Time-resolved spectro-polarimetric analysis of extremely bright GRB 230307A: Possible Evidence of evolution from photospheric to synchrotron dominated emission

TL;DR

This work presents the first detailed time-resolved spectro-polarimetric analysis of the extremely bright GRB 230307A, combining AstroSat/CZTI, Fermi/GBM, and Konus-Wind data to trace spectral and polarization evolution. A hard-to-soft evolution in the low-energy index $α$ accompanies a polarization rise from undetectable to PF $>49\%$ in the decay phase, consistent with a transition from thermal/photospheric to non-thermal synchrotron emission and increasing jet magnetization. The findings imply magnetic-field ordering and energy dissipation processes evolving during the burst, and they demonstrate the value of time-resolved polarimetry for constraining GRB emission models and jet physics. These results motivate future high-sensitivity X/gamma-ray polarimeters to systematically study emission mechanisms in bright GRBs and their magnetic environments.

Abstract

The radiation mechanisms powering Gamma-ray bursts (GRBs) and their physical processes remain one of the unresolved questions in high-energy astrophysics. Spectro-polarimetric observations of exceptionally bright GRBs provide a powerful diagnostic tool to address these challenges. GRB 230307A, the second-brightest long-duration GRB ever detected, exhibits a rare association with a Kilonova, offering a unique and rare probe into the emission processes of GRBs originating from compact object mergers. We present a comprehensive time-averaged and time-resolved spectro-polarimetric analysis of GRB 230307A using joint observations from the $AstroSat$ Cadmium Zinc Telluride Imager (CZTI), the $Fermi$ Gamma-ray Burst Monitor (GBM) and $Konus$-Wind. Spectral analysis reveals a temporal evolution in the low-energy photon index, $α$, transitioning from a hard to a softer state over the burst duration. Time-averaged polarimetric measurements yield a low polarization fraction ($<$ 12.7 %), whereas time-resolved polarization analysis unveils a marked increase in polarization fractions ($>$ 49 %) in the later stages of the emission episode. This spectro-polarimetric evolution suggests a transition in the dominant radiative mechanism: the initial phase, possibly characterized by thermal-dominated photospheric emission (unpolarized or weakly polarized), gives way to a regime dominated by non-thermal synchrotron emission (highly polarized). This transition provides possible evidence for the evolving influence of magnetic fields in shaping the GRB emission process and jet dynamics.

Figures (3)

• Figure 1: The first panel shows the Compton light curve of CZTI data in the 100-600 keV energy range, with the black dashed line indicating the burst interval determined using the Bayesian block technique. The second panel illustrates the light curve from the NaI 10 detector (black), and the green solid lines depict the burst interval of Fermi GBM using the Bayesian block technique. The second through fifth panels illustrate the temporal evolution of spectral parameters throughout the burst (Band function parameters: peak energy $E_p$, low-energy spectral index $\alpha$, and CPL parameters: cut-off energy $E_c$, power-law index $p$). Spectral parameters derived from Konus-Wind data are shown as dark markers with black error bars, while the finer time-bin spectra modeled using Fermi data are displayed as lighter background points. In the third panel, the solid and dashed brown lines indicate the synchrotron limits for slow-cooling (-2/3) and fast-cooling (-3/2) scenarios, respectively. Shaded regions highlight the time-resolved intervals used for the polarization analysis.
• Figure 2: Time-averaged and time-resolved spectro-polarimetric analysis of GRB 230307A: The plot illustrates the evolution of the polarization fraction (PF) in 100-600 keV during the time-resolved (blue hexagon markers) and time-integrated (green hexagon marker) intervals of GRB 230307A. Three distinct time-resolved regions are highlighted with yellow, red, and blue shaded areas. The right vertical axis presents the corresponding variation in the low-energy spectral index ($\alpha$) derived from the Band function fits. The spectral indices obtained using time-resolved spectral analysis of Fermi data are represented by orange squares, while those from Konus-Wind data (for each region) are denoted by red squares. The grey dashed and dot-dashed lines indicate the theoretical bounds for the fast-cooling and slow-cooling synchrotron emission regimes, respectively, providing a reference for evaluating the observed spectral behavior.
• Figure 3: Time-integrated (top-left figure) and time-resolved (top-right figure for region 1, bottom-left figure for region 2, and bottom-right figure for region 3) polarization analysis of GRB 230307A in 100-600 keV using AstroSat CZTI. Each figure is composed of five panels: (1) the Compton light curve for the region of interest (time-integrated, region 1, region 2, and region 3), obtained using Bayesian block analysis (blue-shaded region, top panel); (2) the posterior probability distribution of the detector polarization angle (middle-left panel); (3) the modulation curve of the burst, illustrating the azimuthal dependence of the detected counts (middle-right panel) where in blue is the best fit curve and pink are the 250 randomly selected fitted modulation curves; (4) a two-dimensional contour plot depicting the joint posterior distribution of the detector polarization angle and polarization fraction (denoted as PD in the plots, equivalent to the PF referenced in the text; bottom-left panel); and (5) the posterior probability distribution of the polarization fraction (bottom-right panel). The Bayes factor, quantifying the strength of evidence for polarization, is computed for the 100–600 keV energy range.