2013MNRAS.429L..25Z

Hadronic γ-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 γ-ray emission. Among them, however, it is the only one in which the hadronic emission is proposed to arise from the interaction with a low-density ( ∼ 0.3cm-3) ambient medium. Here we present an alternative hadronic explanation with a modest energy conversion efficiency (of the order of 1 per cent) for this young remnant. With such an efficiency, a normal electron-to-proton ratio (of the order of 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 γ-ray flux. This result is consistent with the multiband evidence of the presence of a dense medium from the north to the east of the Tycho SNR. The SNR-cloud association, in combination with the H i absorption data, helps to constrain the so-far controversial distance to Tycho and leads to an estimate of 2.5 kpc.