2025ApJS..276....9L

The composition and radiation mechanism of gamma-ray bursts (GRBs) within jets continue to be hotly debated. Investigating the joint polarimetric and spectral properties is crucial for understanding the composition and radiation mechanism of GRBs. Various jet properties, such as “kinetic-energy-dominated” (KED), “Poynting-flux-dominated” (PFD), and “hybrid-dominated” (HD) relativistic outflows, have been inferred from observed GRB spectra, with expectations of differing polarization levels among them. In this study, we analyzed a sample of 27 GRBs detected by the Gamma-ray Burst Monitor on board the NASA Fermi Gamma-ray Space Telescope, focusing on 26 bursts with significant polarization measurements. Our analysis revealed that 16 bursts (62%) were predominantly associated with the “PFD” jet type, while 10 bursts (38%) were classified as HD, implying that photosphere emission may also be a possible mechanism powering the high levels of polarization. Notably, no bursts were identified as KED type. We found distinct polarization properties, with HD-type bursts exhibiting consistently higher polarization levels than PFD-type bursts. We proposed models incorporating ordered and random magnetic field configurations specific to hybrid jets.