The correlations between the rest frame peak of the EF_E spectrum of GRBs Epeak and their isotropic energy (E_iso) or luminosity (L_iso) could have several implications for the understanding of the GRB prompt emission. These correlations are presently founded on the time-averaged spectral properties of a sample of 95 bursts, with measured redshifts, collected by different instruments in the last 13 years (pre-Fermi). One still open issue is wether these correlations have a physical origin or are due to instrumental selection effects. By studying 10 long and 14 short GRBs detected by Fermi we find that a strong time-resolved correlation between E_peak and the luminosity L_iso is present within individual GRBs and that it is consistent with the time-integrated correlation. This result is a direct proof of the existence in both short and long GRBs of a similar physical link between the hardness and the luminosity which is not due to instrumental selection effects. The origin of the E_peak-L_iso correlation should be searched in the radiation mechanism of the prompt emission.
Deep Dive into Gamma Ray Bursts Spectral--Energy correlations: recent results.
The correlations between the rest frame peak of the EF_E spectrum of GRBs Epeak and their isotropic energy (E_iso) or luminosity (L_iso) could have several implications for the understanding of the GRB prompt emission. These correlations are presently founded on the time-averaged spectral properties of a sample of 95 bursts, with measured redshifts, collected by different instruments in the last 13 years (pre-Fermi). One still open issue is wether these correlations have a physical origin or are due to instrumental selection effects. By studying 10 long and 14 short GRBs detected by Fermi we find that a strong time-resolved correlation between E_peak and the luminosity L_iso is present within individual GRBs and that it is consistent with the time-integrated correlation. This result is a direct proof of the existence in both short and long GRBs of a similar physical link between the hardness and the luminosity which is not due to instrumental selection effects. The origin of the E_peak
One of the key properties of the prompt emission of gamma ray bursts (GRBs) that is still poorly understood concerns the spectral-energy correlations found when considering the time-integrated spectra of bursts of known redshift. The peak energy of the spectrum, E peak in the νF ν representation, is strongly correlated with the isotropic luminosity L iso (Yonetoku et al. 2004) or with the isotropic energy E iso (Amati et al. 2002), and more tightly with the collimation-corrected energy E γ (Ghirlanda, Ghisellini & Lazzati 2004).
There are two strong motivations for studying these correlations: understand their physics and use them to standardize the GRB energetics, making them cosmological tools.
However, the E peak -E iso and E peak -L iso correlations have been derived considering long GRBs (Fig. 1 filled grey and open red circles) due to the lack of measured redshifts of short GRBs. It has been shown recently (Ghirlanda et al. 2009) that the few short GRBs with measured z (filled blue squares in Fig. 1) do not follow the E peak -E iso correlation (Fig. 1, left panel) but they are consistent with the E peak -L iso correlation (Fig. 1, right panel).
A still debated issue is wether these correlations have a physical origin or they are the result of instrumental selection effects (Nakar & Piran 2005;Band & Preece 2005;Butler et al. 2007, Butler, Kocevski & Bloom 2009;Shahmoradi & Nemiroff 2009 but see Ghirlanda et al. 2005, Bosnjak et al. 2008, Ghirlanda et al. 2008;Nava et al., 2008;Krimm et al. 2009;Amati, Frontera & Guidorzi 2009).
A completely orthogonal possibility (with respect to the debate opened in the literature -e.g. Butler, Kocevski & Bloom 2009 but see e.g. Ghirlanda et al. 2008) to answer this Ghirlanda et al. 2009Ghirlanda et al. , 2010Ghirlanda et al. , 2010a) ) question is to study individual bursts to see whether the luminosity and peak energy at different times during the prompt phase correlate. If they do, and furthermore if the slope of this time-resolved correlation (indicated E t peak -L t iso hereafter) is similar to the time-integrated E peak -L iso correlation found among different bursts, then we should conclude that the spectral energy correlations are surely a manifestation of the physics of GRBs and not the result of instrumental selection effects.
The two questions we want to answer are: (1) is there a time resolved spectral energy correlation within individual GRBs? and (2) is this correlation present in both short and long GRBs?
To answer these questions the time-resolved spectral analysis of GRB is required. Moreover, in order to follow the evolution of the spectrum and its peak energy E peak in time all over the burst duration, a large spectral energy window is desirable like that of the Gamma Burst Monitor (GBM, 8keV-40MeV) onboard the Fermi satellite. Here we present the main results of the study of the spectral evolution of 10 long GRBs and 14 short GRBs detected by Fermi (these results have been published in Ghirlanda et al. 2010Ghirlanda et al. , 2010a)).
We have analyzed the time resolved spectra of 10 long GRBs with measured redshifts detected by Fermi up to July 2009. Their spectral evolution shows that the peak energy tracks the flux and a strong time-resolved E t peak -L t iso correlation is present within individual GRBs (Fig. 2 left panel). The time resolved E t peak -L t iso correlation is similar to the time-integrated one (solid line in Fig. 2 left panel). The same analysis was applied to short GRBs detected by Fermi. Except for GRB 090510 at z = 0.9 (open blue square in Fig. 1), the redshifts of the other Fermi short bursts is not measured. However, we can still study the correlation between the observer frame peak energy and the bolometric flux 1). Small symbols show the evolution of E peak vs Liso. Large symbols are the location in the E peak -Liso plane of the corresponding bursts when the time-integrated spectra are considered (for GRB 081007 and GRB 090423, only the time-integrated spectrum is available). The solid and dotted lines represent the E peak -Liso correlation and its 3σ scatter, respectively, as obtained with the pre-Fermi GRBs (filled grey circles in Fig. 1. The dot-dashed line represents the t to the 10 Fermi GRBs (time-integrated) and the triple-dot-dashed line is the t to the 51 time-resolved spectra (Ghirlanda et al. 2010). Right: time-resolved correlation between the peak energy E peak and the flux of the 163 time resolved spectra of the 13 short Fermi GRBs analyzed in Ghirlanda et al. 2010a. Different symbols/colors correspond to different bursts (as shown in the legend). For all but one short GRBs (090510 -open blue square in Fig. 1) the redshift is not known, this is why their spectral evolution is represented in the observer frame. The solid line (dashed lines) is the E peak -Liso correlation (and its 3σ scatter) of long GRBs transformed in the observer frame assuming z = 1 (z = 0.5 for the grey solid and dotted lines).
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