Particle Acceleration in Relativistic Collisionless Shocks: Fermi Process at Last?

By A. Spitkovsky. 2008

504 citations recorded by the source corpus.

We present evidence that relativistic shocks propagating in unmagnetized plasmas can self-consistently accelerate particles. We use long-term two-dimensional particle-in-cell simulations to study the well-developed shock structure in unmagnetized pair plasma. The particle spectrum downstream of such a shock consists of two components: a relativistic Maxwellian, with a characteristic temperature set by the upstream kinetic energy of the flow, and a high-energy tail, extending to energies >100 times that of the thermal peak. This high-energy tail is best fitted as a power law in energy with index –2.4 ± 0.1, modified by an exponential cutoff. The cutoff moves to higher energies with time of the simulation, leaving a larger power-law range. The number of particles in the tail is ~1% of the downstream population, and they carry ~10% of the kinetic energy in the downstream region. Investigating the trajectories of particles in the tail, we find that the energy gains occur as particles bounce between the upstream and downstream regions in the magnetic fields generated by the Weibel instability. We compare this mechanism to the first-order Fermi acceleration and set a lower limit on the efficiency of the shock acceleration process.

Subfields: Atomic and Molecular Physics, and Optics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/f75829221357c133a6d5277f3809cde9a81c732f

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Atomic and Molecular Physics, and Optics

1. L. Sironi
2. D. Giannios
3. M. Petropoulou
4. U. Keshet
5. M. Lemoine
6. A. Bret
7. Hui Li
8. William S. Daughton

In Nuclear and High Energy Physics

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

Citations in the corpus

• 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 Nine Lives of Cosmic Rays in Galaxies (2015, background): “…by shock waves directly from the thermal gas has been explicitly demonstrated in the ab-initio simulations of MHD turbulence and ions by Spitkovsky (2008) for relativistic shocks (relevant to γ-ray bursts and active galactic nuclei) and by Gargaté & Spitkovsky (2012) and Caprioli &…”
• 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).”
• Smilei : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation (2017, methodology): “The long-standing problem of describing collisionless shock formation has gained renewed interest as PIC simulations provide insight into the micro-physics of these non-linear structures [68, 69, 70].”
• Relativistic Jets Shine through Shocks or Magnetic Reconnection (2015, methodology): “First-order Fermi acceleration in relativistic shocks has been demonstrated from first principles (Spitkovsky 2008a,b; Martins et al. 2009; Haugbølle 2011; Sironi et al. 2013).”
• PARTICLE ACCELERATION AND PLASMA DYNAMICS DURING MAGNETIC RECONNECTION IN THE MAGNETICALLY DOMINATED REGIME (2015, methodology): “The nonthermal-dominated distribution in the simulations is also quite different from distributions in the relativistic shock regions (e.g., Spitkovsky 2008), where the particles are heated at the shock front and form an extended thermal distribution containing most of the dissipated energy.”
• PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS PAIR SHOCKS: DEPENDENCE OF SHOCK ACCELERATION ON MAGNETIC OBLIQUITY (2009, background): “PIC simulations of relativistic unmagnetized shocks have been presented by Spitkovsky (2005, 2008a), Chang et al. (2008) and Keshet et al. (2008); Spitkovsky (2008b) has shown that unmagnetized shocks naturally produce accelerated particles as part of the shock evolution.”
• WEIBEL, TWO-STREAM, FILAMENTATION, OBLIQUE, BELL, BUNEMAN...WHICH ONE GROWS FASTER? (2009, background): “For such settings, filamentation instability has been found leading the linear phase, and the shock formation has so far been analyzed accordingly (Medvedev & Loeb 1999; Spitkovsky 2008a, 2008b).”
• Relativistic Shocks: Particle Acceleration and Magnetization (2015, methodology): “The stream of shock-accelerated particles propagates ahead of the shock, triggering the Weibel instability.”
• Reconciling observed gamma-ray burst prompt spectra with synchrotron radiation? (2010, unknown): “More complex distributions showing several components (e.g. Maxwellian distribution+ non-thermal tail) are observed in some simulations of particle acceleration in ultra-relativistic shocks (Spitkovsky 2008b,a; Martins et al. 2009).”
• PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS ELECTRON–ION SHOCKS (2010, background): “…the incoming plasma can filament, and the shock is mediated by the ion Weibel instability (Weibel 1959; Medvedev & Loeb 1999; Gruzinov & Waxman 1999), which ensures strong electron heating and efficient particle acceleration, for both ions and electrons (Spitkovsky 2008a; Martins et al. 2009).”
• The Astrophysics of Ultrahigh-Energy Cosmic Rays (2011, background): “In order to make progress on these issues, many groups are performing Particle-In-Cell simulations that endeavor to solve self-consistently the field-particle interactions (e.g., Silva et al. 2003; Hededal & Nishikawa 2005; Dieckmann et al. 2008; Spitkovsky 2008; Riquelme & Spitkovsky 2010).”
• THE INTERNAL-COLLISION-INDUCED MAGNETIC RECONNECTION AND TURBULENCE (ICMART) MODEL OF GAMMA-RAY BURSTS (2010, methodology): “Within the standard internal shock model, particles are accelerated at the shock front via the 1st-order Fermi mechanism (Spitkovsky 2008; Baring and Summerlin 2009).”
• QUASI-BLACKBODY COMPONENT AND RADIATIVE EFFICIENCY OF THE PROMPT EMISSION OF GAMMA-RAY BURSTS (2008, unknown): “This was recently shown in the works by, for instance, Spitkovsky (2008); Giannios & Spitkovsky (2009).”
• Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows (2013, background): “It is theorized that these Weibel fields are responsible for the observed gamma-ray burst light curve [3, 4], particle acceleration in shock waves [5], and for providing seed fields for larger-scale cosmological magnetic structures [6].”
• Swift observations of the very intense flaring activity of Mrk 421 during 2006. I. Phenomenological picture of electron acceleration and predictions for MeV/GeV emission (2009, background): “As demonstrated by Spitkovsky (2008), diffusive shock acceleration models rely on the presence of a magnetic turbulence close to the shock responsible for the scattering process, although these models do not clearly determine the possible role of turbulence in the acceleration.”
• THE MAXIMUM ENERGY OF ACCELERATED PARTICLES IN RELATIVISTIC COLLISIONLESS SHOCKS (2013, background): “Shock Structure The physics of electron-ion unmagnetized shocks has been investigated by Spitkovsky (2008a) and Martins et al. (2009) in 2D and by Haugbølle (2011) in 3D.”
• FERMI OBSERVATIONS OF GRB 090510: A SHORT–HARD GAMMA-RAY BURST WITH AN ADDITIONAL, HARD POWER-LAW COMPONENT FROM 10 keV TO GeV ENERGIES (2010, background): “Recent particle in cell simulations of relativistic collisionless shocks (Spitkovsky 2008a,b; Martins et al. 2009) suggest that ξe is fairly small (of the order of a few percent), which may in our case allow hνm of several MeV even for low values of ǫB .”