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Title: NONTHERMAL RADIATION FROM COSMIC-RAY MODIFIED SHOCKS

Abstract

We calculate nonthermal radiation from cosmic-ray (CR) protons and electrons accelerated at CR modified plane and spherical shocks, using time-dependent, diffusive shock acceleration (DSA) simulations that include radiative losses of CR electrons. Strong non-relativistic shocks with physical parameters relevant for young supernova remnants (SNRs) are considered in both the plane-parallel and spherically symmetric geometries, and compared at times when their dynamical and CR properties are concordant. A thermal leakage injection model and a Bohm-like diffusion coefficient are adopted. After DSA energy gains balance radiative losses, the electron spectrum at the plane shock approaches a time-asymptotic spectrum with a super-exponential cutoff above the equilibrium momentum. The postshock electron spectrum cuts off at a progressively lower momentum downstream from the shock due to the energy losses. That results in the steepening of the volume integrated electron energy spectrum by one power of the particle energy. These features evolve toward lower energies in the spherical, SNR shocks. In a CR modified shock, pion decay gamma-ray emission reveals distinct signatures of nonlinear DSA due to the concave proton momentum spectrum. Although the electron momentum spectrum has a much weaker concavity, the synchrotron spectral slope at the shock may flatten by about 0.1-0.3 between radiomore » and X-ray bands. The slope of the volume integrated emission spectrum behaves nonlinearly around the break frequency.« less

Authors:
 [1];  [2];  [3]
  1. Department of Earth Sciences, Pusan National University, Pusan 609-735 (Korea, Republic of)
  2. Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2 (Canada)
  3. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States)
Publication Date:
OSTI Identifier:
22011892
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 745; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; COMPUTERIZED SIMULATION; COSMIC ELECTRONS; COSMIC PROTONS; COSMIC RADIATION; COSMOLOGY; ELECTRON SPECTRA; EMISSION SPECTRA; ENERGY SPECTRA; GAMMA RADIATION; NONLINEAR PROBLEMS; PARTICLE DECAY; PIONS; RELATIVISTIC RANGE; SHOCK WAVES; SUPERNOVA REMNANTS; TIME DEPENDENCE

Citation Formats

Kang, Hyesung, Edmon, Paul P., and Jones, T. W., E-mail: kang@uju.es.pusan.ac.kr, E-mail: pedmon@physics.umanitoba.ca, E-mail: twj@msi.umn.edu. NONTHERMAL RADIATION FROM COSMIC-RAY MODIFIED SHOCKS. United States: N. p., 2012. Web. doi:10.1088/0004-637X/745/2/146.
Kang, Hyesung, Edmon, Paul P., & Jones, T. W., E-mail: kang@uju.es.pusan.ac.kr, E-mail: pedmon@physics.umanitoba.ca, E-mail: twj@msi.umn.edu. NONTHERMAL RADIATION FROM COSMIC-RAY MODIFIED SHOCKS. United States. doi:10.1088/0004-637X/745/2/146.
Kang, Hyesung, Edmon, Paul P., and Jones, T. W., E-mail: kang@uju.es.pusan.ac.kr, E-mail: pedmon@physics.umanitoba.ca, E-mail: twj@msi.umn.edu. 2012. "NONTHERMAL RADIATION FROM COSMIC-RAY MODIFIED SHOCKS". United States. doi:10.1088/0004-637X/745/2/146.
@article{osti_22011892,
title = {NONTHERMAL RADIATION FROM COSMIC-RAY MODIFIED SHOCKS},
author = {Kang, Hyesung and Edmon, Paul P. and Jones, T. W., E-mail: kang@uju.es.pusan.ac.kr, E-mail: pedmon@physics.umanitoba.ca, E-mail: twj@msi.umn.edu},
abstractNote = {We calculate nonthermal radiation from cosmic-ray (CR) protons and electrons accelerated at CR modified plane and spherical shocks, using time-dependent, diffusive shock acceleration (DSA) simulations that include radiative losses of CR electrons. Strong non-relativistic shocks with physical parameters relevant for young supernova remnants (SNRs) are considered in both the plane-parallel and spherically symmetric geometries, and compared at times when their dynamical and CR properties are concordant. A thermal leakage injection model and a Bohm-like diffusion coefficient are adopted. After DSA energy gains balance radiative losses, the electron spectrum at the plane shock approaches a time-asymptotic spectrum with a super-exponential cutoff above the equilibrium momentum. The postshock electron spectrum cuts off at a progressively lower momentum downstream from the shock due to the energy losses. That results in the steepening of the volume integrated electron energy spectrum by one power of the particle energy. These features evolve toward lower energies in the spherical, SNR shocks. In a CR modified shock, pion decay gamma-ray emission reveals distinct signatures of nonlinear DSA due to the concave proton momentum spectrum. Although the electron momentum spectrum has a much weaker concavity, the synchrotron spectral slope at the shock may flatten by about 0.1-0.3 between radio and X-ray bands. The slope of the volume integrated emission spectrum behaves nonlinearly around the break frequency.},
doi = {10.1088/0004-637X/745/2/146},
journal = {Astrophysical Journal},
number = 2,
volume = 745,
place = {United States},
year = 2012,
month = 2
}
  • The solution of the problem of particle acceleration in the non-linear regime, when the dynamical reaction of the accelerated particles cannot be neglected, shows strong shock modification.When stationarity is imposed by hand, the solution may show a prominent energy flux away from the shock towards upstream infinity. This feature is peculiar of cosmic ray modified shocks, while being energetically insignificant in the test particle regime. The escape flux appears also in situations in which it is physically impossible to have particle escape towards upstream infinity, thereby leading to question its interpretation.We show here that the appearance of an escape fluxmore » is due to the unphysical assumption of stationarity of the problem, and in a realistic situation it translates to an increase of the value of the maximum-momentum when the shock velocity is constant. On the other hand, when the shock velocity decreases (for instance during the Sedov-Taylor phase of a supernova explosion), escape to upstream infinity is possible for particles with momenta in a narrow range close to the maximum momentum.« less
  • We use kinetic simulations of diffusive shock acceleration (DSA) to study the time-dependent evolution of plane, quasi-parallel, cosmic-ray (CR) modified shocks. Thermal leakage injection of low-energy CRs and finite Alfven wave propagation and dissipation are included. Bohm diffusion as well as the diffusion with the power-law momentum dependence are modeled. As long as the acceleration timescale to relativistic energies is much shorter than the dynamical evolution timescale of the shocks, the precursor and subshock transition approach the time-asymptotic state, which depends on the shock sonic and Alfvenic Mach numbers and the CR injection efficiency. For the diffusion models we employ,more » the shock precursor structure evolves in an approximately self-similar fashion, depending only on the similarity variable, x/(u{sub s}t). During this self-similar stage, the CR distribution at the subshock maintains a characteristic form as it evolves: the sum of two power laws with the slopes determined by the subshock and total compression ratios with an exponential cutoff at the highest accelerated momentum, p {sub max}(t). Based on the results of the DSA simulations spanning a range of Mach numbers, we suggest functional forms for the shock structure parameters, from which the aforementioned form of CR spectrum can be constructed. These analytic forms may represent approximate solutions to the DSA problem for astrophysical shocks during the self-similar evolutionary stage as well as during the steady state stage if p {sub max} is fixed.« less
  • Reconnection shocks in a magnetically dominated plasma must be compressive. Nonthermal ion acceleration can occur across built-in slow shocks, and across outflow fast shocks when the outflow is supermagnetc and the field is line yield. Electron acceleration may be initiated by injection from the dissipation region. Reconnection and shock acceleration thus cooperate and nonthermal acceleration should be a characteristic feature.
  • We present a semi-analytical kinetic calculation of the process of non-linear diffusive shock acceleration (NLDSA) which includes magnetic field amplification due to cosmic ray induced streaming instability, the dynamical reaction of the amplified magnetic field and the possible effects of turbulent heating. This kinetic calculation allows us to show that the net effect of the amplified magnetic field is to enhance the maximum momentum of accelerated particles while reducing the concavity of the spectra, with respect to the standard predictions of NLDSA. This is mainly due to the dynamical reaction of the amplified field on the shock, which smoothens themore » shock precursor. The total compression factors which are obtained for parameters typical of supernova remnants are R{sub tot} {approx} 7-10, in good agreement with the values inferred from observations. The strength of the magnetic field produced through excitation of streaming instability is found in good agreement with the values inferred for several remnants if the thickness of the X-ray rims are interpreted as due to severe synchrotron losses of high energy electrons. We also discuss the relative role of turbulent heating and magnetic dynamical reaction in smoothening the shock precursor.« less
  • We propose a model for diffusive shock acceleration (DSA) in which stochastic magnetic fields in the shock precursor are generated through purely fluid mechanisms of a so-called small-scale dynamo. This contrasts with previous DSA models that considered magnetic fields amplified through cosmic ray (CR) streaming instabilities, i.e., either by way of individual particles resonant scattering in the magnetic fields, or by macroscopic electric currents associated with large-scale CR streaming. Instead, in our picture, the solenoidal velocity perturbations that are required for the dynamo to work are produced through the interactions of the pressure gradient of the CR precursor and densitymore » perturbations in the inflowing fluid. Our estimates show that this mechanism provides fast growth of magnetic field and is very generic. We argue that for supernovae shocks the mechanism is capable of generating upstream magnetic fields that are sufficiently strong for accelerating CRs up to around 10{sup 16} eV. No action of any other mechanism is necessary.« less