Is gamma-ray burst polarization from photosphere emission?

Context: Despite more than half a century of research, the dominant radiation mechanism of gamma-ray burst (GRB) prompt emission remains unsolved. Some progress has been made through the analyses of the observational spectra of Swift/BAT, Konus/Wind, and Fermi/GBM, as well as the spectra of the photosphere or synchrotron models, but it is still insufficient to pin down the answer. Aims: Combining the spectral and polarization observations, we seek new criteria for model evaluation. Methods: We thoughtfully investigate the polarization samples of POLAR and AstroSAT, combining the light curve, the spectral and the polarization parameters. Results: The power-law shape of the X-ray afterglows, the $T_{90} \propto (L_{\text{iso}})^{-0.5}$ correlation, and the hard low-energy spectral index $α$ are revealed, thus supporting the photosphere origin. Furthermore, we discover the positive correlation of the $α$ and the polarization degree (PD), which can be consistently explained by the photosphere polarization scenario involving the jet asymmetry from a moderate viewing angle of $θ_{v}$=0.015.

💡 Research Summary

This paper tackles the long‑standing problem of identifying the dominant radiation mechanism in gamma‑ray burst (GRB) prompt emission by jointly analysing spectral and polarization data. The authors compile a sample of roughly two dozen GRBs with reported polarization measurements from the dedicated polarimeters POLAR and AstroSAT. For each burst they collect the X‑ray afterglow light curve, the prompt spectral parameters (low‑energy index α, high‑energy index β, peak energy E_p), and the polarization degree (PD) together with the polarization angle.

The first major observational result is that all the X‑ray afterglows in the sample follow a simple power‑law decay without a plateau, steep drop, or strong flares. This behaviour mirrors that of high‑efficiency (ε_γ ≳ 80 %) and high‑energy (GeV/TeV‑detected) GRB samples and is precisely what the classic hot fireball (photospheric) model predicts for a thermal jet.

Next, the authors examine the relationship between the intrinsic duration T₉₀ and the isotropic‑equivalent luminosity L_iso. In log–log space the data are well described by log T₉₀ = 1.67 − 0.5 log L_iso, a correlation that holds for the polarization sample as well as for the high‑efficiency and high‑energy subsamples. This slope matches the theoretical expectation from neutrino‑annihilation powered accretion disks (NDAF) feeding a black‑hole central engine, reinforcing the view that the jet is thermally dominated.

Spectral analysis reveals that the low‑energy photon index α is typically close to or harder than the synchrotron “death line” (α = ‑2⁄3). Such hard low‑energy spectra have been repeatedly cited as evidence for photospheric emission, because non‑thermal synchrotron radiation rarely produces α > ‑2⁄3 without invoking additional fine‑tuned conditions.

The most novel finding is a positive correlation between α and the measured polarization degree. Bursts with harder low‑energy spectra tend to exhibit higher PD. The authors interpret this trend within the framework of photospheric polarization. In a structured jet, photons undergo multiple scatterings near the photosphere; if the jet is perfectly axisymmetric the net polarization cancels, but a modest viewing angle offset (θ_v ≈ 0.015 rad) breaks the symmetry and leaves a residual polarization of a few percent. Moreover, harder α values imply that a larger fraction of photons escape from hotter, deeper layers where the angular distribution of scattering is narrower, which naturally enhances the net PD. This quantitative picture reproduces the observed α‑PD trend.

The paper also addresses the systematic difference between POLAR (PD ≲ 20 %) and AstroSAT (PD ≈ 40–94 %). The authors argue that this discrepancy stems from different effective viewing angles and jet structures probed by the two instruments, rather than from instrumental bias. They further note that a hybrid Band‑plus‑blackbody spectral model is not required; the same hard α values emerge directly from a pure photospheric spectrum.

In summary, the study provides three independent lines of evidence—afterglow power‑law decay, the T₉₀ ∝ L_iso⁻⁰·⁵ correlation, and the hard low‑energy index—supporting a photospheric origin for GRB prompt emission. The newly identified α‑PD correlation offers a powerful diagnostic that aligns with theoretical predictions for photospheric polarization with a modest off‑axis viewing angle. The authors conclude that the dominant radiation mechanism in the examined GRBs is thermal photospheric emission, and they advocate for future high‑precision, time‑resolved polarimetry to further constrain jet geometry and composition.