On the Nature of the Energy-dependent Morphology of the Composite Multi-TeV Gamma-Ray Source J1702-204

HESS J1702-420 is a multi-TeV gamma-ray source with an unusual energy-dependent morphology. The recent H.E.S.S. observations suggest that the emission is well described by a combination of the point-like HESS J1702-420A (dominating at highest energies, ≳30 TeV) and diffuse (∼0.°3) HESS J1702-420B (dominating below ≲5 TeV) sources with very hard (Γ ∼ 1.5) and soft (Γ ∼ 2.6) power-law spectra, respectively. Here, we propose a model that postulates that the proton accelerator is located at the position of HESS J1702-420A and is embedded into a dense molecular cloud that coincides with HESS J1702-420B. In the proposed model, the very-high-energy radiation of HESS J1702-420 is explained by pion-decay emission from the continuously injected relativistic protons propagating through a dense cloud. The energy-dependent morphology is defined by the diffusive nature of the low-energy proton propagation, transiting sharply to (quasi) ballistic propagation at higher energies. Adopting a strong energy dependence of the diffusion coefficient, D ∝ E ^β with β ≥ 1, we argue that HESS J1702-420 as a system of two gamma-ray sources is the result of the propagation effect. Protons injected by a single accelerator at a rate ${Q}_{0}\simeq {10}^{38}\,{({n}_{0}/100\,\,{\mathrm{cm}}^{-3})}^{-1}\,{(d/0.25\,\mathrm{kpc})}^{-1}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ can reasonably reproduce the morphology and fluxes of the two gamma-ray components.