PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS PAIR SHOCKS: DEPENDENCE OF SHOCK ACCELERATION ON MAGNETIC OBLIQUITY

By L. Sironi, A. Spitkovsky. 2009

244 citations recorded by the source corpus.

We investigate shock structure and particle acceleration in relativistic magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell simulations. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. We find that only magnetic inclinations corresponding to “subluminal” shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration. The downstream spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail with exponential cutoff. For increasing magnetic inclination in the subluminal range, the high-energy tail accounts for an increasing fraction of particles (from ∼1% to ∼2%) and energy (from ∼4% to ∼12%). The spectral index of the power law increases with angle from −2.8 ± 0.1 to −2.3 ± 0.1. For nearly parallel shocks, particle energization mostly proceeds via the diffusive shock acceleration process; the upstream scattering is provided by oblique waves which are generated by the high-energy particles that escape upstream. For larger subluminal inclinations, shock-drift acceleration is the main acceleration mechanism, and the upstream oblique waves regulate injection into the acceleration process. For “superluminal” shocks, self-generated shock turbulence is not strong enough to overcome the kinematic constraints, and the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor γ0 ≳ 5) magnetized (σ ≳ 0.03) flows exists only for a very small range of magnetic inclination angles (≲34°/γ0), so relativistic astrophysical pair shocks have to be either nearly parallel or weakly magnetized to generate nonthermal particles. These findings place constraints on the models of pulsar wind nebulae, gamma-ray bursts, and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized shocks.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/010bff2776b8c6f90f8d15c11bf2e2d817335040

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In Astronomy and Astrophysics

1. Bing Zhang
2. B. Metzger
3. L. Sironi
4. D. Giannios
5. J. McKinney
6. A. Spitkovsky
7. Pawan Kumar
8. A. Tchekhovskoy

In Nuclear and High Energy Physics

1. Bing Zhang
2. D. Giannios
3. J. McKinney
4. L. Sironi
5. A. Spitkovsky
6. M. Petropoulou
7. A. Beloborodov
8. J. Arons

Citations in the corpus

• Collisional mechanism for gamma‐ray burst emission (2009, methodology): “The details of this collisionless process are complicated and cbe studied by numerical simulations (e.g. Sironi & Spitkovsky 2009).”
• Relativistic Jets Shine through Shocks or Magnetic Reconnection (2015, background): “…as expected from theoretical arguments (Begelman & Kirk 1990), no particle acceleration takes place when relativistic shocks are quasi-perpendicular, i.e., where the field lines are nearly orthogonal to the shock direction of propagation (Gallant et al. 1992; Sironi & Spitkovsky 2009, 2011).”
• A Reconnection Switch to Trigger gamma-Ray Burst Jet Dissipation (2010, background): “…2009); and if GRB jets contain strong toroidal fields, then the shocks should inefficiently dissipate the kinetic energy (Kennel & Coroniti 1984; Zhang & Kobayashi 2005; Narayan, Kumar & Tchekhovskoy 2011) and inefficiently accelerate particles to high energies (Sironi & Spitkovsky 2009).”
• CONSTRAINING EMISSION MODELS OF LUMINOUS BLAZAR SOURCES (2009, background): “…the most recent analysis of the shock acceleration in a relativistic regime clearly shows the importance of background (plasma) conditions around the shock front in shaping the spectral properties of non-thermal particles generated there (e.g., Niemiec & Ostrowski 2004; Sironi & Spitkovsky 2009).”
• A complete reference of the analytical synchrotron external shock models of gamma-ray bursts (2013, background): “Also numerical simulations (Sironi and Spitkovsky, 2009) suggest that electron acceleration becomes suppressed in a magnetized shock, which would also affect the predicted synchrotron radiation flux.”
• Three-dimensional magnetohydrodynamic simulations of the Crab nebula (2013, background): “But what is even more important is that the recent advanced particle-in-cell (PIC) simulations show that the acceleration of non-thermal particles does not operate at such shocks (Sironi & Spitkovsky 2009, 2011b).”
• Gamma-ray binaries and related systems (2013, background): “Numerical simulations find this occurs in unmagnetised pair plasmas (Spitkovsky 2008) or for nearly parallel magnetised shocks (Sironi & Spitkovsky 2009).”
• THE MAXIMUM ENERGY OF ACCELERATED PARTICLES IN RELATIVISTIC COLLISIONLESS SHOCKS (2013, methodology): “Unmagnetized Shocks As shown by Spitkovsky (2008b) and Sironi & Spitkovsky (2009b) in 2D and by Spitkovsky (2005) in 3D, unmagnetized shocks are mediated by the so-called Weibel instability (Weibel 1959; Medvedev & Loeb 1999; Gruzinov & Waxman 1999).4 The free energy for the instability comes from…”
• PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS ELECTRON–ION SHOCKS (2010, methodology): “Synthetic spectra extracted from PIC simulations of shocks (as done in unmagnetized pair shocks by Sironi & Spitkovsky 2009b) may possibly reveal characteristic signatures that could observationally distinguish such upstream emission from the downstream-based models that are usually employed.”
• ACCELERATION OF PARTICLES AT THE TERMINATION SHOCK OF A RELATIVISTIC STRIPED WIND (2011, methodology): “The case of uniform pre-shock fields has been investigated by Sironi & Spitkovsky (2009) and Sironi & Spitkovsky (2011) via multi-dimensional PIC simulations (for electron-positron and electron-ion plasmas, respectively).”
• INSIGHTS INTO THE HIGH-ENERGY γ-RAY EMISSION OF MARKARIAN 501 FROM EXTENSIVE MULTIFREQUENCY OBSERVATIONS IN THE FERMI ERA (2011, background): “…and numerical simulations presented in the literature indicate a wide possible range for the lowest-energy particle spectral indices (below Ee,min), from sinj ≃ 1.0 − 1.5 down to sinj ≃ −2, depending on the particular shock conditions and parameters (Amato & Arons 2006; Sironi & Spitkovsky 2009).”
• Pulsar Wind Nebulae as Cosmic Pevatrons: A Current Sheet’s Tale (2012, background): “…in the upstream flow with large Larmor radii at encounter with the shock, . . . ) has shown that the shocks themselves generate no such turbulence (Sironi and Spitkovsky 2009 7I discuss here only the young high voltage nebulae, where pressure due to motion through the interstellar medium and to…”
• Fast radio bursts as synchrotron maser emission from decelerating relativistic blast waves (2019, methodology): “Although the maser efficiency could be lower in the physical case of higher dimensions than was found in 1D PIC simulations (Sironi & Spitkovsky 2009), recent multi-dimensional simulations find the 1 The shock magnetization σ is defined as the ratio of incoming Poynting flux to particle energy flux.”
• Relativistic Shocks: Particle Acceleration and Magnetization (2015, methodology): “In contrast, superluminal shocks (θ & θcrit) show negligible particle acceleration (Gallant et al. 1992; Hoshino 2008; Sironi and Spitkovsky 2009a, 2011b; Sironi et al. 2013).”
• A burst in a wind bubble and the impact on baryonic ejecta: high-energy gamma-ray flashes and afterglows from fast radio bursts and pulsar-driven supernova remnants (2016, background): “It is thought that the classical shock acceleration mechanism is inefficient for highly magnetized shocks (e.g., Sironi & Spitkovsky 2009), so the non-thermal emission from the shocked wind would depend on the magnetization parameter of the impulsive flow (σ0 at r0).”
• WEIBEL, TWO-STREAM, FILAMENTATION, OBLIQUE, BELL, BUNEMAN...WHICH ONE GROWS FASTER? (2009, background): “The simplest case consists in a B0 parallel to the flow(s) but normal or oblique orientations have also been considered (Fraix-Burnet & Pelletier 1991; Dieckmann et al. 2008; Bret & Dieckmann 2008a; Sironi & Spitkovsky 2009).”
• Plasmoids in relativistic reconnection, from birth to adulthood: first they grow, then they go (2016, methodology): “…inflow, we employ two moving injectors (receding fromy = 0 at the speed of light along±ŷ) and an expanding simulation box, a technique that we have extensively employed in our studies of relativistic shocks (Sironi & Spitkovsky 2009, 2011b; Sironi et al. 2013) and relativistic reconnection (SS14).”