The polarization-angle flip in GRB prompt emission

Aims. We aim to provide an explanation for the PA rotation in GRBs and find the physical conditions that lead to the rotation by 90 degrees in the toroidal magnetic-field (MF) model. Moreover, we present some observable polarization properties in the MF model that can be tested in the future. Results. We find that the PA rotation in the toroidal MF is primarily related to three critical factors: the viewing angle, the jet opening angle, and the jet Lorentz factor. Additionally, the PA can experience flips of 90 degrees twice. The conditions for the flips are q ≳ 0.5 (except for q≃ 1) and y_j =(Γθ_j)^2 ≳ 4. However, the two flips in the PA might not be concurrently observable due to the constraint of flux. Taking these conditions into account and assuming a random orientation between the jet axis and the line of sight (LOS), we obtain a theoretical upper limit (without any constraints) for the observed rate of GRBs in the X-ray or γ-ray band displaying the flips in PA as R_ch≲ 80%. We further constrain the observed rate as R_ch∼ 16% according to the maximal post-flip polarized flux level, where the observed rate of single and double flips each account for ∼ 8%. It should be noted that the observed rates are different in various wavebands. The observed rate of the second PA flip in the optical bands should be higher than that in the X-ray or γ-ray band since the flux in the optical band declines much slower than that in the X-ray or γ-ray band. Moreover, when the LOS is close to the jet edge (q→ 1), it is the easiest case in which to observe the 90-degree PA flip due to the relatively high post-flip polarized flux level.