Modelling the Gamma-Ray Morphology of HESS J1804-216 from Two Supernova Remnants in a Hadronic Scenario

TL;DR

This study models the gamma-ray morphology of HESS J1804-216 by analyzing cosmic-ray diffusion from nearby supernova remnants, identifying the progenitor SNR of PSR J1803-2137 as the most likely source in a hadronic scenario.

Contribution

First to model the spatial gamma-ray morphology of HESS J1804-216 using arc-minute ISM observations and diffusion equations, identifying the progenitor SNR of PSR J1803-2137 as the likely source.

Findings

Moderately slow diffusion fits the gamma-ray data.

The progenitor SNR of PSR J1803-2137 is the best candidate.

Modeling constrains diffusion parameters for cosmic-ray propagation.

Abstract

HESS J1804-216 is one of the brightest yet most mysterious TeV gamma-ray sources discovered to date. Previous arc-minute scale studies of the interstellar medium (ISM) surrounding this TeV gamma-ray source revealed HESS J1804-216 is likely powered by a mature supernova remnant (SNR) or pulsar, hence its origin remains uncertain. In this paper, we focus on the diffusive escape of cosmic-ray protons from potential SNR accelerators. These cosmic rays interact with the ISM to produce TeV gamma-rays. We utilise the isotropic diffusion equation solution for particles escaping from a shell, to model the energy-dependent escape and propagation of protons into the ISM. This work is the first attempt at modelling the spatial morphology of gamma-rays towards HESS J1804-216, using arc-minute ISM observations from both Mopra and the Southern Galactic Plane Survey. The spectral and spatial distributions of gamma-rays for the two nearby potential SNR counterparts, SNR G8.7-0.1 and the progenitor SNR of PSR J1803-2137, are presented here. We vary the diffusion parameters and particle spectrum and use a grid search approach to find the best combination of model parameters. We conclude that moderately slow diffusion is required for both candidates. The most promising candidate to be powering the TeV gamma-rays from HESS J1804-216 in a hadronic scenario is the progenitor SNR of PSR J1803-213.