The Origin of Self-similar FRED Profiles in Gamma-Ray Bursts Pulses

TL;DR

This work addresses the origin of self-similar self-similar FRED profiles in GRB pulses by proposing a physical mechanism in which an impulsively injected, magnetically dominated jet hosts propagating perturbations that trigger sequential, localized magnetic reconnection as a ring expands outward. By solving electron cooling and synchrotron radiation in each local dissipation site and integrating over a ring-like geometry, the model reproduces key spectrum-time features: self-similar FRED light curves across multiple energy bands, hard-to-soft spectral evolution, local intensity tracking, and increasing subpulse durations due to Doppler-factor decay. Population synthesis shows the $t_p-E$ and $t_w-E$ indices cluster around the diagonal, supporting self-similarity, and an empirical linear relation links observables to physical parameters, enabling estimation of one from the other. The authors also compile a catalogue of 17 general single FRED GRBs from Fermi/GBM, providing measured slopes of $t_p$ and $t_w$ with energy, thereby enabling direct comparison with the model. The findings imply that the GRB light curve reflects jet dissipation dynamics rather than central-engine activity history, with implications for reconciling the merger-short/collapsar-long dichotomy.

Abstract

To understand the physical mechanisms underlying the prompt emission of gamma-ray bursts (GRB), single FRED (Fast-Rise-Exponential-Decay) profile GRBs serve as an ideal sample, as they origin from single epoch central engine activity. These GRBs have been found to exhibit a peculiar morphology-including the elegant self-similarity across energy bands and the recently discovered composite nature challenging nearly all existing radiation mechanisms, sparking widespread curiosity about their origins. Here we propose a physical model which includes radiation locations sequentially triggered by propagating magnetic perturbations. It naturally explains all observed properties of these GRBs, including the self-similar FRED profile, multi-band aligned subpulses, hard-to-soft spectral evolution, local intensity tracking, and increasing subpulse durations. Furthermore, our results demonstrate that the duration of these GRBs is not reflecting the activity timescale of the central engine, reconciling recent challenges to the traditional merger-short/collapsar-long dichotomy of GRBs.

Figures (8)

• Figure 1: The illustration of the model
• Figure 2: The simulated light curve of a GRB in 7 different energy bands, i.e., 6-30keV, 30-70keV, 70-100keV, 100-150keV, 150-200keV, 200-500keV and 500-1000keV.
• Figure 3: The peak time and width of the simulated GRB light curves as a function of energy. The best fit power law are plotted as dashed lines.
• Figure 4: The simulated GRB spectra at 7 different time windows, corresponding to the light curves in figure \ref{['fig:lc']}.
• Figure 5: The $E_p$ as function of time, along with the energy integrated light curve of the simulated GRB.