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Title: Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks

Abstract

There are two distinct breaks in the cosmic ray (CR) spectrum: the so-called “knee” around 3 × 10{sup 15} eV and the so-called “ankle” around 10{sup 18} eV. Diffusive shock acceleration (DSA) at supernova remnant (SNR) shock fronts is thought to accelerate galactic CRs to energies below the knee, while an extragalactic origin is presumed for CRs with energies beyond the ankle. CRs with energies between 3 × 10{sup 15} and 10{sup 18} eV, which we dub the “shin,” have an unknown origin. It has been proposed that DSA at galactic wind termination shocks, rather than at SNR shocks, may accelerate CRs to these energies. This paper uses the galactic wind model of Bustard et al. to analyze whether galactic wind termination shocks may accelerate CRs to shin energies within a reasonable acceleration time and whether such CRs can subsequently diffuse back to the Galaxy. We argue for acceleration times on the order of 100 Myr rather than a few billion years, as assumed in some previous works, and we discuss prospects for magnetic field amplification at the shock front. Ultimately, we generously assume that the magnetic field is amplified to equipartition. This formalism allows us to obtain analytic formulae,more » applicable to any wind model, for CR acceleration. Even with generous assumptions, we find that very high wind velocities are required to set up the necessary conditions for acceleration beyond 10{sup 17} eV. We also estimate the luminosities of CRs accelerated by outflow termination shocks, including estimates for the Milky Way wind.« less

Authors:
;  [1];  [2]
  1. Physics Department, University of Wisconsin–Madison, 1150 University Avenue, Madison, WI 53706 (United States)
  2. Department of Astronomy, University of Wisconsin–Madison, 2535 Sterling Hall, 475 N. Charter Street, Madison, WI 53706 (United States)
Publication Date:
OSTI Identifier:
22664031
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; AMPLIFICATION; COSMIC RADIATION; EVOLUTION; LUMINOSITY; MAGNETIC FIELDS; MILKY WAY; SPECTRA; STELLAR WINDS; SUPERNOVA REMNANTS; VELOCITY

Citation Formats

Bustard, Chad, Zweibel, Ellen G., and Cotter, Cory, E-mail: bustard@wisc.edu. Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/1/72.
Bustard, Chad, Zweibel, Ellen G., & Cotter, Cory, E-mail: bustard@wisc.edu. Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks. United States. doi:10.3847/1538-4357/835/1/72.
Bustard, Chad, Zweibel, Ellen G., and Cotter, Cory, E-mail: bustard@wisc.edu. Fri . "Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks". United States. doi:10.3847/1538-4357/835/1/72.
@article{osti_22664031,
title = {Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks},
author = {Bustard, Chad and Zweibel, Ellen G. and Cotter, Cory, E-mail: bustard@wisc.edu},
abstractNote = {There are two distinct breaks in the cosmic ray (CR) spectrum: the so-called “knee” around 3 × 10{sup 15} eV and the so-called “ankle” around 10{sup 18} eV. Diffusive shock acceleration (DSA) at supernova remnant (SNR) shock fronts is thought to accelerate galactic CRs to energies below the knee, while an extragalactic origin is presumed for CRs with energies beyond the ankle. CRs with energies between 3 × 10{sup 15} and 10{sup 18} eV, which we dub the “shin,” have an unknown origin. It has been proposed that DSA at galactic wind termination shocks, rather than at SNR shocks, may accelerate CRs to these energies. This paper uses the galactic wind model of Bustard et al. to analyze whether galactic wind termination shocks may accelerate CRs to shin energies within a reasonable acceleration time and whether such CRs can subsequently diffuse back to the Galaxy. We argue for acceleration times on the order of 100 Myr rather than a few billion years, as assumed in some previous works, and we discuss prospects for magnetic field amplification at the shock front. Ultimately, we generously assume that the magnetic field is amplified to equipartition. This formalism allows us to obtain analytic formulae, applicable to any wind model, for CR acceleration. Even with generous assumptions, we find that very high wind velocities are required to set up the necessary conditions for acceleration beyond 10{sup 17} eV. We also estimate the luminosities of CRs accelerated by outflow termination shocks, including estimates for the Milky Way wind.},
doi = {10.3847/1538-4357/835/1/72},
journal = {Astrophysical Journal},
number = 1,
volume = 835,
place = {United States},
year = {Fri Jan 20 00:00:00 EST 2017},
month = {Fri Jan 20 00:00:00 EST 2017}
}