HESS J1831$-$098 -- Exploring a pulsar halo scenario with H.E.S.S. data

TL;DR

This study tests whether HESS J1831-098 is a pulsar halo powered by nearby pulsars by reanalyzing H.E.S.S. data with a physically motivated halo template implemented in Gammapy and supplemented by gas-tracer diffuse modeling. A multi-component likelihood fit combines a halo template with standard source components, diffusion physics $D(E)=D_0 (E/E_0)^{\delta}$ (with $\delta=1/3$), and a pulsar-injected spectrum $Q(E)\propto E^{−\Gamma} e^{−E/E_c}$ normalized by the efficiency $\eta$, with $P_0$ encoding the pulsar’s true age; SEDs are computed with Naima. The analysis yields a statistically significant detection and finds that the halo model describes the data well, with best-fit parameters $D_0 = 3.35^{+7.66}_{-1.78}\times10^{27}\ \mathrm{cm^{2}\,s^{-1}}$, $\Gamma = 1.90^{+0.38}_{-0.32}$, $\eta = 0.043^{+0.141}_{-0.015}$, and $P_0 = 42.9^{+17.1}_{-15.9}$ ms, corresponding to a true age $t_{\rm age} ≈ 75.5^{+31.7}_{-50.2}$ kyr; however, a Gaussian description remains a competitive alternative with $\Delta AIC \approx 2.1$. The halo interpretation yields a photon SED consistent with the 1LHAASO KM2A component and aligns with other high-energy sources in the region, implying an average $e^{±}$ energy density of about $0.24$ eV cm$^{-3}$ in the inferred volume. These results support the viability of pulsar-halo scenarios for extended VHE sources but highlight parameter degeneracies that can affect definitive model preference.

Abstract

Pulsar halos are a class of extended very-high-energy (VHE) sources highlighted by the HAWC observatory towards the Geminga pulsar and PSR B0656$+$14. These VHE sources are interpreted as the inverse Compton emission from electrons and positrons diffusing in the interstellar medium at an inhibited rate, having escaped the pulsar wind nebula. Our aim is to search for new pulsar halos using H.E.S.S. data and to constrain their physical properties. Using a physically-motivated model of pulsar halos, we created template-based models of the spatial and energetic distributions of the expected gamma-ray emission using the Gammapy library. A promising candidate source to which this model can be effectively applied is HESS J1831$-$098, an extended VHE source spatially coincident with two energetic pulsars, which also exhibits spectral continuity and morphological compatibility with the ultra-high-energy source 1LHAASO J1831$-$1007u*. It could be powered by the radio pulsar PSR J1831$-$0952 with a characteristic age of 128 kyr. We present a spectro-morphological analysis of this source with H.E.S.S. data, revealing that the emission is well described with a pulsar halo model, although we cannot reject a simple 2D Gaussian morphology. We discuss the implication of the derived physical parameters of the model.

Figures (4)

• Figure 1: Left: H.E.S.S. significance map ($0.5-100~$TeV) with a correlation radius of $0.15^{\circ}$ after subtracting a model containing the background, diffuse emission and all HESS sources apart from HESS J1831$-$098. Circles represent the $1\sigma$ extents of the 2D Gaussians, and crosses represent the positions of point-like sources. The masked left-most region excludes HESS J1837$-$069. Right: Zoom of the left panel, showing the coincident pulsars. The magenta bands represent the $68\%$ confidence interval on the extensions, and the magenta circles the $95\%$ confidence interval on the positions shown with crosses. The black contours are the 2-3-4 $\sigma$ levels.
• Figure 2: Left: Significance residual map ($0.5-100~$TeV) with a correlation radius of $0.15^{\circ}$. Right: Normalized distribution of the map values on the left, compared with the expectation from statistical fluctuations (i.e. a standard normal distribution).
• Figure 3: Likelihood profile scans for each free pulsar halo parameter. During the scan, the parameters of all model components except the one being scanned are free.
• Figure 4: Left: H.E.S.S. significance map from $0.5-100~$TeV after subtracting a model containing the background and diffuse emission only, overlaid with our best-fit components and those reported in the 1LHAASO and 3HWC catalogs. The crosses indicate the $95\%$ confidence interval on the positions. Shaded and hatched bands represent the $68\%$ confidence interval on the Gaussian's $1\sigma$ extent. The X symbols are the H.E.S.S. point-like sources' positions. Right: Fit photon SED and derived flux points of the pulsar halo model, the 1LHAASO components and 3HWC J1831-095. The shaded bands represent the $1\sigma$ uncertainty. The upper limits are given at the $2\sigma$ level.