PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS ELECTRON–ION SHOCKS

By L. Sironi, A. Spitkovsky. 2010

304 citations recorded by the source corpus.

We investigate shock structure and particle acceleration in relativistic magnetized collisionless electron–ion shocks by means of 2.5-dimensional particle-in-cell simulations with ion-to-electron mass ratios (mi/me) ranging from 16 to 1000. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. In “subluminal” shocks, where relativistic particles can escape ahead of the shock along the magnetic field lines, ions are efficiently accelerated via the first-order Fermi process. The downstream ion spectrum consists of a relativistic Maxwellian and a high-energy power-law tail, which contains ∼5% of ions and ∼30% of ion energy. Its slope is −2.1 ±  0.1. The scattering is provided by short-wavelength non-resonant modes produced by Bell's instability, whose growth is seeded by the current of shock-accelerated ions that propagate ahead of the shock. Upstream electrons enter the shock with lower energy than ions (albeit by only a factor of ∼5 ≪ mi/me), so they are more strongly tied to the field. As a result, only ∼1% of the incoming electrons are accelerated at the shock before being advected downstream, where they populate a steep power-law tail (with slope −3.5 ± 0.1). For “superluminal” shocks, where relativistic particles cannot outrun the shock along the field, the self-generated turbulence is not strong enough to permit efficient Fermi acceleration, and the ion and electron downstream spectra are consistent with thermal distributions. The incoming electrons are heated up to equipartition with ions, due to strong electromagnetic waves emitted by the shock into the upstream. Thus, efficient electron heating (≳15% of the upstream ion energy) is the universal property of relativistic electron–ion shocks, but significant nonthermal acceleration of electrons (≳2% by number, ≳10% by energy, with slope flatter than −2.5) is hard to achieve in magnetized flows and requires weakly magnetized shocks (magnetization σ ≲ 10−3), where magnetic fields self-generated via the Weibel instability are stronger than the background field. These findings place important constraints on the models of gamma-ray bursts and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized electron–ion shocks.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/62fd52aaa9e66e623d5c4e3853469f013a5b4c5a

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In Astronomy and Astrophysics

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

In Nuclear and High Energy Physics

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

Citations in the corpus

• Pulsar Wind Nebulae as Cosmic Pevatrons: A Current Sheet’s Tale (2012, methodology): “…within the TWS is of possible importance if the upstream magnetic stripes do survive all the way to the TWS, for then reconnection, driven by the shock induced increase in the pressure, can dissipate the oppositely directed stripes of magnetic field (Lyubarsky 2003; Sironi and Spitkovsky 2011).”
• THE MAXIMUM ENERGY OF ACCELERATED PARTICLES IN RELATIVISTIC COLLISIONLESS SHOCKS (2013, methodology): “…the conclusion that particle acceleration along extended X-lines formed by tearing of the current sheets may contribute to the flat particle distribution (with spectral index p ' 1.5) required to explain the far infrared and radio spectra of PWNe (e.g., Lyubarsky 2003; Sironi & Spitkovsky 2011a).”
• A model for fast extragalactic radio bursts (2014, background): “The necessary magnetization is c© 0000 RAS not large, the ratio of the magnetic to the plasma energy should be σ > 10−3 (Sironi & Spitkovsky 2011).”
• The origin of galactic cosmic rays (2013, unknown): “…(PIC) simulations have been instrumental in reaching a better understanding of the formation of collisionless shocks (both relativistic and non-relativistic) and the initial stages of the acceleration process (Spitkovsky 2008a, 2008b; Sironi and Spitkovsky 2011, Gargaté and Spitkovsky 2012).”
• 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).”
• The physics of gamma-ray bursts & relativistic jets (2014, methodology): “Results of the recent PIC simulations of Sironi and Spitkovsky (2012) for a relativistic striped wind are consistent with this expectation.”
• GAMMA-RAY ACTIVITY IN THE CRAB NEBULA: THE EXCEPTIONAL FLARE OF 2011 APRIL (2011, background): “We note that such a spectrum is also inferred for the radio emitting electrons in the Crab Nebula, and more generally in pulsar wind nebulas (Gaensler & Slane 2006; Sironi & Spitkovsky 2011a).”
• Plasmoids in relativistic reconnection, from birth to adulthood: first they grow, then they go (2016, methodology): “…Nalewajko et al. 2015; Sironi et al. 2015; Werner et al. 2016; Kagan et al. 2016) and three dimensions (3D; e.g., Zenitani & Hoshino 2005, 2008; Yin et al. 2008; Liu et al. 2011; Sironi & Spitkovsky 2011a, 2012; Kagan et al. 2013; Cerutti et al. 2013b; Sironi & Spitkovsky 2014; Guo et al. 2015).”
• 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).”
• RELATIVISTIC RECONNECTION: AN EFFICIENT SOURCE OF NON-THERMAL PARTICLES (2014, background): “Yet, no consensus exists whether relativistic reconnection results self-consistently in non-thermal particle acceleration (Sironi & Spitkovsky 2011a), rather than just heating (Cerutti et al. 2012).”
• 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).”
• Searches for Extended and Point-like Neutrino Sources with Four Years of IceCube Data (2014, background): “…2014; De Marco et al. 2006; Vissani et al. 2011), active galactic nuclei (AGNs) jets (Essey et al. 2010; Kalashev et al. 2013; Murase et al. 2014; Sironi & Spitkovsky 2011; Stecker et al. 1991; Waxman & Bahcall 1999), starburst galaxies (Lacki et al. 2011; Loeb & Waxman 2006; Murase et al.…”
• 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).”
• Fast radio bursts as synchrotron maser emission from decelerating relativistic blast waves (2019, background): “However, electrons may still be heated, ahead of the shock (Lyubarsky 2006; Hoshino 2008) or in the shock layer (Sironi & Spitkovsky 2011).”