On the Hadronic Gamma-ray Emission from Tycho's Supernova Remnant
  Hadronic gamma-ray emission from supernova remnants (SNRs) is an important
tool to test shock acceleration of cosmic ray protons. Tycho is one of nearly a
dozen Galactic SNRs which are suggested to emit hadronic gamma-ray emission.
Among them, however, it is the only one in which the hadronic emission is
proposed to arise from the interaction with low-density (~0.3 cm^{-3}) ambient
medium. Here we present an alternative hadronic explanation with a modest
conversion efficiency (of order 1%) for this young remnant. With such an
efficiency, a normal electron-proton ratio (of order 10^{-2}) is derived from
the radio and X-ray synchrotron spectra and an average ambient density that is
at least one-order-of-magnitude higher is derived from the hadronic gamma-ray
flux. This result is consistent with the multi-band evidence of the presence of
dense medium from the north to the east of the Tycho SNR. The SNR-cloud
association, in combination with the HI absorption data, helps to constrain the
so-far controversial distance to Tycho and leads to an estimate of 2.5 kpc.
