GRB 181201A: Spectropolarimetric Constraints on Prompt Emission and Magnetic Field Structure

Abstract

Gamma-ray bursts (GRBs) have served as testbeds for high-energy astrophysical phenomena since their discovery over 50 years ago. GRB detections reveal diverse properties across events, complicating efforts to constrain their emission mechanisms, magnetic fields, jet structures, and compositions. Spectropolarimetry is a powerful tool utilized to disentangle these properties. INTEGRAL/IBIS has previously detected three polarized GRBs 041219A ($65\pm 26$%), 061122 ($>$33%) and 140206A ($>$28%). We present the time-integrated and time-resolved analyses of the fourth-polarized GRB 181201A. This GRB exhibits high polarization ($81.5^{+17.2}_{-15.7}$%) over three time bins with the highest signal-to-noise ratio, and the polarization angle (PA) shows a modest temporal evolution. The photon index does not cross the synchrotron line-of-death and shows a hard-to-soft evolution. A high degree of polarization (PD) tends to disfavor a photospheric emission model, as previous studies indicate that this model yields PD $<$ 20%. A modest evolution in the PA points towards a non-axisymmetric jet with a multi-patch emission. Our results indicate that under standard synchrotron conditions, the magnetic field is coherent but lacks global structure. Moreover, the field has multiple patches, but only one or a few dominate due to high degrees of polarization, which overlap with the intrinsic maximum of synchrotron emission within 3$σ$ errors. The high PD is also indicative of the Compton Drag scenario, but only under certain conditions, such as viewing the relativistic jet near its edge. However, given our error bars and the scope for improvement in our pipeline, we cannot strongly favor either scenario; both may remain viable.