Spectral index-flux relation for investigating the origins of steep decay in γ-ray bursts

Abstract

Abstractγ-ray bursts (GRBs) are short-lived transients releasing a large amount of energy (1051 − 1053 erg) in the keV-MeV energy range. GRBs are thought to originate from internal dissipation of the energy carried by ultra-relativistic jets launched by the remnant of a massive star’s death or a compact binary coalescence. While thousands of GRBs have been observed over the last thirty years, we still have an incomplete understanding of where and how the radiation is generated in the jet. Here we show a relation between the spectral index and the flux found by investigating the X-ray tails of bright GRB pulses via time-resolved spectral analysis. This relation is incompatible with the long standing scenario which invokes the delayed arrival of photons from high-latitude parts of the jet. While the alternative scenarios cannot be firmly excluded, the adiabatic cooling of the emitting particles is the most plausible explanation for the discovered relation, suggesting a proton-synchrotron origin of the GRB emission.

Evolution patterns of the peak energy in the GRB prompt emission

“…Also, the rapid decay of synchrotron intensity along with R means that the observed spectrum/E p is dominated by the high-latitude emission of the EATS. Similarly, Ronchini et al (2021) find that the spectral evolution becomes dominated by the emission at larger angles for large values of q. Therefore, a chromatic intensity-tracking evolution pattern in E p is produced.…”

“…Recently, Ronchini et al (2021) showed a relation between spectral index and flux by investigating the X-ray tails of bright GRB pulses, which is incompatible with the long-standing scenario that invokes the delayed arrival of photons from highlatitude parts of the jet. It is suggested that a dominating adiabatic cooling process, accompanied by the decay of the magnetic field, could explain this relation after testing several possible cooling mechanisms.…”

“…As the radial component of the magnetic field decreases more rapidly than the toroidal component, the magnetic field is soon toroidally dominated, which decreases as R −1 (Lyubarsky 2009). Therefore, q is always set to be 1 in the main text (Uhm & Zhang 2015;Geng et al 2018b), while the effect of different q (e.g., Ronchini et al 2021) on our results is further discussed in Appendix D. Hence, as a function of decreasing Γ and B , a hard-to-soft evolution pattern for E p is naturally expected according to Eq. ( 1).…”

“…The value of the magnetic field decaying index (q) varies from 1 to 2 due to the geometry of the shell and the structure of the magnetic field (Mészáros & Rees 1999;Spruit et al 2001). Moreover, it was suggested that q could be 0.6 in recent studies (Ronchini et al 2021). To investigate the effect of different q on our results in the main text, we adopt the other three possible values of q (i.e., 0.5, 1.5, and 2) and perform numerical calculations for different cases as above.…”

GRB Prompt Emission with Anisotropic Electron Distribution

“…In Ronchini et al (2021), the observed evolutions of the spectral photon index and the flux density in the steep decay phase of the early afterglow, which is considered as a tail emission of a prompt emission, suggest that adiabatic cooling is the dominant cooling process. Therefore, the suppression of the radiative cooling for low-energy particles seems essential for the GRB emission process.…”