On the Hadronic Gamma-ray Emission from Tycho's Supernova Remnant

TL;DR

This paper proposes an alternative hadronic gamma-ray emission mechanism for Tycho's supernova remnant, suggesting a higher ambient density and a modest cosmic ray acceleration efficiency, consistent with multi-band observations.

Contribution

It introduces a new hadronic emission model with higher ambient density and lower efficiency, challenging previous low-density assumptions for Tycho's SNR.

Findings

Higher ambient density (~1 cm^{-3}) derived from gamma-ray flux.

Modest cosmic ray acceleration efficiency (~1%) proposed.

Distance to Tycho estimated at approximately 2.5 kpc.

Abstract

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.