The Synchrotron Polarization in Decaying Magnetic Field in Gamma-Ray Bursts

Abstract

Polarization can serve as a probe of the radiation mechanism and magnetic field (MF) configuration in gamma-ray bursts (GRBs). In the case of constant MF, the synchrotron polarization in the prompt phase of GRBs has been widely studied. In this paper, we consider the case of the decaying MF. We calculate the time-averaged and instantaneous synchrotron polarizations in a pulse for different viewing angles and for the large-scale decaying MF model, which can explain the so-called Band spectrum. We find that the on-axis time-averaged polarization degree (PD) in the energy band of 50-500 keV for the decaying large-scale MF model ($\sim 0.6$ for typical parameters) is higher than that in the constant MF model ($\sim 0.5$). An interesting result is the instantaneous PD in the off-axis case will experience a turnover, i.e., the PD will evolve from a positive value to a negative one. This suggests the polarization angle (PA) change by an angle of $90^{\circ }$. Such a result is roughly consistent with the discovery of the PA evolution within a pulse in some bursts, such as GRB 170114A and GRB 160821A. Our result implies at least a part of bursts (off-axis bursts) should have the PA evolution in a pulse.

arXiv comment17 pages, 11 figures, 3 tables. Accepted for publication in MNRAS

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• Temporal Evolution of Prompt GRB Polarization
• Spectropolarimetry and photometry of the early afterglow of the gamma-ray burst GRB191221B
• Polarization Predictions in the GRB Prompt Phase with the Internal Shock Model
• Rotation of polarization angle in gamma-ray burst prompt phase
• The polarization-angle flip in GRB prompt emission
• Prospects for Time-Domain and Multi-Messenger Science with eXTP
• Polarization Measurements as a Probe of Axion-Photon Coupling: a Study of GRB 221009A
• SSC Radiation in the ICMART Model: Spectral Simulations and Application to the Record-Breaking GRB 221009A