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Polarization spectra were predicted within the photosphere model. To seek more clues to distinguish between the models, both the time-resolved and time-integrated polarization spectra from optical band to MeV gamma-rays of the magnetic reconnection model are studied here. There are two newly found differences between the two models. First, the time-integrated polarization degree of the magnetic reconnection model would in general increase with frequency for on-axis observations, while it is not monotonous for the photosphere model. Second, the variations of both the time-integrated and the time-resolved polarization angles (PAs) with frequency of the magnetic reconnection model are not random, while the time-integrated PA varies randomly with frequency for the photosphere model. Therefore, future energy-resolved polarization analysis could distinguish between the two models. In addition, the PA rotation spectra are studied for the first time. The rotation value of PA within the burst duration will decrease with the increase of the observational energy band. Most significant PA rotation would happen for slightly off-axis observations in each energy band. The PA would rotate even for on-axis observations in the optical band. Compared with the aligned magnetic field case, the PA rotation is quite rare in the gamma-ray band for the case with a toroidal field in the radiation region.

The time-integrated polarization degree (PD) in the prompt optical band of a gamma-ray burst (GRB) was predicted to be less than 20%, while the time-resolved one can reach as high as 75% in the photosphere model. Polarizations in the optical band during the GRB prompt phase have not previously been studied in the framework of the magnetic reconnection model. Here, a three-segment power law of the energy spectrum is used to reconstruct the Stokes parameters of the magnetic reconnection model. Multiwavelength light curves and polarization curves from the optical band to MeV gamma rays in the GRB prompt phase are studied. We found that, depending mainly on the jet dynamics, there is a long-lasting high-PD phase in all calculated energy bands for the typical parameter sets. The time-resolved PD could be as high as 50%, while the time-integrated one is roughly 17% in the optical band. The time-resolved PD in X-rays can reach 60% and the time-integrated one is around 30%–40%. The evolution of polarization angle (PA) is random in both optical and gamma-ray bands for the photosphere model, while it is roughly constant in the synchrotron models. Therefore, future time-resolved PA observations in the prompt optical or gamma-ray band could distinguish between the photosphere and synchrotron models.

The observed abrupt twice-90° rotations of the polarization angle (PA) in the prompt phase of gamma-ray bursts (GRBs) are difficult to understand within the current one-emitting-shell models. Here, we apply a model with multiple emitting shells to solve this new challenging problem. Two configurations of large-scale ordered magnetic fields in the shells are considered: toroidal and aligned. Together with the light curves and the spectral peak energy evolutions, the twice-90° PA rotations in GRB 170114A and GRB 160821A could be well interpreted with the multishell aligned magnetic field configuration (MFC). Our numerical calculations also show that the multiple shells with the toroidal MFC could not explain the observed twice-90° PA rotations. An aligned MFC in the GRB outflow usually indicates the preference of a magnetar central engine, while a toroidal field configuration is typically related to a central black hole. Therefore, the magnetar central engines for the two GRBs are favored.

In addition to the light curve and energy spectrum, polarization is also important for inferring the physical properties of the gamma-ray burst (GRB). Rotation of the polarization angle (PA) with time will cause depolarization of the time-integrated polarization degree. However, it has rarely been studied before. Here, we use a magnetic reconnection model with a large-scale ordered aligned magnetic field in the emitting region to study the influence of the parameters on the PA rotations in the GRB prompt phase. We find that the half-opening angle of the jet θ _j , the observational angle θ _V , and the bulk Lorentz factor Γ all have significant impacts on the PA rotations. The PA rotations are affected by the product value of θ _j Γ _0 (Γ _0 is the normalization factor of Γ with ${\rm{\Gamma }}{(r)={{\rm{\Gamma }}}_{0}(r/{r}_{0})}^{s}$ ), but are roughly independent of the concrete values of both θ _j and Γ _0 . For the typical parameters, the changes of the PA within T _90 (△PA) would be within (12°, 66°) for slight off-axis observations, where T _90 is the duration of the burst with the accumulated flux density ranging from 5% to 95%. The q range for △PA > 10° becomes smaller with the increase of the product value of θ _j Γ _0 . The most significant PA rotation with △PA ∼ 90° will happen when θ _j Γ _0 > 50 and 1.0 < q ≤ 1.2.

The rotations of the polarization angle (PA) with time (energy) can lead to the depolarization of the time-integrated (energy-integrated) polarization. However, we do not know how and when it will rotate. Here, we consider a magnetic reconnection model to investigate the polarizations, especially the PA rotations of gamma-ray burst (GRB) prompt emission. For a large-scale ordered aligned magnetic field configuration, we find that within T _50 PAs will rotate with time (energy) for slight off-axis observations. Our studies show that the rotations of the PAs are due to changes in the observed image of the emitting region before being averaged over the equal arrival time surface for the unresolved sources. We apply our model to the single-pulse bursts of GRB 170101A and GRB 170114A with time-resolved PA observations. We find it can interpret the violent PA variation of GRB 170101A, while the model could not predict the two 90° PA changes in GRB 170114A. A more detailed model should be considered.

The environment of a γ -ray burst has an important influence on the evolution of jet dynamics and its afterglow. Here we investigate the afterglow polarizations in a stratified medium with the equal arrival time surface (EATS) effect. Polarizations of multiband afterglows are predicted. The effects of the parameters of the stratified medium on the afterglow polarizations are also investigated. We found the influences of the EATS effect on the afterglow polarizations become important for off-axis detections and polarization degree (PD) bumps move to later times with the EATS effect. Even if the magnetic field configurations, jet structure, and observational angles are fixed, polarization properties of the jet emission could still evolve. Here, we assume a large-scale ordered magnetic field in the reverse-shock region and a two-dimensional random field in the forward-shock region. Then, PD evolution is mainly determined by the evolution of the f _32 parameter (the flux ratio between the reverse-shock region and forward-shock region) at the early stage and by the evolution of the bulk Lorentz factor γ at the late stage. Through the influences on the f _32 or γ , the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations.

A central compact object (CCO; e.g., a black hole) with an accretion disk has been suggested as the common central engine of various astrophysical phenomena, such as gamma-ray bursts (GRBs), tidal disruption events (TDEs), and active galactic nuclei (AGNs). A jet powered by such a system might precess due to the misalignment of the angular momenta of the CCO and accretion disk. Some quasiperiodic behaviors observed in the light curves of these phenomena can be well interpreted within the framework of a precessing jet model. In this paper, we study the emission polarization of precessing jets in these three kinds of phenomena. The polarization angle also shows a gradual change for the synchrotron emission in both the random and toroidal magnetic field configurations with the precessing jet, while it can only change abruptly by 90° for the nonprecessing top-hat jet. Polarization properties are periodic due to the assumptions made in our model. The polarization observations are crucial to confirm the precession nature of jets in GRBs, TDEs, and AGNs. [ABSTRACT FROM AUTHOR]

A large-scale ordered magnetic field plays a very important role in the formation and acceleration of a gamma-ray burst (GRB) jet. During the GRB prompt phase, some dissipation processes may occur and disturb the magnetic field, making the field become random to some extent. Therefore, a mixed magnetic field consisting of an ordered component and a random component is plausible for the early afterglow phase. Here we investigate the polarization evolution and light curve of an afterglow under such a three-dimensional mixed magnetic field. Three kinds of ordered component (i.e., aligned, toroidal, and radial) are discussed. We find that the three cases are distinguishable through jet polarization evolution. The polarization angle for a 3D mixed magnetic field with an aligned ordered component can evolve gradually but only changes abruptly by 90° in the toroidal and radial cases. Furthermore, during the reverse shock crossing time, the polarization degree (PD) can be non-zero for the toroidal case but roughly zero for the radial case. Since an aligned component in a jet corresponds to a magnetar central engine and a toroidal component corresponds to a black hole, GRB central engines are distinguishable through polarization observations even if the magnetic field is mixed in a jet. In addition, our polarization calculation can be applied to GRB 170817A associated with GW170817. Using the recently observed PD upper limit 12% of GRB 170817A at t = 244 days, the magnetic field strength ratio of the ordered to random components in this afterglow is constrained to be ≲0.9. [ABSTRACT FROM AUTHOR]

The X-ray afterglows of almost one-half of gamma-ray bursts have been discovered by the Swift satellite to have a shallow decay phase of which the origin remains mysterious. Two main models have been proposed to explain this phase: relativistic wind bubbles (RWBs) and structured ejecta, which could originate from millisecond magnetars and rapidly rotating black holes, respectively. Based on these models, we investigate polarization evolution in the shallow decay phase of X-ray and optical afterglows. We find that in the RWB model, a significant bump of the polarization degree evolution curve appears during the shallow decay phase of both optical and X-ray afterglows, while the polarization position angle abruptly changes its direction by 90°. In the structured ejecta model, however, the polarization degree does not evolve significantly during the shallow decay phase of afterglows whether the magnetic field configuration in the ejecta is random or globally large-scale. Therefore, we conclude that these two models for the shallow decay phase and relevant central engines would be testable with future polarization observations. [ABSTRACT FROM AUTHOR]

The central engine and jet composition of gamma-ray bursts (GRBs) remain mysterious. Here we suggest that observations on the polarization evolution of early optical afterglows may shed light on these questions. We first study the dynamics of a reverse shock and a forward shock that are generated during the interaction of a relativistic jet and its ambient medium. The jet is likely magnetized with a globally large-scale magnetic field from the central engine. The existence of the reverse shock requires that the magnetization degree of the jet should not be high (σ ≤ 1), so that the jet is mainly composed of baryons and leptons. We then calculate the light curves and polarization evolution of early optical afterglows and find that when the polarization position angle changes by 90° during the early afterglow, the polarization degree is zero for a toroidal magnetic field but is very likely to be nonzero for an aligned magnetic field. This result would be expected to provide a probe for the central engine of GRBs because an aligned field configuration could originate from a magnetar central engine and a toroidal field configuration could be produced from a black hole via the Blandford–Znajek mechanism. Finally, for such two kinds of magnetic field configurations, we fit the observed data of the early optical afterglow of GRB 120308A equally well. [ABSTRACT FROM AUTHOR]