On the Efficiency of Fermi Acceleration at Relativistic Shocks

2006 ◽  
Vol 645 (2) ◽  
pp. L129-L132 ◽  
Author(s):  
Martin Lemoine ◽  
Guy Pelletier ◽  
Benoît Revenu
Keyword(s):  
Fermi Acceleration ◽  
Relativistic Shocks

scholarly journals Active Galactic Nuclei: Jets as the Source of Hadrons and Neutrinos

2014 ◽  
Vol 1 (1) ◽  
pp. 269-273
Author(s):  
Athina Meli ◽  
Paolo Ciarcelluti
Keyword(s):  
Active Galactic Nuclei ◽  
Gamma Ray ◽  
Galactic Nuclei ◽  
Cosmic Ray ◽  
High Energy ◽  
Plasma Jets ◽  
Shock Acceleration ◽  
Fermi Acceleration ◽  
Acceleration Mechanism ◽  
Relativistic Shocks

Active galactic nuclei are extragalactic sources, and their relativistic hot-plasma jets are believed to be the main candidates of the cosmic-ray origin, above the so-called knee region of the cosmic-ray spectrum. Relativistic shocks, either single or multiple, have been observed or been theorized to be forming within relativistic jet channels in almost all active galactic nuclei sources. The acceleration of non-thermal particles (e.g. electrons, protons) via the shock Fermi acceleration mechanism, is believed to be mainly responsible for the power-law energy distribution of the observed cosmic-rays, which in very high energies can consequently radiate high energy gamma-rays and neutrinos, through related radiation channels. Here, we will focus on the primary particle (hadronic) shock acceleration mechanism, and we will present a comparative simulation study of the properties of single and multiple relativistic shocks, which occur in AGN jets. We will show that the role of relativistic (quasi-parallel either quasi-perpendicular) shocks, is quite important since it can dramatically alter the primary CR spectral indices and acceleration eciencies. These properties being carried onto gamma-ray and neutrino radiation characteristics, makes the combination of them a quite appealing theme for relativistic plasma and shock acceleration physics, as well as observational cosmic-ray, gamma-ray and neutrino astronomy.

scholarly journals Fermi Acceleration at Relativistic Shocks

2008 ◽  
Author(s):  
Guy Pelletier ◽  
Martin Lemoine ◽  
Alexandre Marcowith ◽  
Felix A. Aharonian ◽  
Werner Hofmann ◽  
...  
Keyword(s):  
Fermi Acceleration ◽  
Relativistic Shocks

ABOUT FERMI ACCELERATION AT RELATIVISTIC SHOCKS AND THE SCATTERING ISSUE

2008 ◽  
Vol 17 (10) ◽  
pp. 1751-1760
Author(s):  
GUY PELLETIER ◽  
MARTIN LEMOINE ◽  
ALEXANDRE MARCOWITH
Keyword(s):  
Cosmic Rays ◽  
Fermi Acceleration ◽  
Turbulence Spectrum ◽  
Magnetic Turbulence ◽  
Short Scale ◽  
Return Probability ◽  
Relativistic Shocks ◽  
Turbulent Spectrum ◽  
Scale Turbulence ◽  
Significant Return

Difficulties arise in the process of Fermi acceleration at relativistic shocks because they are generically in the superluminal regime and they strongly compress turbulence downstream. Under reasonable conditions usually considered for the turbulent spectrum upstream, cosmic rays do not undergo the scattering that would allow them to make several Fermi cycles. An intense short scale turbulence must be excited in order that the Fermi process can be operative. In this paper, we show the requirement for the turbulence spectrum and give a solution to this issue in the form of the generation of intense compressive waves at short scale upstream. We indicate how the magnetic turbulence compression downstream should not be an obstacle for getting a significant return probability. Thus we conclude that, despite the necessity of some further analysis, the Fermi process at relativistic shock is still promising.

scholarly journals Power-Law Spectra from Fermi Acceleration at Relativistic Shocks

1989 ◽  
Vol 134 ◽  
pp. 211-212
Author(s):  
J. G. Kirk
Keyword(s):  
Shock Front ◽  
Power Law ◽  
Hot Spots ◽  
Pitch Angle ◽  
Average Energy ◽  
Acceleration Process ◽  
Fermi Acceleration ◽  
Physical Reason ◽  
Relativistic Shocks ◽  
Angle Scattering

The theory of diffusive acceleration at shock fronts, which applies only if the fluid speed is nonrelativistic, yields a simple formula for the power-law index s of accelerated particles: s = 3r/(r – 1), where r is the compression ratio of the shock front. Although the acceleration process depends on there being effective pitch-angle scattering of the particles in both the upstream and downstream regions, no property associated with this process appears in the formula. Unfortunately, if the velocity of the fluid through the shock front is relativistic, as seems to be the case in the central engines of AGN's, and also in some hot-spots in the outerparts of their jets, this attractive property ceases to hold. To find the index s, it becomes necessary to develop specific models describing the scattering process. The physical reason for this is that the particle distribution close to a relativistic shock is anisotropic. The exact type of anisotropy depends on the properties of the pitch angle scattering and determines both the average energy gain per shock crossing, as well as the probability of escape downstream. In this paper, results are presented for the pitch angle diffusion resulting from scattering in a weakly turbulent plasma with a Kolmogorov spectrum of Alfvén waves moving parallel and antiparallel to the magnetic field. This kind of spectrum has been employed in nonrelativistic models of hot spots [1]. However, the results obtained tend not to vary too dramatically as a function of the turbulence spectrum, being less than about 0.2 in the resulting s (0.1 in the predicted synchroton spectral index) for turbulence spectra between k−1 and k−2 [3].

scholarly journals Extragalactic shocks as cosmic accelerators

2006 ◽  
Vol 2 (14) ◽  
pp. 95-96
Author(s):  
Mikhail V. Medvedev
Keyword(s):  
Active Galactic Nuclei ◽  
Large Scale ◽  
Gamma Ray ◽  
Galactic Nuclei ◽  
High Energy ◽  
Maximum Energy ◽  
Shock Acceleration ◽  
Fermi Acceleration ◽  
High Energy Cosmic Rays ◽  
Relativistic Shocks

AbstractIt is quite well established that shocks accelerate particles via the Fermi mechanism. We discuss common features of various extragalactic sources, ranging from Gamma-Ray Bursts and jets of Active Galactic Nuclei to Large-Scale Structure shocks and address how they affect particle acceleration. In particular, we address constraints on the maximum energy of ultra-high-energy cosmic rays. Interestingly, some recent studies indicate that Fermi acceleration in relativistic shocks (and GRBs, in particular) faces severe difficulties. We will address this issue and demonstrate that the ‘observed’ shock acceleration of electrons may have nothing to do with Fermi acceleration, but may rather be associated with micro-physics of collisionless shocks.

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