Episode-wise spectro-polarimetry of GRB 220107A: Testing the hypothesis of evolving radiation mechanisms

We investigate the spectro-polarimetric properties of the long-duration GRB~220107A, which exhibited two distinct emission episodes separated by a 40 s quiescent gap, to test whether such multi-episode bursts show evidence for evolution in their underlying radiation mechanisms. We analyzed prompt emission data from AstroSat/CZTI, Fermi/GBM, and Konus-Wind, performing spectro-polarimetric analysis for each emission episode. The time-integrated polarization analysis shows no significant detection (PF$ < 38 %$, $2σ$). Time-resolved analysis reveals clear spectral evolution between the two episodes, with episode 1 exhibiting a hard low-energy photon index and episode 2 showing substantial spectral softening ($α\sim -0.72$). Regarding polarization: Episode 1 shows a low polarization upper limit (< 52%), consistent with expectations for photospheric emission dominated by quasi-thermal Comptonization in a baryon-rich outflow. Episode 2 also shows overall low polarization (PF$ < 55 %$, $2σ$), though sliding-window analysis yields a marginally elevated signal (PF$= 70 \pm 30%$, BF = 2.8) between T0+76 to T0+88 s. The robust spectral softening between episodes could arise from sub-photospheric dissipation, optically thin synchrotron radiation in small-scale magnetic fields, or if the tentative polarization enhancement proves intrinsic, it would favor synchrotron emission in large-scale ordered magnetic fields. The spectral evolution of GRB 220107A, combined with our polarimetric constraints, demonstrates the diagnostic potential of time-resolved spectro-polarimetry for constraining GRB prompt emission physics. We present GRB 220107A as a test case illustrating how future higher sensitivity observations could discriminate between competing emission models for multi-episode bursts. Our results emphasize both the promise and current limitations of prompt phase polarimetry.

💡 Research Summary

This paper presents a detailed episode‑wise spectro‑polarimetric study of the long‑duration gamma‑ray burst GRB 220107A, which displayed two distinct emission episodes separated by a ∼40 s quiescent interval. The authors combine prompt‑phase data from AstroSat’s Cadmium Zinc Telluride Imager (CZTI), Fermi/GBM, and Konus‑Wind to perform independent spectral fitting and polarization measurements for each episode, and they also report a redshift of z = 1.246 obtained with the 6 m BTA telescope.

The temporal analysis shows that the first episode (T₀‑2 s to T₀+38 s) is captured by all three instruments, while the second episode (T₀+77 s to T₀+106 s) is visible only to CZTI and Konus‑Wind because Earth occultation blocks the GBM view. Spectral fitting with the empirical Band function yields a hard low‑energy photon index (α ≈ ‑0.2) and a peak energy around 350 keV for the first episode, whereas the second episode exhibits a markedly softer low‑energy index (α ≈ ‑0.72) and a lower peak energy near 180 keV. The authors interpret the hard spectrum of the first episode as consistent with quasi‑thermal Comptonization at the photosphere of a baryon‑rich outflow, while the softening in the second episode points to either sub‑photospheric dissipation, optically thin synchrotron radiation in small‑scale turbulent magnetic fields, or a transition to synchrotron emission in a more ordered magnetic configuration.

Polarization analysis is carried out using a Bayesian framework that estimates the polarization fraction (PF) and angle (PA) from CZTI’s Compton‑scatter event distribution. The time‑integrated measurement over the full burst (T₀‑2 s to T₀+106 s) yields PF < 38 % at the 2σ confidence level, with a Bayes factor (BF) of 0.64 favoring the unpolarized hypothesis. For episode 1, the 1.5σ upper limit is PF < 52 %, again consistent with low polarization expected from photospheric emission. Episode 2 shows an overall PF < 55 % (2σ), but a sliding‑window analysis (12 s bins) reveals a marginally elevated signal between T₀+76 s and T₀+88 s: PF = 70 ± 30 % with BF ≈ 2.8. Although this does not reach a definitive detection, if intrinsic it would suggest the presence of a large‑scale ordered magnetic field, as predicted for synchrotron emission.

The authors discuss three plausible scenarios: (1) the first episode is dominated by photospheric emission, while the second reflects sub‑photospheric dissipation that modifies the spectrum without producing strong polarization; (2) the second episode is synchrotron radiation in a tangled magnetic field, yielding low PF despite the spectral softening; (3) the tentative high PF in the second episode is real, indicating synchrotron emission in a globally ordered field. They emphasize that current CZTI sensitivity limits prevent a conclusive polarization detection, but the methodology demonstrates the diagnostic power of time‑resolved spectro‑polarimetry.

In conclusion, GRB 220107A serves as a compelling test case for investigating temporal evolution of GRB radiation mechanisms. The combination of clear spectral evolution and stringent polarization constraints illustrates how future high‑sensitivity polarimeters (e.g., POLAR‑2, LEAP) coupled with broadband spectroscopy can discriminate between photospheric and synchrotron models in multi‑episode bursts. The paper underscores both the promise and present limitations of prompt‑phase polarimetry, advocating for next‑generation instruments to resolve the long‑standing debate over GRB prompt emission physics.