Statistical properties of Fermi GBM GRBs’ spectra

Abstract

Statistical studies of gamma-ray burst (GRB) spectra may result in important information on the physics of GRBs. The Fermi GBM catalog contains GRB parameters (peak energy, spectral indices, intensity) estimated fitting the gamma-ray SED of the total emission (fluence, flnc), and during the time of the peak flux (pflx ). Using contingency tables we studied the relationship of the models best fitting pflx and flnc time intervals.Our analysis revealed an ordering of the spectra into a power law -Comptonized -smoothly broken power law -Band series. This result was further supported by a correspondence analysis (CA) of the pflx and flnc spectra categorical variables. We performed a linear discriminant analysis (LDA) to find a relationship between categorical (spectral) and model independent physical data. LDA resulted in highly significant physical differences among the spectral types, that is more pronounced in the case of the pflx spectra, than for the flnc spectra. We interpreted this difference as caused by the temporal variation of the spectrum during the outburst. This spectral variability is confirmed by the differences in the low energy spectral index and peak energy, between the pflx and flnc spectra. We found that the synchrotron radiation is significant in GBM spectra. The mean low energy spectral index is close to the canonical value of α = −2/3 during the peak flux. However, α is ∼ −0.9 for the spectra of the fluences. We interpret this difference as showing that the effect of cooling is important only for the fluence spectra.

Fermi GBM GRBs' multivariate statistics

“…The reason for this strange association is probably that the Swift classified the spectra over the entire burst duration similar to the Fermi fluence type, and the spectrum changed in time. Several papers have found similar variability in the spectrum (among others Kumar & Zhang 2015;Mészáros 2006;Rácz et al 2018). The spectral evolution is best shown by the Band spectra, because more GRBs have the Band spectral class rather than peak flux time.…”

“…In our recent work (Rácz et al 2018), we revealed an ordering of the spectra into a power law-Comptonized-smoothly broken power law-Band series with contingency and correspondence analysis.…”

“…The largest discriminated distances appear to be between the Band and the power‐law classes, with the remaining two classes lying between them (Rácz et al ). The fluence spectra contain much more photons, and therefore more information about the GRBs, even though in this work we found the surprising and significant effect that the level of discrimination is higher in case of the peak flux than the fluence spectra.…”

“…In this case, we used the linear discriminant analysis (LDA) originally proposed by Fischer (1936); see McLachlan (2004) for a review. Further mathematical background can be found in Section 3.3.1 of Rácz et al (2018).…”

Multi‐messenger studies of γ‐ray bursts and their cosmic environment

“…We can separate two types of emissions: the radiation from the central engine—the prompt emission—and the afterglow emission from the interaction of the outflow with the environment (Mészáros ). Most γ‐radiation originates from the central engine, and the evolution of the progenitor can be observed by the change in the γ‐spectra (Rácz et al n.d.; Rácz et al ).…”