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.
The prompt emission of gamma-ray bursts extends from the early pulses observed in γ-rays (>15 keV) to very late flares of X-ray photons (0.3–10 keV). The duration of prompt γ-ray pulses is rather ...constant, while the width of X-ray flares correlates with their peak time, suggesting a possibly different origin. However, pulses and flares have similar spectral properties. Considering internal and external shock scenarios, we derive how the energy and duration of pulses scale with their time of occurrence, and we compare this with observations. The absence of an observed correlation between the prompt emission pulse duration and its time of occurrence favours an “internal” origin and confirms earlier results. We show that the energetic and temporal properties of X-ray flares are also consistent with being produced by internal shocks between slow fireballs with a small contrast between their bulk Lorentz factors. These results relax the requirement of a long-lasting central engine to explain the latest X-ray flares.
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 E
peak of the prompt emission, the isotropic ...energetics E
iso and the isotropic luminosity L
iso. This GRB sample is characterized by a high level of completeness in redshift (90 per cent). 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 E
peak-E
iso correlation. Despite this case, we find strong E
peak-E
iso and E
peak-L
iso correlations for the bursts of the complete sample. Their slopes, normalizations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and E
peak. 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 E
peak-E
iso and E
peak-L
iso correlations.
The delay in arrival times between high and low energy photons from cosmic sources can be used to test the violation of the Lorentz invariance (LIV), predicted by some quantum gravity theories, and ...to constrain its characteristic energy scale EQG that is of the order of the Planck energy. Gamma-ray bursts (GRBs) and blazars are ideal for this purpose thanks to their broad spectral energy distribution and cosmological distances: at first order approximation, the constraints on EQG are proportional to the photon energy separation and the distance of the source. However, the LIV tiny contribution to the total time delay can be dominated by intrinsic delays related to the physics of the sources: long GRBs typically show a delay between high and low energy photons related to their spectral evolution (spectral lag). Short GRBs have null intrinsic spectral lags and are therefore an ideal tool to measure any LIV effect. We considered a sample of 15 short GRBs with known redshift observed by Swift and we estimate a limit on EQG ≳ 1.5 × 1016 GeV. Our estimate represents an improvement with respect to the limit obtained with a larger (double) sample of long GRBs and is more robust than the estimates on single events because it accounts for the intrinsic delay in a statistical sense.
The BL Lac object 1ES 0229+200 (z= 0.14) has been detected by the High Energy Stereoscopic System (HESS) during observations taking place in 2005–2006. The TeV spectrum, when corrected for the ...absorption of gamma-ray photons through the interaction with the extragalactic background light, is extremely hard, even if the most conservative level for the background is considered. The case of 1ES 0229+200 is very similar to that of 1ES 1101−232, for which a possible explanation, in the framework of the standard one-zone synchrotron-self Compton (SSC) model, is that the high-energy emission is SSC radiation of electrons distributed as a power law with a large value of the minimum energy. In this scenario, the hard TeV spectrum is accompanied by a very hard synchrotron continuum below the soft X-ray band. We will show that recent Swift observations of 1ES 0229+200 in the critical UV X-ray band support this model, showing the presence of the expected spectral break and hard continuum between the UV and the X-ray bands.
We cross-correlate the Fermi 11-month survey (1FGL) catalogue with the 20-GHz Australia Telescope Compact Array (AT20G) radio survey catalogue composed of 5890 sources at declination < 0°. Among the ...738 Fermi sources distributed in the southern sky, we find 230 highly probable candidate counterparts in the AT20G survey. Of these, 222 are already classified in the Fermi one-year Large Area Telescope (LAT) active galactic nucleus (AGN) catalogue (1LAC) as blazars either flat spectrum radio quasars (FSRQs) or BL Lacertae objects (BL Lacs), AGNs or sources of unknown class but with an associated counterpart, while eight are new associations. By studying the γ-ray and radio properties of these associations, we find a correlation between the γ-ray flux (above 100 MeV) and the 20-GHz flux density. This correlation is more than 3σ statistically significant, both for the population of BL Lacs and for FSRQs considered separately. We also find that the radio counterparts associated with the Fermi sources have, on average, flat radio spectra between 5 and 20 GHz and that Fermiγ-ray sources are not preferentially associated with ‘ultra-inverted spectrum’ radio sources. For two of the eight new associations, we build the broad-band spectral energy distribution combining Fermi, Swift and radio observations. One of these two sources is identified with the high-redshift FSRQ Swift J1656.3–3302 (z= 2.4) and we classify the other source as a candidate new FSRQ. We also study the brightest radio source of the 46 associations without an optical classification and classify it as a new BL Lac candidate ‘twin’ of the prototypical BL Lac OJ 287 if its redshift is larger, z∼ 0.4.
A unifying view of gamma-ray burst afterglows Ghisellini, G.; Nardini, M.; Ghirlanda, G. ...
Monthly Notices of the Royal Astronomical Society,
2009, Letnik:
393, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We selected a sample of 33 gamma-ray bursts detected by Swift, with known redshift and optical extinction at the host frame. For these, we constructed the de-absorbed and K-corrected X-ray and ...optical rest-frame light curves. These are modelled as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circumburst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibilities, as a ‘late prompt’ emission produced by a long-lived central engine with mechanisms similar to those responsible for the production of the ‘standard’ early prompt radiation. Apart from flares or re-brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based, in part, on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first step towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X-ray afterglows in a coherent way, by a relatively simple scenario. Within this context, it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic and why the steepening after the jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall-back material (i.e. ∝t−5/3), suggesting that this can be the reason why the central engine can be active for a long time.
Gamma Ray Bursts (GRBs) are a powerful probe of the high-redshift Universe. We present a tool to estimate the detection rate of high-z GRBs by a generic detector with defined energy band and ...sensitivity. We base this on a population model that reproduces the observed properties of GRBs detected by Swift, Fermi and CGRO in the hard X-ray and γ-ray bands. We provide the expected cumulative distributions of the flux and fluence of simulated GRBs in different energy bands. We show that scintillator detectors, operating at relatively high energies (e.g. tens of keV to the MeV), can detect only the most luminous GRBs at high redshifts due to the link between the peak spectral energy and the luminosity (E
peak–L
iso) of GRBs. We show that the best strategy for catching the largest number of high-z bursts is to go softer (e.g. in the soft X-ray band) but with a very high sensitivity. For instance, an imaging soft X-ray detector operating in the 0.2–5 keV energy band reaching a sensitivity, corresponding to a fluence, of ∼10−8 erg cm−2 is expected to detect ≈40 GRBs yr−1 sr−1 at z ≥ 5 (≈3 GRBs yr−1 sr−1 at z ≥ 10). Once high-z GRBs are detected the principal issue is to secure their redshift. To this aim we estimate their NIR afterglow flux at relatively early times and evaluate the effectiveness of following them up and construct usable samples of events with any forthcoming GRB mission dedicated to explore the high-z Universe.
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