Magnetic Field Amplification in Astrophysical Shocks

1
Magnetic Field Amplification in Astrophysical
Shocks

• There is convincing evidence for large B-fields
  at outer shocks in supernova remnants Bshock
  200-500?G gtgt BISM (e.g., Cowsik Sarkar 80
  Berezhko, Voelk co-workers Vink Laming)
• Broad-band fits radio/TeV ratio gives limits on
  B-field
• Sharp X-ray edges large B ? short electron
  lifetimes ? narrow structures
• Most likely, B-field amplification is an
  intrinsic part of efficient shock acceleration,
  i.e. nonlinear diffusive shock acceleration (DSA)
• Simple basic idea Cosmic ray streaming
  instability creates strong turbulence in
  nonlinear shocks and produces ?B/B gtgt 1. BUT,
  plasma physics hard (impossible?) when ?B/B gtgt 1.
  First attempts with simplified approaches Bell
  Lucek 01,04 Amato Blasi 06 Blasi, Amato
  Caprioli 06 Vladimirov, Ellison Bykov 06
  Blandford Bootstrap model
• B-fields set maximum CR energy, determine
  synchrotron emission, and produce losses for
  relativistic electrons ? understanding
  B-amplification essential for modeling broad-band
  emission from sources ? will determine IC/p-p
  emission ratio at GeV-TeV energies

If physics of particle acceleration in SNRs is
typical, unexpectedly large magnetic fields,
generated by shocks, may exist in radio jets,
shocks in galaxy clusters, GRBs, etc.
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Initial models (with gross approximations) show
that you can start with BISM ? 3?G and end up
with B ? 500?G at the shock, but critical
unresolved issues remain

• Large increases in B can occur, but maximum
  particle energy does not increase in proportion
  to B. Maximum proton energy set by weak B in
  far upstream precursor
• Shape of electron and proton spectra near max.
  energy critical for modeling X-ray synch and
  GeV-TeV observations. BUT, shape depends on
  details of amplification
• How does B-amp influence injection of electrons
  vs. protons? (no clue!)
• At present, PIC simulations are not large
  enough to model B-amp on scales relevant for SNR
  shocks
• Theories must have observations for constraints
  and guidance
• TeV observations particularly important because
  B-amp increases maximum proton energy

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Example GeV-TeV Observations (IC/p-p)
ratio Inverse-Compton (IC) and pion-decay
emission from SNR with large shocked B-fields
Only difference in models is assumed B-field
HESS
GLAST
TeV
Bsk 20?G
Large magnetic fields Large B ? higher energy
pion-decay gamma-rays. Observation of turnover
essential input for theories. Large B ? lower
maximum energy for electrons (synch losses) IC
emission in GLAST range modified by strong losses
in evolving SNR
IC
Bsk 300?G
IC
Broad-band observations to PeV energies essential
for understanding B-field amplification
GeV
TeV
Example with preliminary results for one
particular set of input parameters adapted from
Ellison, Patnaude, Slane, Blasi Gabici et al.
2007 (Note B-amp. NOT calculated in these models)