Instabilities at recollimation shocks in magnetohydrodynamic jets

Springer Link (Chiba Institute of Technology) · 2025-12-09

Context. The internal structure and stability of relativistic jets from active galactic nuclei (AGNs) still presents open questions relevant to high-energy astrophysics, with recollimation shocks often invoked to explain the jet morphology, particle acceleration, and variability. Yet, the role of instabilities triggered downstream of these shocks is not fully understood, particularly in magnetized jets. Aims. We aim to investigate how jet magnetization and other physical parameters influence the development of instabilities beyond the first recollimation shock. In particular, we focus on identifying the conditions under which the centrifugal instability (CFI) is effective, and how it affects the jet propagation and internal dynamics. Methods. We performed high-resolution 2D and 3D simulations using the relativistic magnetohydrodynamics (RMHD) code <ns0:monospace>PLUTO</ns0:monospace>. The jets are initialized with a conical geometry and propagate into an ambient medium, and we followed by axisymmetric simulations how they evolve toward a steady-state. In 2D we explored a range of magnetizations (from 0 to 1), pressure contrasts, and inertia ratios to characterize the formation and evolution of recollimation shocks. The results are further evaluated using linear stability analysis to assess the growth and suppression of CFI. Finally, we performed 3D simulations of unstable and stable jets. Results. We discuss how the different parameters of the axisymmetric steady solutions influence the location and strength of recollimation. We found that, even in moderately magnetized jets, σ = 0.1, the CFI can still develop under suitable local conditions and disrupt the jet structure. This instability is governed by the jet radius, curvature, Lorentz factor, and magnetization, and is not always predictable from injection conditions. While magnetization can delay or locally suppress instability growth, it does not guarantee long-term jet stability. Our 3D results highlight the limitations of 2D models in capturing non-axisymmetric and nonlinear effects, and underline the complex interplay between magnetic confinement and destabilizing mechanisms. These findings have implications for interpreting variability, knot formation, and polarization structure in AGNs jets.

Instabilities at recollimation shocks in MHD jets

arXiv (Cornell University) · 2025-10-02

AGN jet structure and stability remain uncertain; recollimation shocks are linked to morphology and variability, but the role of downstream instabilities is still unclear. We aim to investigate how jet magnetization and other physical parameters influence the development of instabilities beyond the first recollimation shock. In particular, we focus on identifying the conditions under which the centrifugal instability (CFI) is effective. We perform high-resolution 2D and 3D simulations using the relativistic magnetohydrodynamics code PLUTO. The jets are initialized with a conical geometry and propagate into an ambient medium, and we follow by axisymmetric simulations how they evolve towards a steady-state. In 2D we explore a range of magnetizations (from 0 to 1), pressure contrasts, and inertia ratios to characterize the formation and evolution of recollimation shocks. The results are further evaluated using linear stability analysis to assess the growth and suppression of CFI. Finally, we perform 3D simulations of unstable and stable jets. We discuss how the different parameters of the axisymmetric steady solutions influence the location and strength of recollimation. We find that, even in moderately magnetized jets, $σ$=0.1, the CFI can still develop under suitable local conditions and disrupt the jet structure. This instability is governed by the jet radius, curvature, Lorentz factor, and magnetization, and is not always predictable from injection conditions. While magnetization can delay or locally suppress instability growth, it does not guarantee long-term jet stability. Our 3D results highlight the limitations of 2D models in capturing non-axisymmetric and nonlinear effects, and underline the complex interplay between magnetic confinement and destabilizing mechanisms. These findings have implications for interpreting variability, and polarization structure in AGN jets.

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

arXiv (Cornell University) · 2025-12-22

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X<11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

The polarization of the synchrotron radiation from a recollimated jet: Application to high-energy BL Lacs

Astronomy and Astrophysics · 2025-06-10 · 11 citations

Multifrequency polarimetry, recently extended to the X-ray band thanks to the Imaging X-ray Polarimetry Explorer (IXPE) satellite, is an essential tool for understanding blazar jets. High-frequency-peaked BL Lacs (HBLs) and extreme high-frequency-peaked BL Lacs (EHBLs) are especially interesting because the polarimetric properties of their synchrotron emission, extending up to the X-ray band, can be fully tracked by sensitive polarimetric measurements. We investigated the polarization properties of the synchrotron emission of these sources, starting directly from relativistic magnetohydrodynamic simulations of recollimated relativistic jets. To bridge the gap between fluid and kinetic scales, we elaborated a post-processing code based on the Lagrangian macroparticle approach, which models the spectral evolution and emission of nonthermal particles within the jet given the local fluid conditions. When comparing our results with early particle-in-cell (PIC) simulations, we find that shocks formed through jet recollimation are primarily superluminal, limiting particle acceleration in a laminar flow. However, recent PIC simulations suggest that acceleration can occur in the presence of small-scale turbulence or inhomogeneities even in a superluminal configuration. In this case, we reproduce the observed polarization chromaticity (i.e., the polarization degree increases with frequency), along with a stable polarization angle between the X-ray and optical bands. This study sheds light on the role of recollimation shocks in blazar jets and supports the energy-stratified shock model as a plausible explanation for IXPE observations.

How do recollimation-induced instabilities shape the propagation of hydrodynamic relativistic jets?

Astronomy and Astrophysics · 2025-11-12 · 2 citations

Context. Recollimation is a phenomenon of particular importance in the dynamical evolution of jets and in the emission of high-energy radiation. Additionally, the full comprehension of this phenomenon provides insights into fundamental properties of jets in the vicinity of the active galactic nucleus (AGN). Three-dimensional (3D)(magneto)hydrodynamic simulations revealed that the jet conditions downstream of recollimation shocks favor the growth of strong instabilities, challenging the traditional view – supported by two-dimensional (2D) simulations – of confined jets undergoing a series of recollimation and reflection shocks. Aims. In order to investigate the stability of relativistic jets in AGNs at recollimation sites, we performed a set of long duration 3D relativistic hydrodynamic simulations, to focus on the development of hydrodynamical instabilities. We explored the nonlinear growth of the instabilities and their effects on the physical jet properties as a function of the initial jet parameters. Methods. We performed 2D and 3D relativistic hydrodynamic simulations using the state-of-the-art PLUTO code. We assumed that an initially free-expanding jet is collimated by the external medium, and we explored the role of the jet Lorentz factor, temperature, opening angle, and jet-environment density-contrast in the jet deceleration and entrainment. The parameter space was designed to describe low-power, weakly magnetized jets at small distances from the core (around the parsec scale). Results. All of the collimating jets that we simulate develop instabilities. Recollimation instabilities decelerate the jet, heat it, entrain external material, and move the recollimation point to shorter distances from the core. This is true for both conical and cylindrical jets. The instabilities, which are first triggered by the centrifugal instability, appear to be less disruptive in the case of narrower, denser, warmer, and more relativistic jets. These results provide valuable insights into the complex processes governing AGN jets and could be used to model the properties of low-power, weakly magnetized jetted AGNs.

Instabilities at recollimation shocks in magnetohydrodynamic jets

Astronomy and Astrophysics · 2025-11-04 · 3 citations

Context. The internal structure and stability of relativistic jets from active galactic nuclei (AGNs) still presents open questions relevant to high-energy astrophysics, with recollimation shocks often invoked to explain the jet morphology, particle acceleration, and variability. Yet, the role of instabilities triggered downstream of these shocks is not fully understood, particularly in magnetized jets. Aims. We aim to investigate how jet magnetization and other physical parameters influence the development of instabilities beyond the first recollimation shock. In particular, we focus on identifying the conditions under which the centrifugal instability (CFI) is effective, and how it affects the jet propagation and internal dynamics. Methods. We performed high-resolution 2D and 3D simulations using the relativistic magnetohydrodynamics (RMHD) code PLUTO . The jets are initialized with a conical geometry and propagate into an ambient medium, and we followed by axisymmetric simulations how they evolve toward a steady-state. In 2D we explored a range of magnetizations (from 0 to 1), pressure contrasts, and inertia ratios to characterize the formation and evolution of recollimation shocks. The results are further evaluated using linear stability analysis to assess the growth and suppression of CFI. Finally, we performed 3D simulations of unstable and stable jets. Results. We discuss how the different parameters of the axisymmetric steady solutions influence the location and strength of recollimation. We found that, even in moderately magnetized jets, σ = 0.1, the CFI can still develop under suitable local conditions and disrupt the jet structure. This instability is governed by the jet radius, curvature, Lorentz factor, and magnetization, and is not always predictable from injection conditions. While magnetization can delay or locally suppress instability growth, it does not guarantee long-term jet stability. Our 3D results highlight the limitations of 2D models in capturing non-axisymmetric and nonlinear effects, and underline the complex interplay between magnetic confinement and destabilizing mechanisms. These findings have implications for interpreting variability, knot formation, and polarization structure in AGNs jets.

Probing the low-energy particle content of blazar jets through MeV observations

Astronomy and Astrophysics · 2025-01-17 · 1 citations

Many of the blazars observed by Fermi actually have the peak of their time-averaged gamma-ray emission outside the ∼GeV Fermi energy range, at ∼MeV energies. The detailed shape of the emission spectrum around the ∼MeV peak places important constraints on acceleration and radiation mechanisms in the blazar jet and may not be the simple broken power law obtained by extrapolating from the observed X-ray and GeV gamma-ray spectra. In particular, state-of-the-art simulations of particle acceleration by shocks show that a significant fraction (possibly up to ≈90%) of the available energy may go into bulk quasi-thermal heating of the plasma crossing the shock rather than producing a nonthermal power-law tail. Other gentler but possibly more pervasive acceleration mechanisms, such as shear acceleration at the jet boundary, may result in a further build-up of the low-energy ( γ ≲ 10 2 ) particle population in the jet. As already discussed for the case of gamma-ray bursts, the presence of a low-energy Maxwellian-like bump in the jet particle energy distribution can strongly affect the spectrum of the emitted radiation, for example producing an excess over the emission expected from a power-law extrapolation of a blazar’s GeV-TeV spectrum. We explore the potential detectability of the spectral component ascribable to a hot quasi-thermal population of electrons in the high-energy emission of flat-spectrum radio quasars (FSRQs). We show that for the typical physical parameters of FSRQs, the expected spectral signature is located at ∼MeV energies. For the brightest Fermi FSRQ sources, the presence of such a component will be constrained by the upcoming MeV Compton Spectrometer and Imager (COSI) satellite.

Antihelium-3 sensitivity for the GRAMS experiment

Astroparticle Physics · 2025-07-28

The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite mission utilizing a Liquid Argon Time Projection Chamber (LArTPC) detector to measure both MeV gamma rays and antinuclei produced by dark matter annihilation or decay. The GRAMS can identify antihelium-3 events based on the measurements of X-rays and charged pions from the decay of the exotic atoms, Time of Flight (TOF), energy deposition, and stopping range. This paper shows the antihelium-3 sensitivity estimation using a GEANT4 Monte Carlo simulation. For the proposed long-duration balloon (LDB) flight program (35 days × 3 flights) and future satellite mission (2-year observation/10-year observation), the sensitivities become 1.47 × 10 −7 [m 2 s sr GeV/n] −1 and 1.55 × 10 −9 [m 2 s sr GeV/n] −1 / 3 . 10 × 1 0 − 10 [m 2 s sr GeV/n] −1 , respectively. The results indicate that GRAMS can extensively investigate various dark matter models through the antihelium-3 measurements.

Antihelium-3 Sensitivity for the GRAMS Experiment

ArXiv.org · 2025-03-20

The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite mission utilizing a Liquid Argon Time Projection Chamber (LArTPC) detector to measure both MeV gamma rays and antinuclei produced by dark matter annihilation or decay. The GRAMS can identify antihelium-3 events based on the measurements of X-rays and charged pions from the decay of the exotic atoms, Time of Flight (TOF), energy deposition, and stopping range. This paper shows the antihelium-3 sensitivity estimation using a GEANT4 Monte Carlo simulation. For the proposed long-duration balloon (LDB) flight program (35 days $ \times $ 3 flights) and future satellite mission (2-year observation / 10-year observation), the sensitivities become 1.47 $\times$ 10$^{-7}$ [m$^2$ s sr GeV/n]$^{-1}$ and 1.55 $\times$ 10$^{-9}$ [m$^2$ s sr GeV/n]$^{-1}$ / $3.10\times10^{-10}$ [m$^2$ s sr GeV/n]$^{-1}$, respectively. The results indicate that GRAMS can extensively investigate various dark matter models through the antihelium-3 measurements.

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

arXiv (Cornell University) · 2025-12-22

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X&lt;11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.