Arc-minute-scale studies of the interstellar gas towards HESSJ1804$-$216: Still an unidentified TeV $\gamma$-ray source

Abstract

The Galactic TeV $\gamma$-ray source HESSJ1804$-$216 is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7 and 12mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap HESSJ1804$-$216 at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV $\gamma$-ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV $\gamma$-ray emission from HESSJ1804$-$216. For a hadronic scenario, SNRG8.7$-$0.1 and the progenitor SNR of PSRJ1803$-$2137 require cosmic ray (CR) enhancement factors of $\sim 50$ times the solar neighbour CR flux value to produce the TeV $\gamma$-ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8kpc (the dispersion measure distance to PSRJ1803$-$2137) tends to anti-correlate with features of the TeV emission from HESSJ1804$-$216, making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.

arXiv comment29 pages, 23 figures, 5 tables, accepted for publication in PASA

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Cited by in corpus (1)

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