Publications (41)15.88 Total impact
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Article: Afterglow emission in Gamma-Ray Bursts: I. Pair-enriched ambient medium and radiative blast waves
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ABSTRACT: Forward shocks caused by the interaction between a relativistic blast wave and the circum-burst medium are thought to be responsible for the afterglow emission in Gamma-Ray Bursts (GRBs). We consider the hydrodynamics of a spherical relativistic blast wave expanding into the surrounding medium and we generalize the standard theory in order to account for several effects that are generally ignored. In particular, we consider the role of adiabatic and radiative losses on the hydrodynamical evolution of the shock, under the assumption that the cooling losses are fast. Our model can describe adiabatic, fully radiative and semi-radiative blast waves, and can describe the effects of a time-varying radiative efficiency. The equations we present are valid for arbitrary density profiles, and also for a circum-burst medium enriched with electron-positron pairs. The presence of pairs enhances the fraction of shock energy gained by the leptons, thus increasing the importance of radiative losses. Our model allows to study whether the high-energy (>0.1 GeV) emission in GRBs may originate from afterglow radiation. In particular, it is suitable to test whether the fast decay of the high-energy light curve observed in several Fermi LAT GRBs can be ascribed to an initial radiative phase, followed by the standard adiabatic evolution.11/2012; -
Article: GRBs have preferred jet opening angles and bulk Lorentz factors
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ABSTRACT: We recently found that Gamma Ray Burst energies and luminosities, in their comoving frame, are remarkably similar. This, coupled with the clustering of energetics once corrected for the collimation factor, suggests the possibility that all bursts, in their comoving frame, have the same peak energy E'peak (of the order of a few keV) and the same energetics of the prompt emission E'gamma (of the order of 2e48 erg). The large diversity of bursts energies is then due to the different bulk Lorentz factor Gamma and jet aperture angle theta_jet. We investigated, through a population synthesis code, what are the distributions of Gamma and theta_jet compatible with the observations. Both quantities must have preferred values, with log-normal best fitting distributions and ~ 275 and <\theta_jet> ~ 8.7 degree. Moreover, the peak values of the Gamma and theta_jet distributions must be related - theta_jet^2.5 Gamma =const: the narrower the jet angle, the larger the bulk Lorentz factor. We predict that ~6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma. Finally, we estimate that the local rate of GRBs is ~0.3% of all local SNIb/c and ~2.5% of local hypernovae, i.e. SNIb/c with broad absorption lines.11/2012; -
Article: The faster the narrower: characteristic bulk velocities and jet opening angles of Gamma Ray Bursts
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ABSTRACT: The jet opening angle theta_jet and the bulk Lorentz factor Gamma_0 are crucial parameters for the computation of the energetics of Gamma Ray Bursts (GRBs). From the ~30 GRBs with measured theta_jet or Gamma_0 it is known that: (i) the real energetic E_gamma, obtained by correcting the isotropic equivalent energy E_iso for the collimation factor ~theta_jet^2, is clustered around 10^50-10^51 erg and it is correlated with the peak energy E_p of the prompt emission and (ii) the comoving frame E'_p and E'_gamma are clustered around typical values. Current estimates of Gamma_0 and theta_jet are based on incomplete data samples and their observed distributions could be subject to biases. Through a population synthesis code we investigate whether different assumed intrinsic distributions of Gamma_0 and theta_jet can reproduce a set of observational constraints. Assuming that all bursts have the same E'_p and E'_gamma in the comoving frame, we find that Gamma_0 and theta_jet cannot be distributed as single power-laws. The best agreement between our simulation and the available data is obtained assuming (a) log-normal distributions for theta_jet and Gamma_0 and (b) an intrinsic relation between the peak values of their distributions, i.e theta_jet^2.5*Gamma_0=const. On average, larger values of Gamma_0 (i.e. the "faster" bursts) correspond to smaller values of theta_jet (i.e. the "narrower"). We predict that ~6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin(theta_jet)<1/Gamma_0. Finally, we estimate that the local rate of GRBs is ~0.3% of all local SNIb/c and ~4.3% of local hypernovae, i.e. SNIb/c with broad-lines.10/2012; -
Article: A complete sample of bright Swift Gamma-Ray Bursts: X-ray afterglow luminosity and its correlation with the prompt emission
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ABSTRACT: We investigate wheter there is any correlation between the X-ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma-Ray Bursts (GRBs) nearly complete in redshift (~90%). Being free of selection effects (except flux limit), this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray luminosities are computed at four different rest frame times (5 min, 1 hr, 11 hr and 24 hr after trigger) and compared with the prompt emission isotropic energy E_iso, the isotropic peak luminosity L_iso and the rest frame peak energy E_peak. We find that the rest frame afterglow X-ray luminosity do correlate with these prompt emission quantities, but the significance of each correlation decreases over time. This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time, with the prompt and afterglow emission dominating at early and late time, respectively. In particular, we found evidence that the plateau and the shallow decay phase often observed in GRB X-ray light curves are powered by activity from the central engine. The existence of the L_X-E_iso correlation at late times (t_rf > 11 hr) suggests a similar radiative efficiency among different bursts with on average about 6% of the total kinetic energy powering the prompt emission.06/2012; -
Article: The impact of selection biases on the Ep-Liso correlation of Gamma Ray Bursts
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ABSTRACT: We study the possible effects of selection biases on the Ep-Liso correlation caused by the unavoidable presence of flux-limits in the existing samples of Gamma Ray Bursts (GRBs). We consider a well defined complete sample of Swift GRBs and perform Monte Carlo simulations of the GRB population under different assumptions for their luminosity functions. If we assume that there is no correlation between the peak energy Ep and the isotropic luminosity Liso, we are unable to reproduce it as due to the flux limit threshold of the Swift complete sample. We can reject the null hypothesis that there is no intrinsic correlation between Ep and Liso at more than 2.7 sigma level of confidence. This result is robust against the assumptions of our simulations and it is confirmed if we consider, instead of Swift, the trigger threshold of the Batse instrument. Therefore, there must be a physical relation between these two quantities. Our simulations seem to exclude, at a lower confidence level of 1.6 sigma, the possibility that the observed Ep-Liso correlation among different bursts is caused by a boundary, i.e. such that for any given Ep, we see only the largest Liso, which has a flux above the threshold of the current instruments.02/2012; -
Article: A complete sample of bright Swift Long Gamma Ray Bursts: testing the spectral-energy correlations
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ABSTRACT: We use a nearly complete sample of Gamma Ray Bursts (GRBs) detected by the Swift satellite to study the correlations between the spectral peak energy Ep of the prompt emission, the isotropic energetics Eiso and the isotropic luminosity Liso. This GRB sample is characterized by a high level of completeness in redshift (90%). This allows us to probe in an unbiased way the issue related to the physical origin of these correlations against selection effects. We find that one burst, GRB 061021, is an outlier to the Ep-Eiso correlation. Despite this case, we find strong Ep-Eiso and Ep-Liso correlations for the bursts of the complete sample. Their slopes, normalisations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and Ep. This means that the biases present in the total sample commonly used to study these correlations do not affect their properties. Finally, we also find no evolution with redshift of the Ep-Eiso and Ep-Liso correlations.12/2011; -
Article: The Dark Bursts population in a complete sample of bright Swift Long Gamma-Ray Bursts
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ABSTRACT: We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of our sample (52 objects out of 58) allow us to establish the existence of a genuine dark population and we are able to estimate the maximum fraction of dark burst events (~30%) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to the one of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the sub-class of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.12/2011; -
Article: A complete sample of bright Swift Long Gamma-Ray Bursts: Sample presentation, Luminosity Function and evolution
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ABSTRACT: We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.12/2011; -
Article: Short and Long GRBs: same emission mechanism?
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ABSTRACT: We study the spectral evolution on second and sub--second timescales in 11 long and 12 short Gamma Ray Bursts (GRBs) with peak flux >8.5e-6 erg/cm2 s (8 keV-35 MeV) detected by the Fermi satellite. The peak flux correlates with the time-averaged peak energy in both classes of bursts. The peak energy evolution, as a function of time, tracks the evolution of the flux on short timescales in both short and long GRBs. We do not find evidence of an hard-to-soft spectral evolution. While short GRBs have observed peak energies larger than few MeV during most of their evolution, long GRBs can start with a softer peak energy (of few hundreds keV) and become as hard as short ones (i.e. with Ep,obs larger than few MeV) at the peak of their light curve. Six GRBs in our sample have a measured redshift. In these few cases we find that their correlations between the rest frame Ep and the luminosity Liso are less scattered than their correlations in the observer frame between the peak energy Ep,obs and the flux P. We find that the rest frame Ep of long bursts can be as high or even larger than that of short GRBs and that short and long GRBs follow the same Ep-Liso correlation, despite the fact that they likely have different progenitors.09/2011; -
Article: Fermi/GBM and BATSE gamma‐ray bursts: comparison of the spectral properties
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ABSTRACT: The Gamma-ray Burst Monitor (GBM) on board Fermi allows us to study the spectra of gamma-ray bursts (GRBs) over an unprecedented wide energy range (8 keV–35 MeV). We compare the spectral properties of short and long GRBs detected by the GBM (up to 2010 March) with those of GRBs detected by the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO). GBM and BATSE long bursts have similar distributions of fluence (F), Eobspeak and peak flux (P) but GBM bursts have a slightly harder low-energy spectral index α with respect to BATSE GRBs. GBM and BATSE short bursts have similar distributions of fluence, α and peak flux, with GBM bursts having slightly larger Eobspeak. We discuss these properties in light of the correlations found between Eobspeak and the fluence and the peak flux. GBM bursts confirm that these correlations are not determined by instrumental selection effects. Indeed, GBM bursts extend the Eobspeak–F and Eobspeak–P correlations both in fluence/peak flux and in peak energy. No GBM long burst with Eobspeak exceeding a few MeV is found, despite the possibility of detecting it. Similarly to what is found with BATSE, there are 3 per cent of GBM long bursts (and almost all short ones) that are outliers at more than 3σ of the Epeak–Eiso correlation. In contrast, there is no outlier of the Epeak–Liso correlation, for both long and short GBM bursts.Monthly Notices of the Royal Astronomical Society 08/2011; 415(4):3153 - 3162. · 4.90 Impact Factor -
Article: Gamma Ray Bursts in the comoving frame
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ABSTRACT: We estimate the bulk Lorentz factor Gamma_0 of 31 GRBs using the measured peak time of their afterglow light curves. We consider two possible scenarios for the estimate of Gamma_0: the case of a homogeneous circumburst medium or a wind density profile. The values of Gamma_0 are broadly distributed between few tens and several hundreds with average values ~138 and ~66 for the homogeneous and wind density profile, respectively. We find that the isotropic energy and luminosity correlate in a similar way with Gamma_0, i.e. Eiso Gamma_0^2 and Liso Gamma_0^2, while the peak energy Epeak Gamma_0. These correlations are less scattered in the wind density profile than in the homogeneous case. We then study the energetics, luminosities and spectral properties of our bursts in their comoving frame. The distribution of Liso' is very narrow with a dispersion of less than a decade in the wind case, clustering around Liso'=5x10^48 erg/s. Peak photon energies cluster around Epeak'=6 keV. The newly found correlations involving Gamma_0 offer a general interpretation scheme for the spectral-energy correlations of GRBs. The Epeak-Eiso and Epeak-Liso correlations are due to the different Gamma_0 factors and the collimation-corrected correlation, Epeak-Egamma (obtained by correcting the isotropic quantities for the jet opening angle theta_j), can be explained if theta_j^2*Gamma_0=constant. Assuming the Epeak-Egamma correlation as valid, we find a typical value of theta_j*Gamma_0 ~ 6-20, in agreement with the predictions of magnetically accelerated jet models.07/2011; -
Article: Spectral properties of 438 GRBs detected by Fermi/GBM
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ABSTRACT: We present the results of the spectral analysis of the public data of 438 Gamma Ray Bursts (GRBs) detected by the Fermi Gamma ray Burst Monitor (GBM) up to March 2010. For 432 bursts we could fit the time integrated spectrum. In 318 cases we can reliably constrain the peak energy Epeak of their \nu F_\nu spectrum by analyzing their time integrated spectrum between 8 keV and 35 MeV. 80% of these spectra are fitted by a power law with an exponential cutoff, and the remaining with the Band function. Among these 318 GRBs, 274 and 44 belong to the long and short GRB class, respectively. Long GRBs have a typical peak energy Epeak=160 keV and low energy spectral index alpha=-0.92. Short GRBs have harder peak energy (Epeak=490 keV) and harder low energy spectral index (alpha=-0.50) than long bursts. For each Fermi GRB we analyzed also the spectrum corresponding to the peak flux of the burst. On average, the peak spectrum has harder low energy spectral index but similar Epeak than the corresponding time-integrated spectrum for the same burst. The spectral parameters derived in our analysis of Fermi/GBM bursts are globally consistent with those reported in the GRB Cicular Network (GCN) archive after December 2008, while we found systematic differences, concerning the low energy power law index, for earlier bursts.12/2010; -
Article: Spectral evolution of Fermi/GBM short Gamma-Ray Bursts
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ABSTRACT: We study the spectral evolution of 14 short duration Gamma Ray Bursts (GRBs) detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra resolved in time at the level of 4-64 ms in the 8 keV-35 MeV energy range. We find a strong correlation between the observed peak energy Ep and the flux P within individual short GRBs. The slope of the Ep~P^s correlation for individual bursts ranges between ~0.4 and ~1. The rise and decay phase of individual bursts follow the same Ep-P correlation. The same correlation holds also for the precursors present in two GRBs. There is no correlation between the low energy spectral index and the peak energy or the flux. Our results show that in our 14 short GRBs Ep evolves in time tracking the flux. This behavior is similar to what found in the population of long GRBs and it is in agreement with the evidence that long GRBs and (the still few) short GRBs with measured redshifts follow the same rest frame Ep-L correlation. Its origin is most likely to be found in the radiative mechanism that has to be the same in both classes of GRBs. Comment: 5 pages, 1 table, 3 figures. Submitted to MNRAS08/2010; -
Article: Spectral properties of long and short Gamma-Ray Bursts: comparison between BATSE and Fermi bursts
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ABSTRACT: We compare the spectral properties of 227 Gamma Ray Bursts (GRBs) detected by the Fermi Gamma Ray Burst Monitor (GBM) up to February 2010 with those of bursts detected by the CGRO/BATSE instrument. Out of 227 Fermi GRBs, 166 have a measured peak energy E_peak_obs of their \nuF(\nu) spectrum: of these 146 and 20 belong the long and short class, respectively. Fermi long bursts follow the correlations defined by BATSE bursts between their E_peak_obs vs fluence and peak flux: as already shown for the latter ones, these correlations and their slopes do not originate from instrumental selection effects. Fermi/GBM bursts extend such correlations toward lower fluence/peak energy values with respect to BATSE ones whereas no GBM long burst with E_peak_obs exceeding a few MeV is found, despite the possibility of detecting them. Again as for BATSE, $\sim$ 5% of long and almost all short GRBs detected by Fermi/GBM are outliers of the E_peak-isotropic equivalent energy ("Amati") correlation while no outlier (neither long nor short) of the E_peak-isotropic equivalent luminosity ("Yonetoku") correlation is found. Fermi long bursts have similar typical values of E_peak_obs but a harder low energy spectral index with respect to all BATSE events, exacerbating the inconsistency with the limiting slopes of the simplest synchrotron emission models. Although the short GRBs detected by Fermi are still only a few, we confirm that their E_peak_obs is greater and the low energy spectrum is harder than those of long ones. We discuss the robustness of these results with respect to observational biases induced by the differences between the GBM and BATSE instruments. Comment: 10 pages, 8 figures, submitted to A&A04/2010; -
Article: GeV emission from Gamma Ray Bursts: a radiative fireball?
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ABSTRACT: We study the emission observed at energies greater than 100 MeV of 11 Gamma Ray Bursts (GRBs) detected by the Fermi/Large Area Telescope (LAT) until October 2009. The GeV emission has three main properties: (i) its duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor (GBM) onboard Fermi [this confirms earlier results from the Energetic Gamma-Ray Experiment Telescope (EGRET)]; (ii) its spectrum is consistent with F(v) propto v^(-1) and does not show strong spectral evolution; (iii) for the brightest bursts, the flux detected by the LAT decays as a power law with a typical slope: t^(-1.5). We argue that the observed >0.1 GeV flux can be interpreted as afterglow emission shortly following the start of the prompt phase emission as seen at smaller frequencies. The decay slope is what expected if the fireball emission is produced in the radiative regime, i.e. all dissipated energy is radiated away. We also argue that the detectability in the GeV energy range depends on the bulk Lorentz factor Gamma of the bursts, being strongly favoured in the case of large Gamma. This implies that the fraction of bursts detected at high energies corresponds to the fraction of bursts having the largest Gamma. The radiative interpretation can help to explain why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase, despite the fact that the collision with the external medium should be more efficient than internal shocks in producing the radiation we see. Comment: 12 pages, 9 figures, accepted for publication in MNRAS, minor changes, added EGRET light-curve of GRB 94021710/2009; -
Article: The onset of the GeV afterglow of GRB 090510
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ABSTRACT: We study the emission of the short/hard GRB 090510 at energies > 0.1 GeV as observed by the Large Area Telescope (LAT) onboard the Fermi satellite. The GeV flux rises in time as t^2 and decays as t^-1.5 up to 200 s. The peak of the high energy flux is delayed by 0.2 s with respect to the main ~MeV pulse detected by the Fermi Gamma Burst Monitor (GBM). Its energy spectrum is consistent with F(E)=E^-1. The time behavior and the spectrum of the high energy LAT flux are strong evidences of an afterglow origin. We then interpret it as synchrotron radiation produced by the forward shock of a fireball having a bulk Lorentz factor Gamma ~ 2000. The afterglow peak time is independent of energy in the 0.1-30 GeV range and coincides with the arrival time of the highest energy photon (~ 30 GeV). Since the flux detected by the GBM and the LAT have different origins, the delay between these two components is not entirely due to possible violation of the Lorentz invariance. It is the LAT component by itself that allows to set a stringent lower limit on the quantum-gravity mass of 4.7 times the Planck mass. Comment: 4 pages, 3 figures, submitted to ApJ Letters08/2009; -
Article: Spectral-Luminosity relation within individual Fermi GRBs
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ABSTRACT: We study the spectra of all long Gamma Ray Bursts (GRBs) of known redshift detected by the Fermi satellite. Their fluxes and fluences are large enough to allow a time dependent study of their spectral characteristics in the 8 keV-1 MeV energy range. We find that the peak energy Ep of their EL(E) spectrum correlates with the luminosity in a remarkable tight way within individual bursts. This time resolved Ep-Liso correlation is very similar for all the considered bursts, and has a slope and normalisation similar to the analogous Ep-Liso correlation defined by the time integrated spectra of different bursts detected by several different satellites. For a few of the considered GRBs, we could also study the behaviour of the Ep-Liso correlation during the rising and decaying phases of individual pulses within each burst, finding no differences. Our results indicate the presence of a similar physical mechanism, operating for the duration of different GRBs, linking tightly the burst luminosity with the peak energy of the spectrum emitted at different times. Such a physical mechanism is the same during the rise and decay phase of individual pulses composing a GRB. These results, while calling for a robust physical interpretation, strongly indicate that the Ep-Liso spectral energy correlation found considering the time integrated spectra of different bursts is real, and not the result of instrumental selection effects. Comment: 11 pages, 5 figures, 3 tables. Submitted to Astronomy and Astrophysics08/2009; -
Article: Short versus Long Gamma-Ray Bursts: spectra, energetics, and luminosities
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ABSTRACT: We compare the spectral properties of 79 short and 79 long Gamma-Ray Bursts (GRBs) detected by BATSE and selected with the same limiting peak flux. Short GRBs have a low-energy spectral component harder and a peak energy slightly higher than long GRBs, but no difference is found when comparing short GRB spectra with those of the first 1-2 sec emission of long GRBs. These results confirm earlier findings for brighter GRBs. The bolometric peak flux of short GRBs correlates with their peak energy in a similar way to long bursts. Short and long GRBs populate different regions of the bolometric fluence-peak energy plane, short bursts being less energetic by a factor similar to the ratio of their durations. If short and long GRBs had similar redshift distributions, they would have similar luminosities yet different energies, which correlate with the peak energy E_peak for the population of long GRBs. We also test whether short GRBs are consistent with the E_peak-E_iso and E_peak-L_iso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E_peak,obs: while short GRBs are inconsistent with the E_peak-E_iso correlation of long GRBs, they could follow the E_peak-L_iso correlation of long bursts. All the above indications point to short GRBs being similar to the first phases of long bursts. This suggests that a similar central engine (except for its duration) operates in GRBs of different durations. Comment: 14 pages, 7 tables, 5 figures. Accepted by A&A02/2009; -
Article: Time-resolved spectral correlations of long-duration Gamma-Ray Bursts
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ABSTRACT: For a sample of long GRBs with known redshift, we study the distribution of the evolutionary tracks on the rest-frame luminosity-peak energy Liso-Ep' diagram. We are interested in exploring the extension of the `Yonetoku' correlation to any phase of the prompt light curve, and in verifying how the high-signal prompt duration time, Tf, in the rest frame correlates with the residuals of such correlation (Firmani et al. 2006). For our purpose, we analyse separately two samples of time-resolved spectra corresponding to 32 GRBs with peak fluxes >1.8 phot cm^-2 s^-1 from the Swift-BAT detector, and 7 bright GRBs from the CGRO-BATSE detector previously processed by Kaneko et al. (2006). After constructing the Liso-Ep' diagram, we discuss the relevance of selection effects, finding that they could affect significantly the correlation. However, we find that these effects are much less significant in the Liso x Tf-Ep' diagram, where the intrinsic scatter reduces significantly. We apply further corrections for reducing the intrinsic scatter even more. For the sub-samples of GRBs (7 from Swift and 5 from CGRO) with measured jet break time, we analyse the effects of correcting Liso by jet collimation. We find that (i) the scatter around the correlation is reduced, and (ii) this scatter is dominated by the internal scatter of the individual evolutionary tracks. These results suggest that the time, integrated `Amati' and `Ghirlanda' correlations are consequences of the time resolved features, not of selection effects, and therefore call for a physical origin. We finally remark the relevance of looking inside the nature of the evolutionary tracks. Comment: 11 pages, 6 figures, 4 tables. Submitted to MNRAS (Sept 8th), after referee comments11/2008; -
Article: Peak energy of the prompt emission of long gamma‐ray bursts versus their fluence and peak flux
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ABSTRACT: The spectral-energy (and luminosity) correlations in long gamma-ray bursts 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 Eobspeak versus the fluence F or the peak flux P. In a recent paper, we have started to investigate this problem considering all bursts with known redshift and spectral properties. Here, we consider instead all bursts with known Eobspeak, irrespective of redshift, adding to those a sample of 100 faint BATSE bursts representative of a larger population of 1000 objects. This allows us to construct a complete, fluence-limited, sample, tailored to study the selection/instrumental effects we consider. We found that the fainter BATSE bursts have smaller Eobspeak than those of bright events. As a consequence, the Eobspeak 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 six per cent of these bursts are surely outliers of the Epeak–Eiso correlation (updated in this paper to include 83 bursts), since they are inconsistent with it for any redshift. Eobspeak also correlates with the peak flux, with a slope similar to the Epeak–Liso correlation. In this case, there is only one sure outlier. The scatter of the Eobspeak–P correlation defined by the BATSE bursts of our sample is significantly smaller than the Eobspeak–F correlation of the same bursts, while for the bursts with known redshift the Epeak–Eiso correlation is tighter than the Epeak–Liso one. Once a very large number of bursts with Eobspeak and redshift will be available, we thus expect that the Epeak–Liso correlation will be similar to that currently found, whereas it is very likely that the Epeak–Eiso correlation will become flatter and with a larger scatter.Monthly Notices of the Royal Astronomical Society 11/2008; 391(2):639 - 652. · 4.90 Impact Factor
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2007–2008
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Università degli Studi dell'Insubria
Varese, Lombardy, Italy
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