A Common Synchrotron Origin for Prompt Gamma-Ray and Soft X-Ray Emission in GRBs: Evidence from Joint Spectral Analysis

TL;DR

The paper tests whether GRB prompt emission across 0.5–150 keV arises from a single synchrotron-emitting region. By performing joint Swift BAT–XRT spectral fits on 110 pulses from 46 GRBs, it shows that a power-law with a low-energy break or cutoff describes the spectra, with break energies at a few keV and indices consistent with fast-cooling synchrotron emission. The results assign the low-energy break to the cooling frequency $\nu_c$ and the high-energy peak to the minimum frequency $\nu_m$, explaining why X-ray and gamma-ray light curves evolve coherently yet differ in timing. This supports a unified, single-region origin for prompt GRB emission and underscores spectral evolution driven by $\nu_c$ and $\nu_m$, with practical implications for future multi-band GRB observations.

Abstract

The recent launches of the Einstein Probe (EP) and the Space Variable Objects Monitor (SVOM) mission have led to the detection of a growing number of long GRBs with significant, early soft X-ray flux during their gamma-ray emission, prompting the question of whether their multi-band prompt emission shares a common origin in region and mechanism. To address this, we utilize the 20-year Swift archival data, which provides a substantial sample of joint soft X-ray and gamma-ray observations, enabling a systematic joint spectral study. We resolve 110 temporal pulses from 46 GRBs and find that a single power-law model with a low-energy break or cutoff adequately describes the prompt spectra from 150 keV down to 0.5 keV. More than half of the sample pulses require a break around a few keV, with average spectral indices $\langle α_1 \rangle = -0.88$ and $\langle α_2 \rangle = -1.46$ consistent with synchrotron radiation in a marginally fast-cooling regime. The observed spectral evolution and the distribution of indices support a single-emission-region origin, where the varying spectral shapes are largely governed by the evolution of the synchrotron cooling frequency $ν_c$ and the effect of finite emission width. The observed differences in the temporal behavior between X-ray and gamma-ray light curves can be naturally explained by this spectral evolution across the broad band.

Figures (19)

• Figure 1: GRB 241030A: fitted single power-law indices of integrated x-ray spectrum, affected by the selection of different exclusion region radius.
• Figure 2: GRB 241030A: resolved light curve and best-fit spectral parameters
• Figure 3: GRB 241030A: $\nu F_{\nu}$ spectra (SED) and best-fit model of 9 pulse intervals
• Figure 4: SED shapes of the power-law models used in this article. For BPL we denote index $\alpha_1$ and $\alpha_2$ below and above the break. For BPL with an SED peak observed, we denote indices $\alpha$ and $\beta$, with reference to the 'Band' shape. Quoted text refer to the percentage of best-fit models in our sample. There is also a possible subdominant blackbody. Grey shade represent an ideal fast-cooling synchrotron shape with arbitrary normalization.
• Figure 5: Distribution of power-law indices. Different color and linestyle refer to different models. Dashed, dotted lines and shades show typical synchrotron values and range.