L. Nava

Spectral-luminosity relation within individual Fermi gamma rays bursts

We study the spectra of all long gamma ray bursts (GRBs) of known redshift detected by the Fermi satellite untill the end of July 2009. 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 Epeak of their EL(E) spectrum correlates with the luminosity in a remarkably tight way within individual bursts. This time-resolved Epeak − Liso correlation is very similar for all the considered bursts and has a slope and normalisation similar to the analogous Epeak − 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 Epeak − 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, tightly linking 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. While calling for a robust physical interpretation, these results strongly indicate that the Epeak −Liso spectral energy correlation found considering the time-integrated spectra of different bursts is real and not the result of instrumental selection effects.

Short versus long gamma-ray bursts: spectra, energetics, and luminosities

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 Epeak−Eiso and Epeak−Liso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E obs : while short GRBs are inconsistent with the Epeak−Eiso correlation of long GRBs, they could follow the Epeak−Liso 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.

Confirming the γ-ray burst spectral-energy correlations in the era of multiple time breaks

We test the spectral-energy correlation including the new bursts detected (mostly) by Swift with firm measurements of their redshifts and peak energy. The problem of identifying the jet breaks in the complex and multibreak/flaring X-ray light curves observed by Swift is discussed in the complex and multibreak/flaring X-ray light curves observed by Swift. We use the optical data as the most reliable source for the identification of the jet break, since the X-ray flux may be produced by a mechanism different from the external shocks between the fireball and the circumburst medium, which are responsible for the optical afterglow. We show that the presence of an underlying SN event in XRF 050416A requires a break to occur in the afterglow optical light curve at around the expected jet break time. The possible presence of a jet break in the optical light curve of GRB 050401 is also discussed. We point out that, for measuring the jet break, it is mandatory that the optical light curve extends after the epoch where the jet break is expected. The interpretation of the early optical breaks in GRB 050922C and GRB 060206 as jet breaks is controversial because they might instead correspond to the flat-to-steep decay transition common in the early X-ray light curves. All the 16 bursts coming from Swift are consistent with the E Peak - E γ and E peak - E iso - t jet correlation. No outlier is found to date. Moreover, the small dispersion of the E peak - Ey and E peak - E iso - t jet correlation, confirmed also by the Swift bursts, strengthens the case of using GRBs as standard candles.

Peak energy of the prompt emission of long gamma-ray bursts versus their fluence and peak flux

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 E obs peak 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 E obs peak , 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 E obs peak than those of bright events. As a consequence, the E obs peak 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 E peak -E iso correlation (updated in this paper to include 83 bursts), since they are inconsistent with it for any redshift. E obs peak also correlates with the peak flux, with a slope similar to the Ep eak -L iso correlation. In this case, there is only one sure outlier. The scatter of the E obs peak -P correlation defined by the BATSE bursts of our sample is significantly smaller than the E obs peak -F correlation of the same bursts, while for the bursts with known redshift the E peak -E iso correlation is tighter than the E peak -L iso one. Once a very large number of bursts with E obs peak and redshift will be available, we thus expect that the E peak -L iso correlation will be similar to that currently found, whereas it is very likely that the E peak -E iso correlation will become flatter and with a larger scatter.

Optical and X-ray rest-frame light curves of the BAT6 sample

Aims. We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of the Swift long gamma-ray bursts (GRBs), namely, the BAT6 sample.Methods. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift (~95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities, which accounte for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands.Results. We compare the behaviour observed in the X-ray to that in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that rest-frame optical luminosity distribution of the GRBs is not bimodal and is clustered around the mean value Log(LR) = 29.9 ± 0.8 when estimated at a rest-frame time of 12 h. This is in contrast to what is found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events, the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves, which are normalised to the GRB isotropic energy (Eiso), provide evidence for X-ray emission that is still powered by the prompt emission until late times (~hours after the burst event). On the other hand, the same test performed for the Eiso-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.

Re-born fireballs in gamma-ray bursts

We consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to decelerate the fireball efficiently, leading to dissipation of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R∼ 1011 cm, not far from the transparency radius, and with starting bulk Lorentz factors Γc∼ 10. As seen in the observer frame, this ‘re-generated’ fireball appears collimated within an angle θj= 1/Γc. If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help to solve some open issues of the more conventional internal shock scenario for interpreting gamma-ray burst properties.

Selection effects on GRB spectral‐energy correlations

Instrumental selection effects can act upon the estimates of the peak energy Epeakobs, 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. Epeak, Eobs and the peak luminosity Liso) would be questioned. We estimated, as a function of Epeakobs, the minimum peak flux necessary to trigger a GRB and the minimum fluence a burst must have to determine the value of Epeakobs by considering different instruments (BATSE, Swift, BeppoSAX). We find that the latter dominates over the former. We then study the Epeakobs‐fluence (and flux) correlation in the observer plane. GRBs with redshift show well defined Epeakobs‐F and Epeakobs‐P correlations: in this planes the selection effects are present, but do not determine the found correlations. 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...

Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the

Reconciling the diffuse Galactic γ-ray and the cosmic ray spectra

\nu F_\nu

Bulk Lorentz factors of Gamma-Ray Bursts

peak energy should display a power-law behavior with slope