the brightest GRBs detected by BeppoSAX. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. For the X-ray afterglow we found a power-law decay with index alpha = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 1.9. The backward extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus suggesting that both emissions are produced by the same phenomenon. An extensive set of multiwavelength observations for the GRB 990123 afterglow made during the BeppoSAX pointing was collected from the literature. The hard X-ray to radio range coverage allowed to construct a spectral flux distribution and to perform an analysis of the GRB afterglow in the context of the "fireball" model. We also report the results of temporal and spectral analysis of an X-ray source serendipitously observed about 22 arcmin north of the GRB afterglow, along with the optical spectroscopy of its possible counterpart to establish the nature of this source.

astro-ph↗

arXiv2004

Probing the environment in Gamma-ray bursts: the case of an X-ray precursor, afterglow late onset and wind vs constant density profile in GRB011121 and GRB011211

In this paper we present BeppoSax and XMM observations of two long Gamma-ray bursts, the X-ray rich event of December 11, 2001 (GRB011211) and the hard and very bright event of November 21, 2001 (GRB011121). In both events we find evidence of a late X-ray burst, taking place several minutes after the prompt emission. The temporal and spectral behaviour of this phenomenon suggest that it represents the onset of the afterglow. Broad band spectral modelling of the afterglow indicate that the fireball evolution in the December burst takes place in a ISM environment. On the contrary in the November burst the wind case is revealed by an X-ray decay slower than that observed in the optical ($δ_{\rm X}=1.29\pm0.04$ vs $δ_{\rm O}=1.66\pm0.06$). The wind profile should change into a constant density profile at large radii, in order to reconcile late-time radio data with a jet. Two other results are obtained for this burst. An X-ray burst is preceding by about 30 s the much harder GRB. Contrary to the prediction of simple models of precursor activity for collapsars, its spectrum is not consistent with a black body. Finally, a substantial absorption column ($\NH=(7\pm2)\times10^{22}$ cm$^{-2}$) is detected during the early part of the prompt emission. This is much greater than that of the wind, and it is thus likely associated with the region surrounding the burst.

astro-ph↗

arXiv2004

Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27

We report on the X-ray observations of the X-ray flash (XRF) which occurred on 2002 April 27, three days before BeppoSAX was switched off. The event was detected with the BeppoSAX Wide Field Cameras but not with the Gamma ray Burst Monitor. A follow-up observation with the BeppoSAX Narrow Field Instruments was soon performed and a candidate afterglow source was discovered. We present the results obtained. We also include the results obtained from the observations of the XRF field with the Chandra X-ray satellite. The spectral analysis of the prompt emission shows that the peak energy of the EF(E) spectrum is lower than 5.5 keV, with negligible spectral evolution. The X-ray afterglow spectrum is consistent with a power law model with photon index of about 2, while the 2-10 keV flux fades as a power law with a decay index -1.33. Both these indices are typical of GRBs. A very marginal excess around 4.5-5 keV is found in the afterglow spectrum measured by BeppoSAX . As for many GRBs, the extrapolation of the 2-10 keV fading law back to the time of the prompt emission is consistent with the X-ray flux measured during the second part of the event. We estimate a possible range of values of the redshift and discuss our results in the light of current models of XRFs.

astro-ph↗

arXiv2004

Reading a GEM with a VLSI pixel ASIC used as a direct charge collecting anode

In MicroPattern Gas Detectors (MPGD) when the pixel size is below 100 micron and the number of pixels is large (above 1000) it is virtually impossible to use the conventional PCB read-out approach to bring the signal charge from the individual pixel to the external electronics chain. For this reason a custom CMOS array of 2101 active pixels with 80 micron pitch, directly used as the charge collecting anode of a GEM amplifying structure, has been developed and built. Each charge collecting pad, hexagonally shaped, realized using the top metal layer of a deep submicron VLSI technology is individually connected to a full electronics chain (pre-amplifier, shaping-amplifier, sample and hold, multiplexer) which is built immediately below it by using the remaining five active layers. The GEM and the drift electrode window are assembled directly over the chip so the ASIC itself becomes the pixelized anode of a MicroPattern Gas Detector. With this approach, for the first time, gas detectors have reached the level of integration and resolution typical of solid state pixel detectors. Results from the first tests of this new read-out concept are presented. An Astronomical X-Ray Polarimetry application is also discussed.

physics.ins-det↗

arXiv2003

The 2001 April Burst Activation of SGR 1900+14: X-ray afterglow emission

After nearly two years of quiescence, the soft gamma-ray repeater SGR 1900+14 again became burst-active on April 18 2001, when it emitted a large flare, preceded by few weak and soft short bursts. After having detected the X and gamma prompt emission of the flare, BeppoSAX pointed its narrow field X-ray telescopes to the source in less than 8 hours. In this paper we present an analysis of the data from this and from a subsequent BeppoSAX observation, as well as from a set of RossiXTE observations. Our data show the detection of an X-ray afterglow from the source, most likely related to the large hard X-ray flare. In fact, the persistent flux from the source, in 2-10 keV, was initially found at a level $\sim$5 times higher than the usual value. Assuming an underlying persistent (constant) emission, the decay of the excess flux can be reasonably well described by a t$^{-0.9}$ law. A temporal feature - a $\sim$half a day long bump - is observed in the decay light curve approximately one day after the burst onset. This feature is unprecedented in SGR afterglows. We discuss our results in the context of previous observations of this source and derive implications for the physics of these objects.

astro-ph↗

arXiv2002

X-ray Astronomical Polarimetry in the XEUS Era

X-ray Polarimetry is almost as old as X-ray Astronomy. Since the first discovery of X-ray sources theoretical analysis suggested that a high degree of linear polarization could be expected due either to the, extremely non thermal, emission mechanism or to the transfer of radiation in highly asymmetric systems. The actual implementation of this subtopic was, conversely, relatively deceiving. This is mainly due to the limitation of the conventional techniques based on the Bragg diffraction at 45deg, or on Thomson scattering around 90deg. Acually no X-ray Polarimeter has been launched since 25 years. Nevertheless the expectations from such measurement on several astrophysical targets including High and Low Mass X-Ray Binaries, isolated neutron Stars, Galactic and Extragalactic Black Holes is extremely attractive. We developed a new technique to measure the linear polarization of X-ray sources. It is based on the visualization of photoelectron tracks in a, finely subdivided, gas filled detector (micropattern). The initial direction of the photoelectron is derived and from the angular distribution of the tracks the amount and angle of polarization is computed. This technique can find an optimal exploitation in the focus of XEUS-1. Even in a very conservative configuration (basically the already existing prototype) the photoelectric polarimeter could perform polarimetry at % level on many AGNs. Further significant improvements can be expected from a technological development on the detector and with the use of XEUS-2 telescope.

astro-ph↗

arXiv2002

The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB

The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

astro-ph↗

arXiv2002

Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts

We present the main results of a study of spectral and energetics properties of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the redshift estimates and the good-quality BeppoSAX time-integrated spectra we deduce the main properties of GRBs in their cosmological rest frames. All spectra in our sample are satisfactorily represented by the Band model with no significant soft X-ray excesses or spectral absorptions. We find a positive correlation between the estimated total (isotropic) energies in the 1-10000 keV energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are characterized also by larger peak energies E_p of their EF(E) spectra. Furthermore, more distant GRBs appear to be systematically harder in the X-ray band compared to GRBs with lower redshifts. We discuss how selection and data truncation effects could bias our results and give possible explanations for the correlations that we found.

astro-ph↗

arXiv2001

The BeppoSAX View on the 2001 Reactivation of SGR 1900+14

After a couple of years of quiescence, the soft gamma repeater SGR 1900+14 suddenly reactivated on 18 April 2001, with the emission of a very intense, long and modulated flare, only second in intensity and duration to the 27 August 1998 giant flare. BeppoSAX caught the large flare with its Gamma Ray Burst Monitor and with one of the Wide Field Cameras. The Wide Field Cameras also detected X-ray bursting activity shortly before the giant flare. A target of opportunity observation was started only 8 hours after the large flare with the Narrow Field Instruments, composed of two 60-ks long pointings. These two observations show an X-ray afterglow of the persistent SGR 1900+14 source, decaying with time according to a power law of index -0.6.

astro-ph↗

arXiv2001

GRB 990704: the most X-ray rich BeppoSAX gamma-ray burst

We present the X- and $γ$-ray detection of GRB 990704 and the discovery and study of its X-ray afterglow, 1SAX J1219.5-0350. Two pointed BeppoSAX observations with the narrow field instruments were performed on this source, separated in time by one week. The decay of the X-ray flux within the first observation appears unusually slow, being best-fit by a power law with negative index 0.83$\pm$0.16. Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAX-WFC detected sample, unprecedentedly high ratios of X- and gamma-ray peak fluxes (F$_{2-10 keV}$/F$_{40-700 keV}$$\sim$0.6, and F$_{2-26 keV}$/F$_{40-700 keV}$$\sim$1.6) and fluences (S$_{2-10 keV}$/S$_{40-700 keV}$$\sim$1.5 and S$_{2-26 keV}$/S$_{40-700 keV}$$\sim$2.8), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.

astro-ph↗

arXiv2001

BeppoSAX measurements of the bright gamma-ray burst 010222

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of 7.8E53 erg, 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 2-10 keV flux 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 fireball is braked to the non-relativistic regime quickly (within a fraction of day) by a dense 1E6 cm-3 circumburst medium. This implies that, after a mild beaming correction, GRB010222 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.

astro-ph↗