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 \%$, $2σ$). 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.

Figures (10)

• Figure 1: Prompt light curve of GRB 220107A from multiple observatories across various energy bands. Both AstroSat/CZTI and Konus-Wind detected emission from the first and second episodes of the burst, whereas Fermi/GBM detected only the initial episode due to Earth occultation. The Konus-Wind light curve aligns with AstroSat/CZTI observations, confirming emission during the period when GBM was obscured. The vertical line marks the GBM trigger time, and the shaded regions indicate the durations of the first (T$_{\rm 0}$-2 to T$_{\rm 0}$+ 38 sec) and second (T$_{\rm 0}$+77 to T$_{\rm 0}$+ 106 sec) episodes.
• Figure 2: Analysis of the Fermi/GBM field of view for GRB 220107A, illustrating the visibility of the GRB position relative to Earth occultation. The upper panel shows a full-sky map in celestial coordinates showing the location of GRB 220107A, marked by a pink star. The pointing directions of all 14 GBM detectors at the trigger time are indicated by light-gray circles, which represent the detector normal directions and do not correspond to the detectors' fields of view. The Galactic plane is shown as a thick gray band, with the Galactic center marked by a circle. The Earth, as seen from the Fermi spacecraft in orbit, is shown as a blue polygon, indicating the Earth-occulted region of the sky. The position of the Sun is marked by a yellow smiley symbol. The lower panel shows the geolocation of the Fermi spacecraft at the time of the trigger, marked by a red circle with the mission elapsed time (MET). The red shaded region highlights the South Atlantic Anomaly (SAA), while the solid blue line indicates the orbital trajectory of the Fermi spacecraft.
• Figure 3: AstroSat/CZTI Compton light curve of GRB 220107A, illustrating the temporal distribution of photon counts as a function of time since the T$_{\rm 0}$. Bayesian block analysis is employed to delineate the time-integrated duration, as well as the specific intervals for episode 1 and episode 2, which are selected for spectro-polarimetric analysis. The vertical green lines mark the start and end times of these intervals, while the shaded beige regions highlight the durations of the first and second episodes, respectively. The red dashed line represents the baseline count rate, with peak intensities observed around -2-38 seconds and 77-106 seconds post-trigger, indicating significant emission episodes.
• Figure 4: Time-integrated $\nu F_{\nu}$ spectrum of GRB 220107A (episode 1; $T_{0}-2$ to $T_{0}+38$ sec) fitted with the cutoff power law plus blackbody (CPL+BB) model using Fermi/GBM data. The red curve shows the cutoff power law component, the blue curve shows the blackbody emission, and the green curve represents the total model. The shaded gray regions denote the $1\sigma$ uncertainties of each spectral component. The fit suggests a thermal blackbody component at $kT \sim 14$ keV in addition to the non-thermal CPL continuum, which significantly improves the fit relative to purely non-thermal models.
• Figure 5: Spectro-polarimetric evolution of GRB 220107A: Time-resolved spectro-polarimetric analysis of GRB 220107A. Top panel: Konus-Wind light curve in the 20–1300 keV band (red) with flux evolution (orange-red) shown on the right axis. Shaded regions indicate the two main episodes of the burst. Vertical gray dashed lines mark finer time bins used for spectral analysis using Konus-Wind data. Second panel: Evolution of $E_{\rm p}$ with time, showing the peak energy of the emission in each time bin. Third panel: Evolution of the low-energy spectral index $\alpha$ with synchrotron slow- ($\alpha=-2/3$) and fast-cooling ($\alpha=-3/2$) reference lines indicated by dashed lines. Pink points indicate the polarization fraction measured with AstroSat CZTI, including upper limits. Bottom panel: Evolution of the blackbody temperature $kT$ for models including a thermal component. The combination of spectral and polarimetric measurements highlights the hard-to-soft evolution of the first to second episode and provides constraints on the emission mechanisms of GRB 220107A.