Double-peak spectral energy density of GRBs and the true identity of GRB 031203

A double-peak spectral-energy-density of gamma-rays, similar to that observed in blazars, is expected in gamma-ray bursts (GRBs) produced in supernova (SN) explosions. The second peak, which is formed by inverse Compton scattering of ambient SN light by cosmic ray electrons accelerated by the jets from the SN explosion, has a much higher peak-energy than the first ordinary peak. However, in X-ray flashes (XRFs), which in the cannonball (CB) model are normal GRBs viewed farther off axis, the first peak-energy is shifted to the soft X-ray region while the second peak-energy moves to the MeV range. In far-off-axis GRBs, such as GRBs 980425 and 031203, the second peak may have been confused with the normal GRB peak. In most GRBs, which have been observed so far, the gamma-ray detectors ran out of statistics far below the second peak. However, in bright GRBs, the two peaks may be resolved by simultaneous measurements with SWIFT and GLAST.Comment: The estimated soft X-ray fluence in GRB 031203 was replaced in the text and Fig. 3 with the correct estimate by Vaughan et al. Corrected typo

Extremely energetic Fermi Gamma-Ray Bursts obey spectral energy correlations

The extremely energetic Fermi GRBs 080916C, with its Eiso of ~ 10^{55} erg in 1 keV - 10 GeV and intense GeV emission, and 090323 give us a unique opportunity to test the reliability and extension of spectral energy correlations. Based on Konus/WIND and Fermi spectral measurements, we find that both events are fully consistent with the updated (95 events as of April 2009) Ep,i - Eiso correlation, thus further confirming and extending it and pointing against a possible flattening or increased dispersion at very high energies. This also suggests that the physics behind the emission of peculiarly bright and hard GRBs is the same as for softer and weaker ones. In addition, we find that the normalization of the correlation obtained by considering these two GRBs and the other long ones for which Ep,i was measured with high accuracy by the Fermi/GBM are fully consistent with those obtained by other instruments (e.g., BeppoSAX, Swift, Konus-WIND), thus indicating that the correlation is not affected significantly by detectors limited thresholds and energy bands. Prompted by the extension of the spectrum of GRB 080916C up to several GeVs without any excess or cut-off, we also investigated if the evaluation of Eiso in the commonly adopted 1 keV - 10 MeV energy band may bias the Ep,i - Eiso correlation contributing to its scatter. By computing Eiso from 1 keV to 10 GeV, the slope of the correlation becomes slightly flatter, while its dispersion does not change significantly. Finally, we find that GRB 080916C is also consistent with most of the other spectral energy correlations derived from it, with the possible exception of the Ep,i - Eiso - tb correlation.Comment: 9 pages, 5 figures, final revised version accepted for pubblication in Astronomy & Astrophysics (main Journal

Interpretation of High Energy String Scattering in terms of String Configurations

High energy string scattering at fixed momentum transfer, known to be dominated by Regge trajectory exchange, is interpreted by identifying families of string states which induce each type of trajectory exchange. These include the usual leading trajectory $\alpha(t)=\alpha^\prime t+1$ and its daughters as well as the ``sister'' trajectories $\alpha_m(t)=\alpha(t)/m-(m-1)/2$ and their daughters. The contribution of the sister $\alpha_m$ to high energy scattering is dominated by string excitations in the $m^{th}$ mode. Thus, at large $-t$, string scattering is dominated by wee partons, consistently with a picture of string as an infinitely composite system of ``constituents'' which carry zero energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-

A cosmological independent calibration of the Ep,i-Eiso correlation for Gamma Ray Bursts

The relation connecting the emitted isotropic energy and the rest-frame peak energy of the \nuF\nu spectra of Gamma-Ray Bursts (the Amati relation), strictly depends on the cosmological model, so we need a method to obtain an independent calibration of it. Using the Union Supernovae Ia catalog, we obtain a cosmographic luminosity distance in the y-redshift and demonstrate that this parametrization approximates very well the fiducial standard comsomlogical model \LambdaCDM. Furthermore, by this cosmographic luminosity distance dl, it is possible to achieve the Amati relation independent on the cosmological model. The cosmographic Amati relation that we obtain agrees, in the errors, with other cosmological-independent calibrations proposed in the literature. This could be considered a good indication in view to obtain standard candles by Gamma-Ray Bursts Key words. Gamma rays : bursts - CosmologyComment: 6 pages, 3 figure

A search for pulsations in short gamma-ray bursts to constrain their progenitors

We searched for periodic and quasiperiodic signal in the prompt emission of a sample of 44 bright short gamma-ray bursts detected with Fermi/GBM, Swift/BAT, and CGRO/BATSE. The aim was to look for the observational signature of quasiperiodic jet precession which is expected from black hole-neutron star mergers, but not from double neutron star systems. Thus, this kind of search holds the key to identify the progenitor systems of short GRBs and, in the wait for gravitational wave detection, represents the only direct way to constrain the progenitors. We tailored our search to the nature of the expected signal by properly stretching the observed light curves by an increasing factor with time, after calibrating the technique on synthetic curves. In none of the GRBs of our sample we found evidence for periodic or quasiperiodic signals. In particular, for the 7 unambiguously short GRBs with best S/N we obtained significant upper limits to the amplitude of the possible oscillations. This result suggests that BH-NS systems do not dominate the population of short GRB progenitors as described by the kinematic model of Stone, Loeb, & Berger (2013).Comment: 7 pages, 5 figures, accepted to ApJ, added reference