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

16 pages, 10 figures, 6 tables, accepted for the publication in Astronomy and Astrophysics

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