2025A&A...693A.156M

Context. For over two decades, gamma-ray burst (GRB) prompt emission spectra were modeled with smoothly broken power laws (Band function), and a positive and tight correlation between the spectral rest-frame peak energy, E_p,z, and the total isotropic-equivalent luminosity, L_iso, was found, constituting the so-called Yonetoku relation. However, more recent studies show that many prompt emission spectra are well described by the synchrotron radiation model, and hence significantly deviate from the Band function.
Aims. In this work, we test the impact of a more refined spectral model such as an idealized synchrotron spectrum from nonthermal electrons on the Yonetoku relation and its connection with physical parameters.
Methods. We selected GRBs with measured redshift observed by Fermi/GBM together with high-energy observations (> 30 MeV), and performed a spectral analysis, dividing them in two samples: the single-bin sample, using the light curve peak spectrum of each GRB, and the multiple-bin sample, for which we explored the whole duration of 13 bright bursts with time-resolved spectral analysis.
Results. We observed that the E_p,z of synchrotron spectra in a fast-cooling regime (ν_m/ν_c ≫ 1) is generally larger than the one provided by the Band function. For this reason, we do not find any E_p,z-L_iso correlation in our samples except for the GRBs in an intermediate-cooling regime (1< ν_m/ν_c< 3); namely, where peak and break energies are very close. We instead find in both our samples a new tight correlation between the rest-frame cooling frequency, ν_c, z_, and L_iso: $ \nu_{c,z} \propto {L_{\mathrm{iso}}}^{(0.53 \pm 0.06)} $.
Conclusions. These results suggest that, assuming that prompt emission spectra are produced by synchrotron radiation, the physical relation is between ν_c, z_ and L_iso. The fit of the Band function to an intrinsic synchrotron spectrum returns peak energy values of $ E_{p,z}^{\mathrm{Band}} \sim \nu_{c,z} $. This may explain why the systematic interpretation of prompt spectra through the Band function returns the E_p, z_ - L_iso relation.