arXiv subjects

Giancarlo Ghirlanda

Publications and source records attributed to Giancarlo Ghirlanda.

59 records · Page 4Linked to original sources

arXiv2009

Selection effects on GRB spectral-energy correlations

Instrumental selection effects can act upon the estimates of the peak energy Ep, the fluence F and the peak flux P of GRBs. If this were the case, then the correlations involving the corresponding rest frame quantities (i.e. Ep, Eiso and the peak luminosity Liso) would be questioned. We estimated, as a function of Ep, the minimum peak flux necessary to trigger a GRB and the minimum fluence a burst must have to determine the value of Ep by considering different instruments (BATSE, Swift, BeppoSAX). We find that the latter dominates over the former. We then study the Ep-fluence (and flux) correlation in the observer plane. GRBs with redshift show well defined Ep-F and Ep-P correlations: in this planes the selection effects are present, but do not determine the found orrelations. This is not true for Swift GRBs with redshift, for which the spectral analysis threshold does affect their distribution in the observer planes. Extending the sample to GRBs without z, we still find a significant Ep-F correlation, although with a larger scatter than that defined by GRBs with redshift. We find that 6% are outliers of the Amati correlation. The Ep-P correlation of GRBs with or without redshift is the same and no outlier is found among bursts without redshift.

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arXiv2008

Peak energy of the prompt emission of long Gamma Ray Bursts vs their fluence and peak flux

The spectral-energy and (luminosity) correlations in long GRBs are being hotly debated to establish, first of all, their reality against possible selection effects. These are best studied in the observer planes, namely the peak energy E_peak_obs vs the fluence F or the peak flux P. In a recent paper we started to attack this problem considering all GRBs with known z and spectral properties. Here we consider instead all bursts with known E_peak_obs, irrespective of z, adding to those a sample of 100 faint BATSE bursts representative of a larger population. This allows us to construct a complete, fluence limited, sample, to study the selection/instrumental effects. We found that fainter bursts have smaller E_peak_obs than those of bright events. As a consequence, the E_peak_obs of these bursts is correlated with the fluence, though with a slope flatter than that defined by bursts with z. Selection effects, which are present, are shown not to be responsible for the existence of such a correlation. About 6% of these bursts are surely outliers of the E_peak-E_iso correlation (updated to include 83 bursts), since they are inconsistent with it for any z. E_peak_obs correlates also with P, with a slope similar to the E_peak-L_iso correlation.In this case there is only one sure outlier.The scatter of the E_peak_obs-P correlation defined by the BATSE bursts of our sample is smaller than the E_peak_obs-F correlation of the same bursts, while for the bursts with known z the E_peak-E_iso correlation is tighter than the E_peak-L_iso one. Once a very large number of bursts with E_peak_obs and z will be available, we thus expect that the E_peak-L_iso correlation will be similar to that currently found, whereas it is likely that the E_peak-E_iso correlation will become flatter and with a larger scatter.

astro-ph↗

arXiv2007

On the spectra and distance scale of short Gamma Ray--Bursts

I discuss the spectral properties of short GRBs detected by BATSE and compare them with long events. The detailed modeling of short GRB spectra shows that their being (on average) harder than long events (as it has been found by comparing their hardness ratios) is indeed due to a harder low energy spectral component (i.e. the powerlaw of the Band or cutoff-powerlaw model) in short GRBs which, instead, have a peak energy similar to long events. Another open issue is the distance scale to short GRBs. While the few redshift measurements suggests that they are at z<1, statistical studies (with local galaxies or X-ray selected clusters) suggest that they should be even more local.

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arXiv2005

SGR giant flares in the BATSE short GRB catalogue: constraints from spectroscopy

The giant flare observed on Dec. 27th 2004 from SGR 1806-20 has revived the idea that a fraction of short (<2 s) Gamma Ray Bursts (GRBs) is due to giant flares from Soft Gamma Ray Repeaters located in nearby galaxies. One of the distinguishing characteristics of these events is the thermal (black body) spectrum with temperatures ranging from ~50 to ~180 keV, with the highest temperature observed for the initial 0.2 s spike of the Dec. 27th 2004 event. We analyzed the spectra of a complete sample of short GRBs with peak fluxes greater than 4 photon s^(-1) cm^(-2) detected by BATSE. Of the 115 short GRBs so selected only 76 had sufficient signal to noise to allow the spectral analysis. We find only 3 short GRBs with a spectrum well fitted by a black body, with 60 1 sec) and a more complex light curve than the Dec. 27th 2004 event. This implies a stringent limit on the rate of extragalactic SGR giant flares with spectral properties analogous to the Dec. 27th flare. We conclude that up to 4 per cent of the short GRBs could be associated to giant flares (2 sigma confidence). This implies that either the distance to SGR 1806-20 is smaller than 15 kpc or the rate of Galactic giant flares is lower than the estimated 0.033 per year.

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arXiv2003

Extremely hard GRB spectra prune down the forest of emission models

We consider the evidence for very hard low energy spectra during the prompt phase of Gamma-Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness (i.e. extremely steep spectral slopes below the $EF_{E}$ peak) by applying different tests. These bursts, detected by the Burst And Transient Source Experiment (BATSE) in the $\sim 30$ keV -- 2 MeV energy range, are sufficiently bright to allow time resolved spectral studies on time intervals of the order of tenths of a second. We discuss the hard spectra of these bursts and their evolution in the context of several non--thermal emission models, which all appear inadequate to account for these cases. The extremely hard spectra, which are detected in the early part of the BATSE light curve, are also compared with a black body spectral model: the resulting fits are remarkably good, except for an excess at high energies (in several cases) which could be simply accounted for by the presence of a supra--thermal component. The findings on the possible thermal character of the evolving spectrum and the implications on the GRB physical scenario are considered in the frameworks of photospheric models for a fireball which is becoming optically thin, and of Compton drag models, in which the fireball boosts "ambient" seed photons by its own bulk motion. Both models, according to simple estimates, appear to be qualitatively and quantitatively consistent with the found spectral characteristics, although their possible caveats are discussed.

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