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

Context. The radiation mechanisms powering the prompt emission of Gamma-ray bursts (GRBs) remain a long-standing question, with both synchrotron and photospheric models offering plausible explanations. Time-resolved spectropolarimetric measurements are crucial, as they directly constrain the emission mechanism, magnetic field structure, and jet composition. Aims. We investigate the spectropolarimetric properties of the GRB 220107A, which exhibited two distinct emission episodes separated by a ∼40 s quiescent gap, to test whether such multi-episode bursts show evidence of evolution in their underlying radiation mechanisms. Methods. We analyzed prompt emission data from AstroSat /CZTI, Fermi /GBM, and Konus -Wind, performing spectropolarimetric analysis for each emission episode. In addition, we report a redshift of z = 1.246 using the 6 m BTA telescope. Results. The time-integrated polarization analysis (T 0 −2 to T 0 + 106 s) we performed shows no significant detection (PF < 38%, 2 σ confidence; BF = 0.64), whereas the time-resolved analysis revealed 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 ∼ −0.72). Regarding polarization, episode 1 shows a low polarization upper limit (1.5 σ upper limit < 52%), consistent with expectations for photospheric emission dominated by quasi-thermal Comptonization in a baryon-rich outflow. Episode 2 also shows low polarization overall (PF < 55%, 2 σ ; BF ∼ 1), though our sliding-window analysis yielded a marginally elevated signal (PF = 70 ± 30%, BF = 2.8) between T 0 + 76 and T 0 + 88 s. We interpret these measurements cautiously: The robust spectral softening between episodes could arise from sub-photospheric dissipation or optically thin synchrotron radiation in small-scale magnetic fields. Alternatively, if the tentative polarization enhancement proves intrinsic, it would favor synchrotron emission in large-scale ordered magnetic fields. Conclusions. The spectral evolution of GRB 220107A, combined with our polarimetric constraints, demonstrates the diagnostic potential of time-resolved spectropolarimetry for constraining GRB prompt emission physics. While our polarization measurements remain below definitive detection thresholds, we present GRB 220107A as a test case that illustrates 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.