Constraining the coherence scale of the interstellar magnetic field
  using TeV gamma-ray observations of supernova remnants
  Galactic cosmic rays (CRs) are believed to be accelerated at supernova
remnant (SNR) shocks. In the hadronic scenario the TeV gamma-ray emission from
SNRs originates from decaying pions that are produced in collisions of the
interstellar gas and CRs. Using CR-magnetohydrodynamic simulations, we show
that magnetic obliquity-dependent shock acceleration is able to reproduce the
observed TeV gamma-ray morphology of SNRs such as Vela Jr. and SN1006 solely by
varying the magnetic morphology. This implies that gamma-ray bright regions
result from quasi-parallel shocks (i.e., when the shock propagates at a narrow
angle to the upstream magnetic field), which are known to efficiently
accelerate CR protons, and that gamma-ray dark regions point to
quasi-perpendicular shock configurations. Comparison of the simulated gamma-ray
morphology to observations allows us to constrain the magnetic coherence scale
$\lambda_B$ around Vela Jr. and SN1006 to $\lambda_B \simeq 13_{-4.3}^{+13}$ pc
and $\lambda_B >200_{-10}^{+80}$ pc, respectively, where the ambient magnetic
field of SN1006 is consistent with being largely homogeneous. We find
consistent pure hadronic and mixed hadronic-leptonic models that both reproduce
the multi-frequency spectra from the radio to TeV gamma rays and match the
observed gamma-ray morphology. Finally, to capture the propagation of a SNR
shock in a clumpy interstellar medium, we study the interaction of a shock with
a dense cloud with numerical simulations and analytics. We construct an
analytical gamma-ray model for a core collapse SNR propagating through a
structured interstellar medium, and show that the gamma-ray luminosity is only
biased by 30% for realistic parameters.
