arXiv subjects

L. Nicastro

Publications and source records attributed to L. Nicastro.

At least 73 records · Page 4Linked to original sources

arXiv2004

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

We report observation results of the prompt X-- and $γ$--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.

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arXiv2004

Spectral and timing properties of the X-ray emission from the millisecond pulsar PSR B1821-24

We report results on the timing and spectral analysis of observations of the millisecond pulsar PSR B1821-24 with RXTE, BeppoSAX and Chandra. The X-ray light curve is characterized by two narrow peaks at a phase distance of 0.452+/-0.002. The average pulsed emission, over the range 1.6-20 keV, is well represented by a single power law with a photon index alpha=1.30 +0.05 -0.02 and unabsorbed (2-10 keV) pulsed X-ray flux of 3.9x10^(-13) erg cm^(-2) s^(-1). We searched for a possible bunching of X-ray photons to verify if the X ray emission has a time structure similar to that of giant pulses and found a negative result.

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arXiv2004

Multiwavelength study of the very long GRB 020410

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 νF_νspectrum 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''.

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arXiv2004

The Sky Polarization Observatory

SPOrt is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7 degrees. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 microK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

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arXiv2003

BeppoSAX observation of PSR B1937+21

We present the results of a BeppoSAX observation of the fastest rotating pulsar known: PSR B1937+21. The ~200 ks observation (78.5 ks MECS/34 ks LECS on-source time) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The pulse profile is clearly double peaked at energies > ~4 keV. Peak widths are compatible with the instrumental time resolution and the second pulse lags the main pulse 0.52 in phase, like is the case in the radio. In the 1.3-4 keV band we detect a ~45% DC component; conversely the 4-10 keV pulsed fraction is consistent with 100%. The on-pulse spectrum is fitted with an absorbed power-law of spectral index ~1.2, harder than that of the total flux which is ~1.9. The total unabsorbed (2-10 keV) flux is F_{2-10} = 4.1 10^-13 cgs, implying a luminosity of L_X = 5.0 10^31 Θ(d/3.6 kpc)^2 erg s^-1 and a X-ray efficiency of η= 4.5 10^-5 Θ, where Θis the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA and a more recent Rossi-XTE observation. The hydrogen column density N_H ~2 10^22 cm^-2 is ~10 times higher than expected from the radio dispersion measure and average Galactic density of e-. Though it is compatible (within 2σ) with the Galactic (HI derived) value of ~1 10^22 cm^-2, inspection of dust extinction maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4 GHz radio map shows that the nearby (likely unrelated) HII source 4C21.53W is part of a circular emission region ~4' across.

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arXiv2003

The phase of the radio and X-ray pulses of PSR B1937+21

We present timing and spectral results of PSR B1937+21, the fastest known millisecond pulsar (P about 1.56 ms), observed with RXTE. The pulse profile, detected up to 20 keV, shows a double peak with the main component much stronger than the other. The peak phase separation is 0.526 +/- 0.002 and the pulsed spectrum over the energy range 2--25 keV is well described by a power law with a photon index equal to 1.14 +/- 0.07. We find that the X-ray pulses are closely aligned in phase with the giant pulses observed in the radio band. This results suggest that giant radio pulses and X-ray pulses originate in the same region of the magnetosphere due to a high and fluctuating electron density that occasionally emits coherently in the radio band. The X-ray events, however, do not show any clustering in time indicating that no X-ray flares are produced.

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arXiv2003

X/gamma-ray measurements of the faint GRB 020321

GRB 020321 is a faint GRB that received wide follow-up attention in X-rays (BeppoSAX/NFI, Chandra/ACIS-S, XMM-Newton), radio (ATCA) and optical (ESO, HST). We identify a weak X-ray afterglow by a combined study of the Chandra and XMM-Newton observations. Its decay index of 1.2 is typical for GRB afterglows. Inside the 4"-accurate error box there is a weak optical counterpart candidate with a much shallower decay index.

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arXiv2003

XMM-Newton Observations of the Field of Gamma-Ray Burst 980425

The error box of GRB980425 has been observed by XMM-Newton in March 2002, with the aim of measuring the late epoch X-ray emission of the supernova 1998bw and of clarifying its supposed association with the GRB itself. We present here the preliminary results obtained with the EPIC PN camera. Our observations confirm the association between SN 1998bw and GRB980425. The EPIC PN measurement of the SN 1998bw flux is significantly below the extrapolation of the power-law temporal trend fitted to the BeppoSAX points and implies a faster temporal decay. We propose different physical interpretations of the SN X-ray light curve, according to whether it is produced by one or more radiation components.

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arXiv2003

Optical and near-infrared observations of the GRB020405 afterglow

(Abridged) We report on observations of the optical and NIR afterglow of GRB020405. Ground-based optical observations started about 1 day after the GRB and spanned a period of ~10 days; archival HST data extended the coverage up to 70 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of emission lines in the optical spectrum indicates that the GRB is located at z = 0.691. Absorptions are also detected at z = 0.691 and at z = 0.472. The latter system is likely caused by clouds in a galaxy located 2 arcsec southwest of the GRB host. Hence, for the first time, the galaxy responsible for an intervening absorption system in the spectrum of a GRB afterglow is identified. Optical and NIR photometry indicates that the decay in all bands follows a single power law of index alpha = 1.54. The late-epoch VLT and HST points lie above the extrapolation of this power law, so that a plateau is apparent in the VRIJ light curves at 10-20 days after the GRB. The light curves at epochs later than day ~20 after the GRB are consistent with a power-law decay with index alphaprime = 1.85. We suggest that this deviation can be modeled with a SN having the same temporal profile as SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A polarimetric R-band measurement shows that the afterglow is polarized, with P = 1.5 % and theta = 172 degrees. Optical-NIR spectral flux distributions show a change of slope across the J band which we interpret as due to the presence of nu_c. The analysis of the multiwavelength spectrum within the fireball model suggests that a population of relativistic electrons produces the optical-NIR emission via synchrotron in an adiabatically expanding blastwave, and the X-rays via IC.

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arXiv2003

Multiwavelength observations of the GRB000615 field

We report on prompt and afterglow observations of GRB 000615 detected by BeppoSAX. The study of the high-energy prompt event is presented along with the search for its X-ray and optical afterglow. Spectral fits seem to suggest a temporal evolution of the GRB prompt emission. We possibly find evidence for intrinsic N_H (at 90% confidence level) and for a transient spectral emission feature around 8 keV (at 98% confidence level). The X-ray to gamma-ray fluence ratio of 1.73 +/- 0.22 is one of the largest among BeppoSAX GRBs. A weak X-ray source is also detected in the MECS, between 1.6 and 4.5 keV, and its position is compatible with the WFC error box. The behaviour of this source may be compatible with that of an afterglow. Low significance signal is detected in the 0.2-1.5 keV at a position consistent with the WFC and MECS error boxes. The S/N ratio is insufficient to speculate on the nature of this source. There is no evidence of an optical transient down to R ~ 22.

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arXiv2003

The dark burst 010214 with BeppoSAX: possible variable absorption and jet X-ray emission

We report on the prompt and afterglow emission observations of the dark burst GRB010214 with BeppoSAX. The prompt emission shows possible evidence of variable absorption from N_H = 3.0^{+5.1}_{-2.0} x 10^23 cm-2 in the first 6 s of the event to a value consistent with the Galactic column density (N_H^(G) = 2.66 x 10^20 cm-2) in the GRB direction. An X-ray afterglow emission in the 2-10 keV energy band was detected with BeppoSAX, but an analogue search at lower wavelengths (optical, IR and radio) was unsuccessful. The X-ray afterglow spectrum is consistent with a power-law with Galactic absorption. The light curve shows a complex decay, if the tail of the prompt emission is assumed as the onset of the afterglow: if the origin of the afterglow is coincident with the GRB onset, a bump before ~ 3 x 10^4 s is inferred, while if the afterglow is assumed to start later, a steepening of the power-law light curve at t ~ 3 x 10^4 s is deduced. We discuss these results in the light of the current models of afterglows and the possible origin of the GRB darkness. Finally, we tentatively derive an estimate of the burst redshift.

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arXiv2003

The BaR-SPOrt Experiment

BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an experiment to measure the linearly polarized emission of sky patches at 32 and 90 GHz with sub-degree angular resolution. It is equipped with high sensitivity correlation polarimeters for simultaneous detection of both the U and Q stokes parameters of the incident radiation. On-axis telescope is used to observe angular scales where the expected polarization of the Cosmic Microwave Background (CMBP) peaks. This project shares most of the know-how and sophisticated technology developed for the SPOrt experiment onboard the International Space Station. The payload is designed to flight onboard long duration stratospheric balloons both in the Northern and Southern hemispheres where low foreground emission sky patches are accessible. Due to the weakness of the expected CMBP signal (in the range of microK), much care has been spent to optimize the instrument design with respect to the systematics generation, observing time efficiency and long term stability. In this contribution we present the instrument design, and first tests on some components of the 32 GHz radiometer.

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arXiv2002

A comparative study of the X-ray afterglow properties of optically bright and dark GRBs

We have examined the complete set of X-ray afterglow observations of dark and optically bright GRBs performed by BeppoSAX until February 2001. X-ray 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 0.794\pm 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 equal to or higher than 2.6 sigma. 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 possible causes of their behaviour, including a possible occurrence in high density clouds or origin at very high redshift and a connection with ancient, Population III stars.

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arXiv2002

SPOrt: an Experiment Aimed at Measuring the Large Scale Cosmic Microwave Background Polarization

SPOrt (Sky Polarization Observatory) is a space experiment to be flown on the International Space Station during Early Utilization Phase aimed at measuring the microwave polarized emission with FWHM = 7deg, in the frequency range 22-90 GHz. The Galactic polarized emission can be observed at the lower frequencies and the polarization of Cosmic Microwave Background (CMB) at 90 GHz, where contaminants are expected to be less important. The extremely low level of the CMB Polarization signal (< 1 uK) calls for intrinsically stable radiometers. The SPOrt instrument is expressly devoted to CMB polarization measurements and the whole design has been optimized for minimizing instrumental polarization effects. In this contribution we present the receiver architecture based on correlation techniques, the analysis showing its intrinsic stability and the custom hardware development carried out to detect such a low signal.

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arXiv2002

The 2-10 keV emission properties of PSR B1937+21

We present the results of a BeppoSAX observation of the fastest pulsar known: PSR B1937+21. The ~ 200 ks observation (78.5 (34) ks MECS (LECS) exposure times) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The absorbed power law spectral model gave a photon index of ~ 1.7 and N_H ~ 2.3 x 10^22 cm^-2. These values explain both a) the ROSAT non-detection and b) the deviant estimate of a photon index of ~ 0.8 obtained by ASCA. The pulse profile appears, for the first time, clearly double peaked with the main component much stronger than the other. The statistical significance is 10 sigma (main peak) and 5 sigma (secondary peak). The 1.6-10 keV pulsed fraction is consistent with 100%; only in the 1.6-4 keV band there is a ~ 2 sigma indication for a DC component. The secondary peak is detected significantly only for energies above 3 / 4 keV. The unabsorbed (2-10 keV) flux is F_2-10 = 3.7 x 10^-13 erg cm^-2 s^-1, implying a luminosity of L_X = 4.6 x 10^31 Theta (d/3.6 kpc)^2 erg s^-1 and an X-ray efficiency of eta = 4 x 10^-5 Theta, where Theta is the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA.

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arXiv2002

Polarization angular spectra of Galactic synchrotron emission on arcminute scales

We study the angular power spectra of the polarized component of the Galactic synchrotron emission in the 28-deg^2 Test Region of the Southern Galactic Plane Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact Array and allow us to investigate angular power spectra down to arcminute scales. We find that, at this frequency, the polarization spectra for E- and B-modes seem to be affected by Faraday rotation produced in compact foreground screens. A different behavior is shown by the angular spectrum of the polarized intensity PI=\sqrt{Q^2+U^2}. This is well fitted by a power law with slope \~1.7, which agrees with higher frequency results and can probably be more confidently extrapolated to the cosmological window.

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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.

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arXiv2002

SDAMS: SPOrt Data Archiving and Management System

SDAMS is the ensemble of database + software packages aimed to the archiving, quick-look analysis, off-line analysis, network accessibility and plotting of the SPOrt produced data. Many of the aspects related to data archiving, analysis and distribution are common to almost all the astronomical experiments. SDAMS ambition is to face and solve problems like accessibility and portability of the data on any hardware/software platform in a way as simpler as possible, though effective. The system is conceived in a way to be used either by the scientific community interested in background radiation studies or by a wider public with low or null knowledge of the subject. The user authentication system allows us to apply different levels of access, analysis and data retrieving. SDAMS will be accessible through any Web browser though the most efficient way to use it is by writing simple programs. Graphics and images useful for outreach purposes will be produced and put on the Web on a regular basis.

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