Showing papers by "F. Frontera published in 2004"

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

Abstract: 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 ($\delta_{\rm X}=1.29\pm0.04$ vs $\delta_{\rm O}=1.66\pm0.06$). 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.

A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows

Abstract: We present the first high resolution (R=20000--45000, corresponding to 14 km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. These spectra show that the inter--stellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spanning a total velocity range of up to about 3000 km/s. Several narrow components are resolved down to a width of a few tens of km/s. In the case of GRB021004 we detected both low and high ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

On the Soft Excess in the X-ray Spectrum of Cir X-1: Revisitation of the Distance to Cir X-1

Abstract: We report on a 300 ks BeppoSAX (0.12-200 keV) observation of Circinus X-1 (Cir X-1) at phases between 0.62 and 0.84 and on a 90 ks BeppoSAX observation of Cir X--1 at phases 0.11-0.16. Using the canonical model adopted until now to fit the energy spectrum of this source large residuals appear below 1 keV. These are well fitted using an equivalent hydrogen column of $0.66 \times 10^{22}$ cm$^{-2}$, adding absorption edges of O VII, O VIII and Ne IX in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of O VII, O VIII, Mg XI and Mg XII and absorption lines of O VIII and Mg XII in the spectra extracted from the observation at phases 0.11-0.16. During the observation at phases 0.62--0.84 the electron density associated to the ionized matter is $\sim 10^{13}$ cm$^{-3}$ remaining quit constant going away from the compact object. During the observation at phases 0.11-0.16 the electron density profile varies along the distance going from $\sim 6 \times 10^{13}$ cm$^{-3}$ at $\sim 10^{11}$ cm to $\sim 9 \times 10^{10}$ cm$^{-3}$ at $\sim 10^{13}$ cm. The equivalent hydrogen column towards Cir X-1 is thre times lower than the value obtained from previous models. This low value would imply that Cir X-1 is at a distance of 4.1 kpc.

A DECREASING COLUMN DENSITY DURING THE PROMPT EMISSION FROM GRB 000528 OBSERVED WITH BeppoSAX

Abstract: We report observation results of the prompt X- and gamma-ray emission from GRB 000528. This event was detected with the Gamma-Ray Burst Monitor and one of the two Wide Field Cameras aboard the BeppoSAX satellite. The gamma-ray burst (GRB) was promptly followed on with the BeppoSAX narrow field instruments and with ground optical and radio telescopes. The X-ray afterglow source was identified, but no optical or radio counterpart found. We report here results from the prompt and afterglow emission analysis. The main feature derived from spectral evolution of the prompt emission is a high hydrogen equivalent column density with evidence of its decrease with time. We model this behavior in terms of a time-dependent photoionization of the local circumburst medium, finding that a compact and dense environment is required by the data. We also find a fading of the late part of the 2‐10 keV prompt emission, which is consistent with afterglow emission. We discuss this result in the light of the external shock model scenario. Subject headingg gamma rays: bursts — gamma rays: observations — X-rays: bursts

The prompt x-ray emission of grb 011211: possible evidence of a transient absorption feature

Abstract: We report on observation results of the prompt X– and –ray emission from GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart of the GRB was soon identified and its redshift determined (z = 2.140), while with the XMM-Newton satellite, the X–ray afterglow emission was detected. Evidence of soft X–ray emission lines was reported by Reeves et al. (2002), but not confirmed by other authors. In investigating the spectral evolution of the prompt emission we find the possible evidence of a transient absorption feature at 6.9 +0.6 −0.5 keV during the rise of the primary event. The significance of the feature is derived with non parametric tests and numerical simulations, finding a chance probability which ranges from 3 × 10 −3 down to 4 × 10 −4 . The feature shows a Gaussian profile and an equivalent width of 1.2 +0.5 −0.6 keV. We discuss our results and their possible interpretation.

A BeppoSAX study of the Galactic Z-source GX 340+0

Abstract: We present the results of a BeppoSAX broad band (0.1–200 keV) observation of the Z-source GX 340+0. The 1.8–30 keV continuum is well described by a blackbody ( kT BB ∼0.5 keV) plus a Comptonized component with seed photons temperature ∼ 1 keV and electron temperature ∼ 3 keV. A hard tail dominates the spectrum above 30 keV. It can be fitted using a bremsstrahlung component or, equivalently, a powerlaw (with a low-energy cutoff). We detect also a Gaussian line at ∼6.8 keV and an absorption edge at ∼9.2 keV. A low-energy (∼1 keV) unresolved feature needs further investigations.

A decreasing column density during the prompt emission from GRB000528 observed with BeppoSAX

Abstract: We report observation results of the prompt X-- and $\gamma$--ray emission from GRB 000528. This event was detected with the Gamma-Ray Burst Monitor and one of the two Wide Field Cameras aboard the BeppoSAX satellite. The GRB was promptly followed on with the BeppoSAX Narrow Field Instruments and with ground optical and radio telescopes. The X-ray afterglow source was identified, but no optical or radio counterpart found. We report here results from the prompt and afterglow emission analysis. The main feature derived from spectral evolution of the prompt emission is a high hydrogen-equivalent column density with evidence of its decrease with time. We model this behavior in terms of a time-dependent photoionization of the local circum-burst medium, finding that a compact and dense environment is required by the data. We also find a fading of the late part of the 2--10 keV prompt emission which is consistent with afterglow emission. We discuss this result in the light of the external shock model scenario.

Multiwavelength study of the very long GRB 020410

Abstract: GRB 020410 is by far the longest gamma-ray burst (with a duration of about 1600 s) to have been followed up from the X-ray through the radio regime. Afterglow emission was detected in X-rays and at optical wavelengths while no emission was detected at 8 GHz brighter than 120 microJy. The decaying X-ray afterglow, back extrapolated to 11 hr after the burst, had a flux of 7.9 10^-12 cgs (2-10 keV); the brightest detected so far. No direct redshift determination is available yet for this GRB, but according to the empirical relationship between the peak energy in the u F_ u spectrum and the isotropic energy output, z is constrained in the range 0.9-1.5. The reconstructed optical afterglow light curve implies at least two breaks in the simple power-law decay. This may be related to emergence of a SN, or refreshment of the external shock by a variation in the circumstellar medium. By comparing the backward extrapolation of the 2-10 keV afterglow decay it is shown that the long duration of the prompt emission is not related to an early onset of afterglow emission, but must be related to prolonged activity of the ``central engine''.

The prompt X-ray emission of GRB011211: possible evidence of a transient absorption feature

Abstract: We report on observation results of the prompt X- and gamma-ray emission from GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart to the GRB was soon identified and its redshift determined (z = 2.140), while with the XMM-Newton satellite, the X-ray afterglow emission was detected. Evidence of soft X-ray emission lines was reported by Reeves et al. (2002), but not confirmed by other authors. In investigating the spectral evolution of the prompt emission we find the possible evidence of a transient absorption feature at 6.9^{+0.6}_{-0.5} keV during the rise of the primary event. The significance of the feature is derived with non parametric tests and numerical simulations, finding a chance probability which ranges from 3x10^{-3} down to 4x10^{-4}. The feature shows a Gaussian profile and an equivalent width of 1.2^{+0.5}_{-0.6} keV. We discuss our results and their possible interpretation.

The X-Ray Facility of the Physics Department of the Ferrara University

Abstract: We will report on the equipment and performance of the X-ray facility of the University of Ferrara. Initially developed to test the PDS (Phoswich Detection System) instrument aboard the BeppoSAX satellite and to perform reflectivity measurements of mosaic crystal samples of HOPG (Highly Oriented Pyrolytic Graphite), with time the facility has been improved and its applications extended. Now these applications include test and calibration of hard X-ray (> 10 keV) detectors, reflectivity measurements of hard X-ray mirrors, reflectivity tests of crystals and X-ray transparency measurements. The facility is being further improved in order to determine the optical axis mosaic crystals in Laue configuration within a project devoted to develop a hard X-ray (> 60 keV) focusing optics (Pisa, A. et al.: in press, Feasibility study of a Laue lens for hard X-rays for space astronomy, SPIE Proc., 5536).

A new BeppoSAX observation of the Z Source GX 349+2

Abstract: We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in February 2001 and covering the broad energy range 012–200 keV The average spectrum is well described by a soft blackbody (kTBB∼05 keV) and a Comptonized component having a seed-photon temperature of kT0∼1 keV, an electron temperature of kTe∼27 keV, and optical depth τ∼11 To well fit the energy spectrum three gaussian lines are needed at 12 keV, 26 keV, and 67 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39 eV, probably associated to L-shell emission of Fe XXIV, Lyα S XVI, and Fe XXV, respectively These lines may be produced at different distances from the neutron star, which increase when the count rate of the source increases An absorption edge is also needed at 9 keV with an optical depth of ∼3×10−2 From the Color-Color Diagram (CD) we selected five zones from which we extracted the corresponding energy spectra

Broadband spectra of XTE J1650-500 with BeppoSAX

Abstract: We report on preliminary results of three Target of Opportunity (TOO) observations of the X-ray transient and black hole candidate XTE J1650-500 performed on 2001 September 11-12, September 21-23, and October 3-4 respectively, with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. Inspection of the light curves in different energy bands clearly shows an extreme variability both temporal and spectral. This variability has the consequence that spectra provide acceptable fits only if integrated on a temporal scale of order 104s or even less. However, all spectra are fit by the same model with some parameters that remain unchanged during the 3 TOOs, and others that dramatically change on the above time scale. The model used is a two-phase accretion disk corona model, in which a hot corona with a hybrid electron distribution comptonizes seed photons from a disk. Reflection is important and ionization seems to be very small or even zero. During the first TOO a broad Fe Kαline is clearly seen.

Twin shaping filter techniques for compensating the signals in CZT/CdTe detectors

Abstract: Our group is currently involved in the proposal of a Gamma Ray Burst Monitor for the LOBSTER experiment, approved by ESA for a Phase A study for a future flight (2009) aboard the International Space Station (ISS) The detector will be based on CZT array modules, cooled along with its front-end electronics, at a temperature not lower than 250 K, using Peltier effect or passive cooling systems To improve the detector performance several hardware and software techniques are being tested In addition to the strip readout technique for rejecting charged particles which interact with the detector, we have investigated a method that utilizes a pair of pulse shaping active filters (one slow and one fast) to analyze the same signal from the charge sensitive preamplifier This technique could be particularly suitable for implementation in a readout integrated circuit for multipixel detectors We present the experimental results obtained with the application of the biparametric technique on CdZnTe/CdTe detectors with planar electrodes and segmented in order to study the distribution of the correction parameters and to verify the reliability of applying a mean correction on groups of pixels Furthermore we will compare the results obtained applying this technique, both using the differentiator and integrator stage of the shaping amplifier and switching off the integrator stage for the fast signal formation to achieve a more direct information on electron rise time

The two large flares from SGR1900+14 observed with the BeppoSAX Gamma-Ray Burst Monitor: new results

Abstract: We present the results of a thorough timing and spectral analysis of the BeppoSAX Gamma-Ray Burst Monitor data of the two large flares from SGR1900+14: the giant one of August 27, 1998 and the intermediate one of April 18, 2001. We compare the two flares, showing interesting common spectral and temporal properties, despite their apparent different profiles and fluences. New findings concerning the presence of timing noise and the time-averaged energy spectra are discussed and interpreted in the light of the magnetar model.

The optical/near-IR spectral energy distribution of the GRB 000210 host galaxy

Abstract: We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of spectral templates to its Spectral Energy Distribution (SED), we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The best fit to the SED is obtained with a blue starburst template with an age of 0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.

GRB 030131: a long Gamma-Ray Burst detected with INTEGRAL during a satellite slew

Abstract: A long Gamma-Ray Burst (GRB) was detected with the instruments on board the INTEGRAL satellite on January 31 2003. Although most of the GRB, which lasted ∼150 seconds, occurred during a satellite slew, the automatic software of the INTEGRAL Burst Alert System was able to detect it in near-real time. Here we report the results obtained with the IBIS instrument, which detected GRB 030131 in the 15 keV - 200 keV energy range, The burst displays a complex time profile with numerous peaks. The peak spectrum can be described by a single power law with photon index Γ≃1.7 and has a flux of ∼2 photons cm −2 s −1 in the 20-200 keV energy band. The high sensitivity of IBIS has made it possible for the first time to perform detailed time-resolved spectroscopy of a GRB with a fluence of 7×10 −6 erg cm −2 (20-200 keV).

Presence of a soft excess between 0.6 keV and 0.9 keV in the energy spectrum of Cir X-1

Abstract: We report on the results of a new BeppoSAX (0.12–200 keV) observation of the peculiar X–ray binary source Circinus X–1 (Cir X–1) near the apastron. We produced a color-color diagram and selected seven different zones. We fitted the spectra obtained from each zone using a model consisting of a blackbody component, at a temperature of ∼0.5 keV, and a Comptonized component, with a seed-photon temperature of ∼1 keV, electron temperature of ∼2.7 keV and optical depth of ∼11. A soft excess between 0.6 keV and 0.9 keV is present in four out of the seven extracted spectra. To fit the soft excess we tried several continuum emission models, and we find good results only adding a further blackbody component absorbed by a different equivalent hydrogen column, an order of magnitude lower than the absorption of the other two components; its physical interpretation is however hard to address.

Monte Carlo evaluation of polarimetric capabilities of a hard X-ray CdTe focal plane

Abstract: High energy polarimetry in astrophysics is a quite unexplored field, as well as optical solutions for polarimeters Herein we propose and compare two different CdTe based polarimeter optics for X-ray and /spl gamma/-ray astrophysics The first configuration that we propose is a CdTe pixelised detector for multilayer telescope operating between 10 and 100 keV In this case we are dealing with a 2 to 5 mm thick and small area detector with a very fine spatial resolution: pixel pitch of 05 mm, because typically the point spread function is of the order of 2-3 mm (FWHM) The disadvantages of this solution are mainly two: the effect of the multilayer mirror reflection on the incoming photons polarisation state and the limited energy range for Compton scattering in CdTe The second configuration proposed is a CdTe thick pixelised matrix detector for Laue optics based telescope In this case, it is required better detection efficient at high energy (from 60 to 400 keV) therefore a CdTe thickness from 3 to 10 mm is required, but for pixel scales of the order of a few mm because the expected point spread function is about 30 mm (FWHM) for photon energies of about 200 keV Modulation Q factors and minimum detectable polarisation are calculated from Monte Carlo simulation of the two type CdTe focal plane detector for the Crab pulsar, as on axes reference source using the analytical response of the two optics type

BeppoSAX study of the spectral and temporal behavior of the black hole candidate XTE J1118+480

Abstract: The high galactic latitude black–hole candidate XTE J1118+480 was followed–up by BeppoSAX during its outburst and transition to quiescence with four TOOs from April to December 2000. In this paper, we summarize and briefly discuss the results of the spectral and temporal data analysis of these observations.

The INTEGRAL LMXRB Monitoring Programme

Abstract: Our collaboration is responsible for the study of a sample of 72 low mass X-ray binaries (LMXRB) using the INTEGRAL Core Programme data. In this paper we describe the monitoring programme we have started and the current variability and spectral results on a sample of 8 persistently bright LMXRBs hosting a neutron star (Z and Atoll sources). Current results show that among our sample of sources there seems to be no important difference in the variability of Z sources with respect to Atolls and the first colour-colour and hardness intensity diagrams built in the "traditional" energy bands display the expected patterns. Z sources seem to be harder than the bright Atolls of our sample (above 20 keV) and present no evident cut-off until about 50 keV. A hint of a non-thermal hard tail is seen in Sco X-1 with ISGRI and SPI, similarly to what was previously detected by D'Amico et al. (2001) with RXTE. These results, even if preliminary, show the importance of such a programme and the potential underlying it to understand these sources as a class.

Polarimetric performances of a wide field CZT monitor for X-and /spl gamma/-ray astrophysics

Abstract: Polarimetry in astrophysics had no much development in the hard X- and soft /spl gamma/-ray energy range; in fact no dedicated polarimeters had ever been launched in space. Previous Monte Carlo simulations and prototype experimental measurements were made in order to evaluate the polarimetric performances of pixelised CZT matrices. The results showed that CZT based polarimeters are able to perform polarimetric measurements of short duration polarised emissions like gamma ray bursts. On the behalf of a Gamma Ray Burst Monitor (GRBM) proposal for the LOBSTER experiment, approved by ESA for a Phase A study for a future flight (2009) aboard the International Space Station (ISS), we propose a Monte Carlo polarimetric study of the 4 detection units that compose this instrument. Each of these units is a 24/spl times/12 matrix of CZT elementary crystals. Each pixel has a cross section of 8/spl times/8 mm/sup 2/ , therefore a detection unit has an active area of about 184 cm/sup 2/. The detector thickness is 3 mm as baseline, and 5 mm in the case this thickness is crucial to exploit the GRBM as polarimeter as well. The 4 detection units have a rectangular FOV of 55/spl deg//spl times/35/spl deg/ FVVHM and their axes misaligned with each other by 45/spl deg/ in the direction perpendicular to the ISS motion and 10/spl deg/ along the ISS direction of motion. Energy dependent polarimetric Q factor and detection efficiencies will be determined. Expected GRBM minimum detectable polarisation will be presented and discussed.