Intrinsic spectra and energetics of BeppoSAX Gamma–Ray Bursts with known redshifts

Abstract

Abstract. We present the main results of a study of spectral and energetics properties of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the redshift estimates and the good-quality BeppoSAX time-integrated spectra we deduce the main properties of GRBs in their cosmological rest frames. All spectra in our sample are satisfactorily represented by the Band model, with no significant soft X-ray excesses or spectral absorptions. We find a positive correlation between the estimated total (isotropic) energies in the 1-10 000 keV energy range (E rad ) and redshifts z. Interestingly, more luminous GRBs are characterized also by larger peak energies E p s of their EF(E) spectra. Furthermore, more distant GRBs appear to be systematically harder in the X-ray band compared to GRBs with lower redshifts. We discuss how selection and data truncation effects could bias our results and give possible explanations for the correlations that we found.

Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE

“…The distribution of E 0 and α Figure 3 displays α as a function of the cutoff energy E 0 for the 35 GRBs studied here. We have also plotted for comparison 9 GRBs/XRFs discussed in Kippen et al 2001 and 12 GRBs described in Amati et al (2002). This figure shows a good agreement between the values measured by BeppoSAX and FREGATE.…”

“…Finally, we note that Amati et al (2002) find a correlation between the intrinsic (redshift corrected) E p of 12 GRBs with known redshifts, and E 52 , their isotropic-equivalent energy radiated in gamma-rays, in units of 10 52 ergs: E p = 100 E 0.52 52 keV. This correlation, if it extends over a sufficient range of redshifts could certainly explain the hardness-intensity correlation we observe.…”

“…If we assume that the correlation found by Amati et al (2002) extends down to E p as low as 20 keV, it would imply that the isotropic-equivalent energy radiated by a GRB with E p = 20 keV is about 80 times smaller than the isotropic-equivalent energy radiated by a "typical" GRB with E p = 200 keV. Combined with the conclusion of Frail et al (2000) that the total energy of GRBs is roughly constant, it implies that the jet opening angle of X-ray rich GRBs are substantially larger than the jet opening angle of "typical" GRBs.…”

“…In a clean fireball the initial Lorentz factor is higher but the internal shocks take place at larger distances from the central source where the density and the magnetic fields are smaller, leading to the emission of less energetic photons. Further theoretical studies are needed to assess whether one of these conditions can also explain the correlation found by Amati et al (2002), the lack of bright optical afterglows and the unusual properties of the X-ray afterglows of X-ray rich GRBs (Frontera et al 2000b;Feroci et al 2001).…”

GRB Prompt X-ray Emission

“…4.3. E peak -E iso and E peak -L iso relations The empirical spectral relations between E peak and the isotropically radiated γ-ray energy E iso (E peak -E iso relation; Amati et al 2002) and between E peak and the peak isotropic luminosity L iso (E peak -L iso relation; Yonetoku et al 2004) are well discussed relationships of the prompt emission. As shown in Figure 5 and 6, most of the long GRBs are consistent with the E peak -E iso and the E peak -L iso relations including the SN-GRBs except GRB 980425.…”