Features of the gamma-ray pulsar halo HESS J1831−098

Abstract

Gamma-ray pulsar halos are ideal indicators of cosmic-ray propagation in localized regions of the Galaxy and electron injection from pulsar wind nebulae. HESS $\mathrm{J}1831\ensuremath{-}098$ is a candidate pulsar halo observed by both the H.E.S.S. and HAWC experiments. We adopt the flux map of the H.E.S.S. Galactic plane survey and the spectrum measurements of H.E.S.S. and Fermi-LAT to study HESS $\mathrm{J}1831\ensuremath{-}098$. We find that HESS $\mathrm{J}1831\ensuremath{-}098$ meets all the criteria for a pulsar halo. The diffusion coefficient inside the halo and the conversion efficiency from the pulsar spin-down energy to the electron energy are both similar to the Geminga halo, a canonical pulsar halo. The injection spectrum can be well described by an exponentially cutoff power law. However, the needed power-law term is very hard with $p\ensuremath{\lesssim}1$ if the diffusion coefficient is spatially and temporally independent. Considering the possible origins of the slow-diffusion environment, we adopt the two-zone diffusion model and the time-delayed slow-diffusion model. Both the models can interpret the H.E.S.S. and Fermi-LAT results with a milder $p$. A modified injection time profile may have a similar effect.