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

¹ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
² Centre for High Performance Computing, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram 695551, India
³ School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram 695551, India
⁴ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg, 194021, Russia
⁵ Kavli Institute of Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain
⁷ Ingeniería de Sistemas y Automática, Universidad de Málaga, Unidad Asociada al CSIC por el IAA, Escuela de Ingenierías Industriales, Arquitecto Francisco Peñalosa, 6 Campanillas 29071, Málaga, Spain
⁸ Special Astrophysical Observatory of Russian Academy of Sciences, Nizhniy Arkhyz, Russia
⁹ Crimean Astrophysical Observatory, Russian Academy of Sciences, Nauchnyi 298409, Russia
¹⁰ Bhabha Atomic Research Center, Mumbai, Maharashtra 400094, India
¹¹ Homi Bhabha National Institute, Mumbai, Maharashtra 400094, India
¹² Inter-University Center for Astronomy and Astrophysics, Pune, Maharashtra 411007, India
¹³ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, India
¹⁴ Department of Physics, Ashoka University, Sonipat, Haryana 131029, India
¹⁵ Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
¹⁶ Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra 400076, India
¹⁷ Physical Research Laboratory, Ahmedabad, Gujarat 380009, India
¹⁸ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, km 0.7, I-09042 Monserrato, Italy

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Received: 24 November 2025
Accepted: 1 March 2026

Abstract

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 (T₀ −2 to T₀ + 106 s) we performed shows no significant detection (PF < 38%, 2σ 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 (1.5σ 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%, 2σ; BF ∼ 1), though our sliding-window analysis yielded a marginally elevated signal (PF = 70 ± 30%, BF = 2.8) between T₀ + 76 and T₀ + 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.