Identifying the poynting-flux-dominated outflow of Fermi GRBs with non-thermal spectrum and its energy-resolved light curve fitting

TL;DR

This study systematically probes the jet composition of GRBs by combining spectral decomposition and energy-dependent timing analyses for 88 Fermi/GBM GRBs with redshift. Through Bayesian fits of multiple spectral models, 80 pulses exhibit purely non-thermal spectra, enabling lower-limit estimates of the magnetization parameter $\sigma$; about 40% of these pulses have $σ>5$, indicating Poynting-flux-dominated outflows consistent with the ICMART model. Independent validation comes from energy-resolved light-curve fitting, where 13 of 15 high-$σ$ cases show an inverse pulse-width–energy relation, aligning with mini-jet dynamics in magnetized outflows. Together, these results imply a substantial fraction of bright GRBs are powered by magnetic energy dissipation, with significant implications for GRB jet physics and prompt emission mechanisms. Limitations include dependence on the assumed photospheric radius $R_0$, the single-pulse time-integrated spectra, and the need for higher-energy observations for definitive confirmation.

Abstract

The jet compositions of gamma-ray bursts (GRBs) are very important to understand the energy dissipation and radiation mechanisms, but it remains an open question in GRB physics. In this paper, we present a systematic analysis to search for 88 bright GRBs that include a total of 129 pulses observed by Fermi/GBM with redshift measured, and extract the spectra of each pulse with Band function (Band), cutoff power-law (CPL), blackbody (BB), non-dissipative photospheric (NDP), Band+BB, as well as CPL+BB. We find that 80 pulses, 35 pulses, and 14 pulses present purely non-thermal, hybrid, and thermal spectra, respectively. By focusing on those 80 pulses with purely non-thermal spectra, one can estimate the lower limits of magnetization factor ($σ$) via suppressing the pseudo-thermal component. It is found that 30 pulses in 21 GRBs are the lower limit of $σ>5$ at the photosphere by adopting $R_{0}=10^{10}$ cm. It suggests that at least the outflow of those GRB jets with high $σ$ is dominated by Poynting-flux. On the other hand, we also perform the light curve fitting with a fast-rise-exponential-decay (FRED) model for 15 bright GRBs with a high magnetization factor in our sample, and find that a correlation between pulse width ($w$) and energy of 13 GRBs really exists in their energy-resolved light curves. It is also a piece of independent evidence for those GRBs with a high value $σ$ to support the origin of the Poynting flux outflow.

Figures (17)

• Figure 1: An example of time-integrated spectral fitting of GRB 080916C with Band function model. Left: Observed and modeled photon count spectra. Right: the parameter constraints of the spectral fits.
• Figure 2: Spectral and temporal analysis of an example GRB 160519A for group (I). (a‌). The predicted lower limits of the photosphere spectra (dashed lines) with $R_{0}=10^{10}$ cm and observed non-thermal spectrum (solid line). (b). The light curves of the prompt emission of GRB 160519A (gray) in the different energy ranges and the FRED model fitting (red solid lines). (c). The pulse width ($w$) is derived from FRED model fitting as a function of energy and the power-law fitting (solid red line).
• Figure 3: Spectral and temporal analysis of an example GRB 080916C for group (II). (a‌). The predicted lower limits of the photosphere spectra (dashed lines) with $R_{0}=10^{10}$ cm and observed non-thermal spectrum (solid line). (b). The light curves of the prompt emission of GRB 080916C (gray) in the different energy ranges and the FRED model fitting (red and blue solid lines). (c). The pulse width ($w$) is derived from the FRED model fitting for sub-pulse and whole pulse as a function of energy and the power-law fitting (red and blue solid lines).
• Figure 4: Spectral and temporal analysis of an example GRB 140512A for group (III). (a‌). The predicted lower limits of the photosphere spectra (dashed lines) with $R_{0}=10^{10}$ cm and observed non-thermal spectrum (solid lines). (b). The light curves of prompt emission of GRB 140512A (gray) in the different energy ranges and the FRED model fitting (red solid lines). (c). The pulse width ($w$) is derived from the FRED model fitting for the bright pulse as a function of energy and the power-law fitting (red solid lines).‌‌
• Figure 5: 2-D distributions of $\sigma$ and $kT_{\rm max}$. The solid circles are the lower limit of estimated $\sigma$ for pulses with non-thermal spectrum.