On the hadronic γ -ray emission from tycho's supernova remnant

Abstract

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 (~0.3 cm-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 withthe 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. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.