arXiv subjects

Davide Lazzati

Publications and source records attributed to Davide Lazzati.

137 records · Page 8Linked to original sources

arXiv1999

Polarization lightcurves and position angle variation of beamed gamma-ray bursts

The recently detected linear polarization in the optical lightcurve of GRB 990510 renewed the interest on how polarization can be produced in gamma-ray burst fireballs. Here we present a model based on the assumption that we are seeing a collimated fireball, observed slightly off-axis. This introduces some degree of anisotropy, and makes it possible to observe a linearly polarized flux even if the magnetic field is completely tangled in the plane orthogonal to the line of sight. We construct the lightcurve of the polarization flux, showing that it is always characterized by two maxima, with the polarization position angle changing by 90 deg. between the first and the second maximum. The very same geometry here assumed implies that the total flux initially decays in time as a power law, but gradually steepens as the bulk Lorentz factor of the fireball decreases.

astro-ph↗

arXiv1999

GRB Redshift determination in the X-ray band

If gamma-ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X-ray spectrum. Such features can be detected by existing and planned X-ray satellites, as long as the X-ray afterglow is observed after a few minutes from the burst. Detection of these X-ray features will make possible the determination of the redshift of gamma-ray bursts even when their optical afterglows are severely dimmed by extinction.

astro-ph↗

arXiv1999

Iron line in the afterglow: a key to the progenitor

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectrum of GRB 970508 and GRB 970828, if confirmed, would be a major breakthrough for understanding the nature the progenitor of gamma-ray bursts. We show that a large mass of iron very close to the burster is necessary to produce the emission line. This in itself strongly limits the possible progenitor of the gamma-ray event, suggesting the former explosion of a supernova, as predicted in the Supranova model (Vietri & Stella 1998). The line emission process and the line intensity depend strongly on the age, density and temperature of the remnant. The simultaneous observation of the iron line and of a power-law optical afterglow lasted for one year strongly suggest that the burst emission is isotropic. Recent observations of GRB 990123 are also discussed.

astro-ph↗

arXiv1999

Iron line afterglows: general constraints

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors, strongly suggesting the presence of a large, iron rich, mass in the vicinity of the burst event. Model-independent limits to the size and the mass of the the iron line emitting region are derived and discussed. We also discuss how these results can be used to constrain the amount of beaming or anisotropy of the burst emission.

astro-ph↗

arXiv1999

Iron line afterglows: how to produce them

We discuss how a powerful iron line emission can be produced if ~1-5 iron rich solar masses are concentrated in the close vicinity of the burst. Recombination, thermal and fluorescent reflection are discussed. We find that recombination suffers the high Compton temperature of the plasma while the other two scenarios are not mutually exclusive and could account for the claimed iron line detected in two afterglows.

astro-ph↗

arXiv1999

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm

We present a new detection algorithm based on the wavelet transform for the analysis of high energy astronomical images. The wavelet transform, due to its multi-scale structure, is suited for the optimal detection of point-like as well as extended sources, regardless of any loss of resolution with the off-axis angle. Sources are detected as significant enhancements in the wavelet space, after the subtraction of the non-flat components of the background. Detection thresholds are computed through Monte Carlo simulations in order to establish the expected number of spurious sources per field. The source characterization is performed through a multi-source fitting in the wavelet space. The procedure is designed to correctly deal with very crowded fields, allowing for the simultaneous characterization of nearby sources. To obtain a fast and reliable estimate of the source parameters and related errors, we apply a novel decimation technique which, taking into account the correlation properties of the wavelet transform, extracts a subset of almost independent coefficients. We test the performance of this algorithm on synthetic fields, analyzing with particular care the characterization of sources in poor background situations, where the assumption of Gaussian statistics does not hold. For these cases, where standard wavelet algorithms generally provide underestimated errors, we infer errors through a procedure which relies on robust basic statistics. Our algorithm is well suited for the analysis of images taken with the new generation of X-ray instruments equipped with CCD technology which will produce images with very low background and/or high source density.

astro-ph↗

arXiv1999

The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. II: application to the HRI and first results

The wavelet detection algorithm (WDA) described in the accompanying paper by Lazzati et al. is made suited for a fast and efficient analysis of images taken with the High Resolution Imager (HRI) instrument on board the ROSAT satellite. An extensive testing is carried out on the detection pipeline: HRI fields with different exposure times are simulated and analysed in the same fashion as the real data. Positions are recovered with few arcsecond errors, whereas fluxes are within a factor of two from their input values in more than 90% of the cases in the deepest images. At variance with the ``sliding-box'' detection algorithms, the WDA provides also a reliable description of the source extension, allowing for a complete search of e.g. supernova remnant or cluster of galaxies in the HRI fields. A completeness analysis on simulated fields shows that for the deepest exposures considered (~120 ks) a limiting flux of \~3x10^{-15} erg/cm2/s can be reached over the entire field of view. We test the algorithm on real HRI fields selected for their crowding and/or presence of extended or bright sources (e.g. cluster of galaxies and of stars, supernova remnants). We show that our algorithm compares favorably with other X-ray detection algorithms such as XIMAGE and EXSAS. A complete catalog will result from our analysis: it will consist of the Brera Multi-scale Wavelet Bright Source Catalog (BMW-BSC) with sources detected with a significance >4.5 sigma and of the Faint Source Catalog (BMW-FSC) with sources at >3.5 sigma. A conservative estimate based on the extragalactic log(N)-log(S) indicates that at least 16000 sources will be revealed in the complete analysis of the whole HRI dataset.

astro-ph↗

arXiv1999

Iron line in the afterglow: a key to unveil Gamma-Ray Burst progenitors

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors. Piro et al. (1999) and Yoshida et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB 970828, respectively. We discuss how such a strong line could be produced in the various scenarios proposed for the event progenitor. We show that the observed line intensity requires a large iron mass, concentrated in the vicinity of the burst. The previous explosion of a supernova, predicted in the Supranova scenario, is the most straightforward way to account for such a large amount of matter. We discuss three different physical processes that could account for the line: recombination, reflection and thermal emission. Among these, reflection and thermal emission may explain the observed line features: reflection should be important if the remnant is optically thick, while thermal lines can be produced only in a thin plasma. The recombination process requires extremely high densities to efficiently reprocess the burst photons, whereas this process could work during the X-ray afterglow. Future key observations for discriminating the actual radiating process are discussed.

astro-ph↗

arXiv1998

EISily looking for distant galaxy clusters

We present a new algorithm to search for distant clusters of galaxies on catalogues deriving from imaging data, as those of the ESO Imaging Survey. We discuss the advantages of our technique relative to the ones developed before and present preliminary results of its application to the I--band data of the survey's patch A.

astro-ph↗

arXiv1998

On the location of X-ray peaks and dominant galaxies in clusters

We analyze the structure of the X-ray emission of a sample of 22 Abell clusters of galaxies with a cD in their centre, observed with the ROSAT PSPC. Utilizing the multi-scale power of the Wavelet Transform we detect significant (~ 50 h^{-1} kpc) offsets between the large scale centroid and the peak of X-ray emission. Despite the uncertainties on the satellite pointing, the X-ray to optical correlation indicates a likely association between the X-ray peak and the dominant galaxy. We develop a model in which the offset is produced by small amplitude oscillations of the cD galaxy around the bottom of the cluster potential well and successfully compare it to the observed distribution of offsets. Within this scenario the offsets are not due to dynamic instabilities and the number of structured clusters is greatly reduced.

astro-ph↗

arXiv1996

On the relationship between the periodic and aperiodic variability of accreting X-ray pulsars

Besides the narrow peaks originating from the periodic signal, the power spectra of accreting X-ray pulsars display continuum components usually increasing towards low frequencies; these arise from the source aperiodic variability. Most studies so far adopted the view that the periodic and aperiodic variations are independent. However any aperiodic variability in the emission from the accretion column(s) towards the magnetic neutron star should be modulated at the X-ray pulsar period, by virtue of the same rotation-induced geometric effects which give rise to the periodic signal. We develop here a simple shot noise model to test the presence of a coupling between the periodic and aperiodic variability of X-ray pulsars. The model power spectrum is fit to the power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62 and Cen X-3 observed with EXOSAT. In the first two cases we find that a highly significant coupling is present, as testified by a substantialbroadening in the wings of the power spectrum peaks due to the periodic modulation. We find also that these wings can mimick the presence of a knee in the continuum power spectrum components around the fundamental of the periodic modulation, therefore questioning the correlation reported by Takeshima (1992) between the X-ray pulsar frequency and the knee frequency, beyond which the continuum power spectral component steepens.

astro-ph↗