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 s 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 Epeak for the population of long GRBs. We also test whether short GRBs are consistent with the $E_{\rm peak}{-}E_{\rm iso}$ and $E_{\rm peak}{-}L_{\rm iso}$ correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and $E^{\rm obs}_{\rm peak}$: while short GRBs are inconsistent with the $E_{\rm peak}{-}E_{\rm iso}$ correlation of long GRBs, they could follow the $E_{\rm peak}{-}L_{\rm iso}$ 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.
Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the 3 months of survey had an average γ-ray luminosity above 1048 erg s−1. For 17 out of the 23 sources we ...found and analysed X-ray and optical–ultraviolet data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high-energy states of single sources, like 0528+134 and 3C 454.3, for which multiple good sets of multiwavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disc emission. We can then constrain the black hole mass and the disc luminosity. Both are large (i.e. masses equal or greater than 109 M ⊙ and disc luminosities above 10 per cent of Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disc luminosity, and proportional to the mass accretion rate.
We investigate the physical properties of 10 blazars at redshift greater than 2 detected in the 3-yr all-sky survey performed by the Burst Alert Telescope (BAT) on board the Swift satellite. We find ...that the jets of these blazars are among the most powerful known. Furthermore, the mass of their central black hole, inferred from the optical–ultraviolet bump, exceeds a few billions of solar masses, with accretion luminosities being a large fraction of the Eddington one. We compare their properties with those of the brightest blazars of the 3-month survey performed by the Large Area Telescope (LAT) on board the Fermi satellite. We find that the BAT blazars have more powerful jets, more luminous accretion discs and larger black hole masses than LAT blazars. These findings can be simply understood on the basis of the blazar sequence, which suggests that the most powerful blazars have a spectral energy distribution with a high-energy peak at MeV (or even sub-MeV) energies. This implies that the most extreme blazars can be found more efficiently in hard X-rays, rather than in the high-energy γ-ray band. We then discuss the implications of our findings for future missions, such as the New Hard X-ray Mission (NHXM) and especially the Energetic X-ray Imaging Survey Telescope (EXIST) mission which, during its planned 2-yr all-sky survey, is expected to detect thousands of blazars, with a few of them at z≳ 6.
In this paper, we compute rest-frame extinctions for the afterglows of a sample of Swift gamma-ray bursts (GRBs) complete in redshift. The selection criteria of the sample are based on observational ...high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of GRBs. Our main result is that the optical/near-infrared extinction of GRB afterglows in our sample does not follow a single distribution. 87 per cent of the events are absorbed by less than 2 mag, and 50 per cent suffer from less than 0.3-0.4 mag extinction. The remaining 13 per cent of the afterglows are highly absorbed. The true percentage of GRB afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star-forming regions associated with the GRB progenitor or along the afterglow line of sight, and/or due to massive dusty host galaxies. No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z ∼ 1.5-2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown, although the observed N
H appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.
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, which are bright in the 15-150 keV Swift/BAT band, i.e., with 1-s peak photon fluxes in excess to 2.6 photons s super(-1) cm super(-2). The sample is composed of 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 an unbiased way. We find that strong evolution in luminosity ( delta sub()l = 2.3 + or - 0.6) or in density ( delta sub()d = 1.7 + or - 0.5) is required in order to account for the observations. The derived redshift distributions in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distributions. 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.
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), E
obs
peak 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 E
obs
peak. We discuss these properties in light of the correlations found between E
obs
peak 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 E
obs
peak-F and E
obs
peak-P correlations both in fluence/peak flux and in peak energy. No GBM long burst with E
obs
peak 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 E
peak-E
iso correlation. In contrast, there is no outlier of the E
peak-L
iso correlation, for both long and short GBM bursts.
The structure of gamma-ray burst (GRB) jets impacts on their prompt and afterglow emission properties. The jet of GRBs could be uniform, with constant energy per unit solid angle within the jet ...aperture, or it could be structured, namely with energy and velocity that depend on the angular distance from the axis of the jet. We try to get some insight about the still unknown structure of GRBs by studying their luminosity function. We show that low (1046−48 erg s−1) and high (i.e. with L ≥ 1050 erg s−1) luminosity GRBs can be described by a unique luminosity function, which is also consistent with current lower limits in the intermediate luminosity range (1048−50 erg s−1). We derive analytical expressions for the luminosity function of GRBs in uniform and structured jet models and compare them with the data. Uniform jets can reproduce the entire luminosity function with reasonable values of the free parameters. A structured jet can also fit adequately the current data, provided that the energy within the jet is relatively strongly structured, i.e. E ∝ θ−k
with k ≥ 4. The classical E ∝ θ−2 structured jet model is excluded by the current data.
ABSTRACT
We study the spectral evolution on second and subsecond time‐scales in 11 long and 12 short gamma‐ray bursts (GRBs) with peak flux >8.5 × 10−6 erg cm−2 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 time‐scales in both short and long GRBs. We do not find evidence of a 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 Eobspeak 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 Epeak and the luminosity Liso are less scattered than their correlations in the observer frame between the peak energy Eobspeak and the flux P. We find that the rest frame Epeak of long bursts can be as high or even larger than that of short GRBs and that short and long GRBs follow the same Epeak(t)–Liso(t) correlation, despite the fact that they likely have different progenitors.
The jet opening angle θjet and the bulk Lorentz factor Γ0 are crucial parameters for the computation of the energetics of gamma-ray bursts (GRBs). From the ∼30 GRBs with measured θjet or Γ0 it is ...known that (i) the real energetic E
γ, obtained by correcting the isotropic equivalent energy E
iso for the collimation factor ∼ θ2
jet, is clustered around 1050-1051 erg and it is correlated with the peak energy E
p of the prompt emission and (ii) the comoving frame E′p and E′γ are clustered around typical values. Current estimates of Γ0 and θjet 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 Γ0 and θjet can reproduce a set of observational constraints. Assuming that all bursts have the same E′p and E′γ in the comoving frame, we find that Γ0 and θjet 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 θjet and Γ0 and (b) an intrinsic relation between the peak values of their distributions, i.e. θjet
2.5Γ0 = const. On average, larger values of Γ0 (i.e. the 'faster' bursts) correspond to smaller values of θjet (i.e. the 'narrower'). We predict that ∼6 per cent of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin θjet < 1/Γ0. Finally, we estimate that the local rate of GRBs is ∼0.3 per cent of all local Type Ib/c supernova (SNIb/c) and ∼4.3 per cent of local hypernovae, i.e. SNIb/c with broad lines.
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