Bulk Lorentz Factor Research Articles

A Simulation Study of Low-power Relativistic Jets: Flow Dynamics and Radio Morphology of FR-I Jets

Radio galaxies are classified into two primary categories based on their morphology: center-brightened FR-I and edge-brightened FR-II. It is believed that the jet power and interactions with the ambient medium govern the deceleration and decollimation of the jet-spine flows, which, in turn, influence this dichotomy. Using high-resolution, three-dimensional relativistic hydrodynamic simulations, we follow the development of flow structures on sub-kiloparsec to kiloparsec scales in kinetically dominant low-power relativistic jets. We find that the bulk Lorentz factor of the jet spine and the advance speed of the jet head, which depend on the energy injection flux and the jet-to-background density contrast, primarily determine the dynamics and structures of the jet-induced flows. The entrainment of ambient gas and the background density and pressure gradient may also play significant roles. To emulate radio morphology, we produce the synthetic maps of the synchrotron surface brightness for the simulated jets, by employing simple models for magnetic field distribution and nonthermal electron population and considering relativistic beaming effects at different inclination angles. Both the flow structures and radio maps capture the longitudinal and transverse structures of the jet-spine and shear layer, consistent with observations. We also compare different background effects and argue that the loss of pressure confinement beyond the galactic core may be a key factor in the flaring and disruption of FR-I jets. Our results confirm that mildly relativistic jets could explain the one-sidedness or asymmetries with the boosted main jet and deboosted counterjet pairs.

Probing broadband spectral energy distribution and variability of Mrk 501 in the low flux state

We conducted a multi-wavelength analysis of the blazar Mrk501, utilizing observations from AstroSat (SXT, LAXPC), Swift-UVOT, and Fermi-LAT during the period August 15, 2016 to March 27, 2022. The resulting multi-wavelength light curve revealed relatively low activity of the source across the electromagnetic spectrum. Notably, logparabola and broken power-law models provided a better fit to the joint X-ray spectra from AstroSat-SXT/LAXPC instruments compared to the power-law model. During the low activity state, the source showed the characteristic “harder when brighter” trend at the X-ray energies. To gain insights into underlying physical processes responsible for the broadband emission, we performed a detailed broadband spectral analysis using the convolved one-zone leptonic model with different forms of particle distributions such as logparabola (LP), broken power-law (BPL), power-law model with maximum energy (ξmax), and energy-dependent acceleration (EDA) models. Our analysis revealed similar reduced-χ2 values for the four particle distributions. The LP and EDA models exhibited the lowest jet powers. The correlation analyses conducted for the LP and BPL models revealed that there is a positive correlation between jet power and bulk Lorentz factor. Specifically, in the LP model, jet power proved independent of γmin, whereas in the broken power-law model, jet power decreased with an increase in γmin. The jet power in the LP/EDA particle distribution is nearly 10 percent of the Eddington luminosity of a 107 M⊙ black hole. This result suggests that the jet could potentially be fueled by accretion processes.

Synchrotron self-Compton in a radiative-adiabatic fireball scenario: Modelling the multiwavelength observations in some Fermi/LAT bursts

Abstract Energetic GeV photons expected from the closest and the most energetic Gamma-ray bursts (GRBs) provide an unique opportunity to study the very-high-energy emission as well as the possible correlations with lower energy bands in realistic GRB afterglow models. In the standard GRB afterglow model, the relativistic homogeneous shock is usually considered to be fully adiabatic, however, it could be partially radiative. Based on the external forward-shock scenario in both stellar wind and constant-density medium. We present a radiative-adiabatic analytical model of the synchrotron self-Compton (SSC) and synchrotron processes considering an electron energy distribution with a power-law index of 1 < p < 2 and 2 ≤ p. We show that the SSC scenario plays a relevant role in the radiative parameter ε, leading to a prolonged evolution during the slow cooling regime. In a particular case, we derive the Fermi/LAT light curves together with the photons with energies ≥100 MeV in a sample of nine bursts from the second Fermi/LAT GRB catalog that exhibited temporal and spectral indices with ≳ 1.5 and ≈2, respectively. These events can hardly be described with closure relations of the standard synchrotron afterglow model, and also exhibit energetic photons above the synchrotron limit. We have modeled the multi-wavelength observations of our sample to constrain the microphysical parameters, the circumburst density, the bulk Lorentz factor and the mechanism responsible for explaining the energetic GeV photons.

High energy neutrino production in gamma-ray bursts: dependence of the neutrino signal on the jet composition

Heavy nuclei can be synthetized or entrained in gamma-ray bursts (GRBs) with implications on the high-energy neutrino emission. By means of a Monte-Carlo algorithm, we model nuclear cascades and investigate their impact on the neutrino production considering kinetic dominated jets (in the internal shock model, including a dissipative photosphere) as well as Poynting flux dominated jets (for a jet model invoking internal-collision-induced magnetic reconnection and turbulence, ICMART). We find that the ICMART model allows for efficient nuclear cascades leading to an overall larger neutrino fluence than in the other two jet models. The survival of nuclei and inefficient nuclear cascades lead to an overall reduction of the neutrino fluence up to one order of magnitude. However, if nuclei are disintegrated, the neutrino fluence may be comparable to the one emitted from a jet loaded with protons. Exploring the parameter space of jet properties, we conclude that the composition and the bulk Lorentz factor have significant impact on the efficiency of nuclear cascades as well as the spectral shape of the expected neutrino fluence. On the other hand, the neutrino spectral distribution is less sensitive to the power-law index of the accelerated population of protons or heavier nuclei. For what concerns the diffuse emission of neutrinos from GRBs, we find that the uncertainty due to the jet composition can be at most comparable to the one related to the GRB cosmological rate.

Rotation of Polarization Angle in Gamma-Ray Burst Prompt Phase. II. The Influence of the Parameters

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 Γ0 (Γ0 is the normalization factor of Γ with Γ(r)=Γ0(r/r0)s ), but are roughly independent of the concrete values of both θ j and Γ0. 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 Γ0. The most significant PA rotation with △PA ∼ 90° will happen when θ j Γ0 > 50 and 1.0 < q ≤ 1.2.

Characterizing the γ-Ray Emission from FR0 Radio Galaxies

FR0 galaxies constitute the most abundant jet population in the local Universe. With their compact jet structure, they are broadband photon emitters and have been proposed as multimessenger sources. Recently, these sources have been detected for the first time in γ rays. Using a revised FR0 catalog, we confirm that the FR0 population as a whole are γ-ray emitters, and we also identify two significant sources. For the first time, we find a correlation between the 5 GHz core radio luminosity and γ-ray luminosity in the 1–800 GeV band, having a 4.8σ statistical significance. This is clear evidence that the jet emission mechanism is similar in nature for FR0s and the well-studied canonical FR (FRI and FRII) radio galaxies. Furthermore, we perform broadband spectral energy distribution modeling for the significantly detected sources as well as the subthreshold source population using a one-zone synchrotron self-Compton model. Within the maximum jet power budget, our modeling shows that the detected γ rays from the jet can be explained as inverse Compton photons. To explain the multiwavelength observations for these galaxies, the modeling results stipulate a low bulk Lorentz factor and a jet composition far from equipartition, with the particle energy density dominating over the magnetic field energy density.

Jet collimation and acceleration in the flat spectrum radio quasar 1928+738

Using time-resolved multifrequency Very Long Baseline Array data and new KaVA (KVN and VERA Array) observations, we study the structure and kinematics of the jet of the flat spectrum radio quasar (FSRQ) 1928+738. We find two distinct jet geometries as a function of distance from the central black hole, with the inner jet having a parabolic shape, indicating collimation, and the outer jet having a conical shape, indicating free expansion of the jet plasma. Jet component speeds display a gradual outward acceleration up to a bulk Lorentz factor Γmax ≈ 10 followed by a deceleration further downstream. The location of the acceleration zone matches the region where the jet collimation occurs. Therefore, this is the first direct observation of an acceleration and collimation zone (ACZ) in an FSRQ. The ACZ terminates approximately at a distance of 5.6 × 106 gravitational radii, which is in good agreement with the sphere of gravitational influence of the supermassive black hole, implying that the physical extent of the ACZ is controlled by the black hole gravity. Our results suggest that confinement by an external medium is responsible for the jet collimation and that the jet is accelerated by converting Poynting flux energy to kinetic energy.

Multiwavelength Modeling for the Shallow Decay Phase of Gamma-Ray Burst Afterglows

We simulate the emission in the shallow decay phase of gamma-ray burst afterglows using a time-dependent code. We test four models: the energy injection model, evolving the injection efficiency of nonthermal electrons, evolving the amplification of the magnetic field, and the wind model with a relatively low bulk Lorentz factor. All of the four models can reproduce the typical X-ray afterglow lightcurve. The spectral shape depends on not only the parameter values at the time corresponding to the observer time but also the past evolution of the parameters. The model differences appear in the evolution of the broadband spectrum, especially in the inverse Compton component. Future gamma-ray observations with imaging atmospheric Cherenkov telescopes such as the Cherenkov Telescope Array will reveal the mechanism of the shallow decay phase.

Evidence of a toroidal magnetic field in the core of 3C 84

The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic (RMHD) jet launching simulations and by most very long baseline interferometry (VLBI) observations differ by an order of magnitude. Bridging the gap between these RMHD simulations and VLBI observations requires selecting nearby active galactic nuclei (AGN), the parsec-scale region of which can be resolved. The radio source 3C 84 is a nearby bright AGN fulfilling the necessary requirements: it is launching a powerful, relativistic jet powered by a central supermassive black hole, while also being very bright. Using 22 GHz globe-spanning VLBI measurements of 3C 84 we studied its sub-parsec region in both total intensity and linear polarisation to explore the properties of this jet, with a linear resolution of ∼0.1 parsec. We tested different simulation set-ups by altering the bulk Lorentz factor Γ of the jet, as well as the magnetic field configuration (toroidal, poloidal, helical). We confirm the persistence of a limb brightened structure, which reaches deep into the sub-parsec region. The corresponding electric vector position angles (EVPAs) follow the bulk jet flow inside but tend to be orthogonal to it near the edges. Our state-of-the-art RMHD simulations show that this geometry is consistent with a spine-sheath model, associated with a mildly relativistic flow and a toroidal magnetic field configuration.

Insights into the physics of gamma-ray bursts from the high-energy extension of their prompt emission spectra

The study of the high-energy part (MeV-GeV) of the spectrum of gamma-ray bursts (GRBs) can play a crucial role in investigating the physics of prompt emission, but it is often hampered by low statistics and the paucity of GeV observations. In this work, we analyze the prompt emission spectra of the 22 brightest GRBs which have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning six orders of magnitude in energy. The high-energy photon spectra can be modeled with a power-law N(E)∝E−β possibly featuring an exponential cutoff. We find that, with the inclusion of the LAT data, the spectral index β is softer than what is typically inferred from the analysis of Fermi/GBM data alone. Under the assumption that the emission is synchrotron, we derived a median value of the index p ∼ 2.79 of the power-law energy distribution of accelerated particles (N(γ)∝γ−p). In nine out of 22 GRB spectra, we find a significant presence of an exponential cutoff at high energy, ranging between 14 and 298 MeV. By interpreting the observed cutoff as a sign of pair-production opacity, we estimate the jet bulk Lorentz factor Γ, finding values in the range 130–330. These values are consistent with those inferred from the afterglow light curve onset time. Finally, by combining the information from the high-energy prompt emission spectrum with the afterglow light curve, we exploited a promising method to derive the distance R from the central engine where the prompt emission occurs. The distances (R &gt; 1013 − 15 cm) inferred for the only two GRBs in our sample that are suitable for the application of this method, which have only lower limits on their cutoff energies, suggest large emitting regions, although they are still compatible with the standard model. Larger samples of GRBs with measured cutoff energies and afterglow deceleration time will allow for more informative values to be derived. These results highlight the importance of including high-energy data, when available, in the study of prompt spectra and their role in addressing the current challenges of the GRB standard model.

The On-axis Jetted Tidal Disruption Event AT2022cmc: X-Ray Observations and Broadband Spectral Modeling

AT2022cmc was recently reported as the first on-axis jetted tidal disruption event (TDE) discovered in the last decade, and the fourth on-axis jetted TDE candidate known so far. In this work, we present NuSTAR hard X-ray (3–30 keV) observations of AT2022cmc, as well as soft X-ray (0.3–6 keV) observations obtained by NICER, Swift, and XMM-Newton. Our analysis reveals that the broadband X-ray spectra can be well described by a broken power law with f ν ∝ ν −0.5 (f ν ∝ ν −1) below (above) the rest-frame break energy of E bk ∼ 10 keV at the observer frame t obs = 7.8 and 17.6 days since discovery. At t obs = 36.2 days, the X-ray spectrum is consistent with either a single power law or a broken power law. By modeling the spectral energy distribution from radio to hard X-ray across the three NuSTAR observing epochs, we find that the submillimeter/radio emission originates from external shocks at large distances ≳1017 cm from the black hole, the UV/optical light comes from a thermal envelope with radius ∼1015 cm, and the X-ray emission is consistent with synchrotron radiation powered by energy dissipation at intermediate radii within the (likely magnetically dominated) jet. We constrain the bulk Lorentz factor of the jet to be of the order 10–100. Our interpretation differs from the model proposed by Pasham et al. where both the radio and X-rays come from the same emitting zone in a matter-dominated jet. Our model for the jet X-ray emission has broad implications on the nature of relativistic jets in other sources such as gamma-ray bursts.

GRB 220304A: Another Gamma-Ray Burst Dominated by Thermal Radiation

We report a recently detected long-duration gamma-ray burst (GRB) event by Fermi-GBM, GRB 220304A. The spectral analysis of the burst by the Band function shows that both the time-integrated and time-resolved spectra are very narrow, with low-energy spectral index 〈α〉 = −0.05 ± 0.30 and high-energy spectral index 〈β〉 = −3.53 ± 0.30. It is reminiscent of GRB 090902B, a special GRB with photosphere radiation characteristics. Then, we perform spectral analysis using the Planck function (blackbody, BB) and the multicolor BB model. It is found that the spectra within −1 ∼ 3 s is well fit by the BB model, indicating that the observation within the first 4 s is a pure thermal event. Apart from that most of the spectra can be well modeled as a multicolor BB. We conclude that GRB 220304A is a GRB dominated by thermal radiation. We also find that the spectral widths of the time-resolved spectrum of the burst gradually increase with time. Based on the Amati relation, we infer the redshift to be 0.23, the physical properties of the relativistic outflow, and find that the relationship between the bulk Lorentz factor and the isotropic luminosity Γ–L iso,52 still exists.

Superluminal Motion and Jet Parameters in the Gamma-ray-Emitting Narrow-Line Seyfert 1 Galaxy TXS 1206+549

Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). Among them, TXS 1206+549 belongs to a small group of radio-loud and γ-ray-emitting NLS1 galaxies. We focus on the radio properties of this galaxy by analysing archival, high-resolution, very long baseline interferometry (VLBI) imaging observations taken at 8 GHz frequency in six epochs between 1994 and 2018. Using the milliarcsecond-scale radio structure, we can resolve a core and a jet component whose angular separation increases by (0.055±0.006) mas yr−1. This corresponds to an apparent superluminal jet component motion of (3.5±0.4)c. From the core brightness temperature and the jet component proper motion, we determine the characteristic Doppler-boosting factor, the bulk Lorentz factor, and the jet viewing angle. We find no compelling evidence for a very closely aligned blazar-type jet. The parameters for TXS 1206+549 resemble those of radio-loud quasar jets with a moderate Lorentz factor (Γ≈4) and ϑ≈24∘ inclination to the line of sight.

Revisiting a Core–Jet Laboratory at High Redshift: Analysis of the Radio Jet in the Quasar PKS 2215+020 at z = 3.572

The prominent radio quasar PKS 2215+020 (J2217+0220) was once labelled as a new laboratory for core–jet physics at redshift z=3.572 because of its exceptionally extended jet structure traceable with very long baseline interferometric (VLBI) observations up to a ∼600 pc projected distance from the compact core and a hint of an arcsec-scale radio and an X-ray jet. While the presence of an X-ray jet could not be confirmed later, this active galactic nucleus is still unique at high redshift with its long VLBI jet. Here, we analyse archival multi-epoch VLBI imaging data at five frequency bands from 1.7 to 15.4 GHz covering a period of more than 25 years from 1995 to 2020. We constrain apparent proper motions of jet components in PKS 2215+020 for the first time. Brightness distribution modeling at 8 GHz reveals a nearly 0.02 mas yr−1 proper motion (moderately superluminal with apparently two times the speed of light), and provides δ=11.5 for the Doppler-boosting factor in the inner relativistic jet that is inclined within 2∘ to the line of sight and has a Γ=6 bulk Lorentz factor. These values qualify PKS 2215+020 as a blazar, with rather typical jet properties in a small sample of only about 20 objects at z&gt;3.5 that have similar measurements to date. According to the 2-GHz VLBI data, the diffuse and extended outer emission feature at ∼60 mas from the core, probably a place where the jet interacts with and decelerated by the ambient galactic medium, is consistent with being stationary, albeit slow motion cannot be excluded based on the presently available data.

Parameter Inference of a State-of-the-Art Physical Afterglow Model for GRB 190114C

A state-of-the-art semi-analytic gamma-ray burst (GRB) afterglow model with synchrotron self-Compton (SSC) emission has been applied for the first time for parameter inference using real GRB data. We analyzed the famous GRB 190114C as a case study. GRB 190114C, characterized by its long duration and high luminosity, was observed by many ground-based and orbiting telescopes spanning a wide range of electromagnetic wavelengths, from radio to GeV gamma rays. We used two advanced algorithms for inference: a nested sampling algorithm called UltraNest and an MCMC algorithm emcee. Evoking the standard afterglow model, the inference result and the best-fit values lead to an initial bulk Lorentz factor (a rough estimate of Γ=526), which aligns with the values often seen in GRBs identified by the Fermi-LAT instrument. Similarly to the best-fit values of other studies in the literature, the derived values of microphysical parameters, the circumburst density, and the kinetic efficiency are consistent with those found after modeling the multi-wavelength observations in GRB 190114C. We show that the SSC from the forward-shock region can only describe the highest-energy photons above a few GeVs.

Jet Structure and Burst Environment of GRB 221009A

We conducted a comprehensive investigation of the brightest-of-all-time GRB 221009A, using new insights from very high-energy (VHE) observations from LHAASO and a complete multiwavelength afterglow data set. Through data fitting, we imposed constraints on the jet structure, radiation mechanisms, and burst environment of GRB 221009A. Our findings reveal a structured jet morphology characterized by a core+wing configuration. A smooth transition of energy within the jet takes place between the core and wing, but with a discontinuity in the bulk Lorentz factor. The jet structure differs from both the case of the short GRB 170817A and the results of numerical simulations for long-duration bursts. The VHE emission can be explained by the forward shock synchrotron self-Compton radiation of the core component, but requiring a distinctive transition of the burst environment from uniform to wind-like, suggesting the presence of complex pre-burst mass ejection processes. The low-energy multiwavelength afterglow is mainly governed by the synchrotron radiation from the forward and reverse shocks of the wing component. Our analysis indicates a magnetization factor of 5 for the wing component. Additionally, by comparing the forward shock parameters of the core and wing components, we find a potential correlation between the electron acceleration efficiency and both the Lorentz factor of the shock and the magnetic field equipartition factor. We discuss the significance of our findings, potential interpretations, and remaining issues.

Inverse Compton Emission and Cooling of Relativistic Particles Accelerated at Shear Boundary Layers in Relativistic Jets

Both observational evidence and theoretical considerations from magnetohydrodynamic simulations of jets suggest that the relativistic jets of active galactic nuclei (AGN) are radially stratified, with a fast inner spine surrounded by a slower-moving outer sheath. The resulting relativistic shear layers are a prime candidate for the site of relativistic particle acceleration in the jets of AGNs and gamma-ray bursts (GRBs). In this article, we present outcomes of particle-in-cell simulations of magnetic-field generation and particle acceleration in the relativistic shear boundary layers (SBLs) of jets in AGNs and GRBs. We investigate the effects of inverse Compton cooling on relativistic particles that are accelerated in the SBLs of relativistic jets, including the self-consistent calculation of the radiation spectrum produced by inverse Compton scattering of relativistic electrons in an isotropic external soft photon field. We find that the Compton cooling can be substantial, depending on the characteristic energy (blackbody temperature and energy density) of the external radiation field. The produced Compton emission is highly anisotropic and more strongly beamed along the jet direction than the characteristic 1/Γ pattern expected from intrinsically isotropic emission in the comoving frame of an emission region moving along the jet with a bulk Lorentz factor Γ. We suggest that this may resolve the long-standing problem of the Doppler factor crisis.

Numerical Simulation of Photospheric Emission in Long Gamma-Ray Bursts: Prompt Correlations, Spectral Shapes, and Polarizations

We explore the properties of photospheric emission in the context of long gamma-ray bursts (GRBs) using three numerical models that combine relativistic hydrodynamical simulations and Monte Carlo radiation transfer calculations in three dimensions. Our simulations confirm that photospheric emission gives rise to correlations between the spectral peak energy and luminosity that agree with the observed Yonetoku, Amati, and Golenetskii correlations. It is also shown that the spectral peak energy and luminosity correlate with the bulk Lorentz factor, as indicated in the literature. On the other hand, synthetic spectral shapes tend to be narrower than those of the observations. This result indicates that an additional physical process that can provide nonthermal broadening is needed to reproduce the spectral features. Furthermore, the polarization analysis finds that, while the degree of polarization is low for the emission from the jet core (Π < 4%), it tends to increase with viewing angle outside of the core and can be as high as Π ∼ 20%–40% in an extreme case. This suggests that the typical GRBs show systematically low polarization compared to softer, dimmer counterparts (X-ray-rich GRBs and X-ray flashes). Interestingly, our simulations indicate that photospheric emission exhibits large temporal variation in the polarization position angle (Δψ ∼ 90°), which may be compatible with those inferred in observations. A notable energy dependence of the polarization property is another characteristic feature found in the current study. Particularly, the difference in the position angle among different energy bands can be as large as ∼90°.

Investigation of the correlation between optical and γ-ray flux variations in the blazar Ton 599

ABSTRACT The correlation between optical and γ-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and γ-ray emissions in the blazar Ton 599 over a span of approximately 15 yr, from 2008 August to 2023 March. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of 2023 January. To investigate the connection between changes in optical and γ-ray flux, we have designated five specific time periods, labelled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The γ-ray counterparts to these optical flares are present during periods B, C, and E; however, during period D, the γ-ray counterpart is either weak or absent. We conducted a broad-band spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical–ultraviolet emission primarily emanated from the accretion disc in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and γ-ray emissions, like correlated optical and γ-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor.

Prompt GRB polarization from non-axisymmetric jets

ABSTRACT Time-resolved linear polarization (Π) measurements of the prompt gamma-ray burst emission can reveal its dominant radiation mechanism. A widely considered mechanism is synchrotron radiation, for which linear polarization can be used to probe the jet’s magnetic-field structure, and in turn its composition. In axisymmetric jet models, the polarization angle (PA) can only change by 90°, as Π temporarily vanishes. However, some time-resolved measurements find a continuously changing PA, which requires the flow to be non-axisymmetric in at least one out of its emissivity, bulk Lorentz factor, or magnetic field. Here, we consider synchrotron emission in non-axisymmetric jets, from an ultrarelativistic thin shell, comprising multiple radially expanding mini-jets (MJs) or emissivity patches within the global jet, that yield a continuously changing PA. We explore a wide variety of possibilities with emission consisting of a single pulse or multiple overlapping pulses, presenting time-resolved and integrated polarization from different magnetic field configurations and jet angular structures. We find that emission from multiple incoherent MJs/patches reduces the net polarization due to partial cancellation in the Stokes plane. When these contain a large-scale ordered field in the plane transverse to the radial direction, Π always starts near maximal and then declines over the single pulse or shows multiple highly polarized peaks due to multiple pulses. Observing $\Pi \lesssim 40~{{\ \rm per\ cent}}$ (15 per cent) integrated over one (several) pulse(s) will instead favour a shock-produced small-scale field either ordered in the radial direction or tangled in the plane transverse to it.