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

TL;DR

This study analyzes the spectro-polarimetric properties of GRB 220107A's two emission episodes to investigate evolving radiation mechanisms, highlighting the potential of time-resolved spectro-polarimetry in constraining GRB physics.

Contribution

It provides the first detailed spectro-polarimetric analysis of a multi-episode GRB, demonstrating how spectral evolution and polarization constraints inform emission models.

Findings

Spectral softening observed between the two episodes.

No significant polarization detected overall, with marginal polarization hints.

Spectral evolution suggests sub-photospheric dissipation or synchrotron emission as possible mechanisms.

Abstract

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 \%$, $2\sigma$). 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 ($\alpha \sim -0.72$). 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 \%$, $2\sigma$), 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.