Investigating the emission signatures of pulsar halo candidate HESS J1813-126

TL;DR

This study tests whether HESS J1813-126 is a pulsar halo powered by PSR J1813-1246 by combining a synchro-curvature (SC) model for the pulsar's magnetospheric emission with time-dependent diffusion-loss transport for the halo under three scenarios: two-zone isotropic suppressed diffusion (2ZISD), ballistic-to-diffusion (B2D), and anisotropic diffusion (AD). The authors show that the observed GeV–TeV spectral energy distributions and the extended TeV morphology can be reproduced across these models, but each yields distinct surface-brightness profiles and aperture-dependent emission, enabling observational discrimination with future imaging atmospheric Cherenkov telescopes. They provide predictions for SBPs and gamma-ray maps across energy ranges and apertures, and assess the high-energy pair multiplicities in both magnetosphere and halo to argue for energetic self-consistency without invoking exotic mechanisms. The work strengthens the pulsar-halo interpretation for HESS J1813-126, highlights the transport physics that govern halo formation, and outlines concrete observational tests for upcoming facilities like CTA and SWGO.

Abstract

Extended gamma-ray sources surrounding middle-aged pulsars, primarily observed at teraelectronvolt energies, have been interpreted as pulsar halos, where relativistic $e^\pm$ diffuse into the interstellar medium and produce inverse-Compton (IC) emission. HESS J1813-126, associated with the energetic, radio-quiet gamma-ray pulsar PSR J1813-1246, has been suggested as a candidate pulsar halo, though its nature remains uncertain. We interpreted the high-energy emission of PSR J1813-1246 using the synchro-curvature (SC) radiation model and tested whether the gamma-ray spectral energy distribution (SED) of HESS J1813-126 can be explained as a pulsar halo powered by PSR J1813-1246. We explain the X-ray and gamma-ray SEDs of the pulsar using the SC framework. We further computed the transport and losses of $e^\pm$ injected by the pulsar through time-dependent diffusion-loss equations, exploring various common pulsar halo transport models. The resulting IC emission was compared with \textit{Fermi}-LAT, H.E.S.S., HAWC, and LHAASO data. We present predictions for the surface brightness profiles (SBPs) and the aperture-dependent emission for the different transport models, providing key diagnostics for assessing the observability of HESS J1813-126 with current and future instruments. The SC framework successfully reproduces the emission of PSR J1813-1246. The SED of HESS J1813-126 can be consistently reproduced within different pulsar halo frameworks, albeit with distinct predictions across different transport models. The corresponding SBP predictions and aperture-dependent emission offer testable signatures for future imaging atmospheric Cherenkov telescopes, which will be crucial for discriminating between the transport models. We further examined the link between the pulsar central engine and its extended halo by comparing the pair multiplicities in the magnetospheric and halo regions.

Figures (8)

• Figure 1: Significance maps of the HESS J1813-126 region. Left: Significance map of 4FGL J1813.4-1246 from the phase-averaged analysis. Middle: Same but for PS J1813.3-1246, detected during the off-pulse phase of the 4FGL source. Right: H.E.S.S. significance map. The color bar in each plot provides the significance level. The source extent for H.E.S.S., HAWC, and LHAASO (KM2A and WCDA) is shown as colored circles.
• Figure 2: Weighted pulse profile of PSR J1813-1246 (4FGL J1813.4-1246) over one rotational cycle (pulse phase 0-1). It was constructed using 100 uniform phase bins per period from events between 0.1 and 500 GeV with the model weight from bruel19. The blue histogram shows the weighted photon counts per phase bin, while the solid red line indicates the Bayesian block decomposition of statistically significant structures in the weighted pulse profile. The red shaded region signifies the off-pulse interval estimated from this work, whereas the magenta and green shaded regions signify the same obtained in abdo13 and ackermann11, respectively.
• Figure 3: Distance variation of different SC model parameters and the spectrum of PSR J1813-1246: Lorentz factor ($\Gamma$; first panel), SC parameter ($\xi$; second panel), and pitch angle ($\mathrm{sin} \ \alpha$; third panel). In the second panel, the dashed red line marks $\xi = 1$, i.e., the transition point between synchrotron- and curvature-dominated regimes. In the last panel, the model spectrum is plotted against the observed SED obtained from the Fermi-LAT data analysis of 4FGL J1813.4-1246 and the X-ray SED and upper limit from XMM-Newton and INTEGRAL data, respectively, as obtained from marelli14 and guevel25. In the last panel, decomposed emission from different parts of the emitting region is also shown; the colorbar signifies the distance from the inner boundary ($x_{\rm in}$) in units of the light-cylinder radius ($R_{\rm lc}$).
• Figure 4: Exploration of SEDs and SBPs of HESS J1813-126 in the 2ZISD framework. Left: SEDs corresponding to different choices of inner zone diffusion coefficients. The $D_0$ in the plot signifies the normalization of the inner zone diffusion coefficient. Middle: Angle-integrated SEDs with increasing aperture sizes starting from 0.34$^{\circ}$, assuming $D_{\mathrm{0}} = 5 \times10^{27} \ \mathrm{cm^2 \ s^{-1}}$. In these two panels, the brown squares and green triangles (along with the upper limits) correspond to the off-pulse emission from ackermann11 and this work (PS J1813.3-1246), respectively. The blue data points and butterfly plot correspond to the H.E.S.S. data, whereas the purple, pink, and sky blue butterfly plots correspond to the HAWC, LHAASO WCDA, and KM2A observations, respectively. We also provide the sensitivity curves for the CTA-North (gray), CTA-South (yellow-green), and SWGO (red) for comparison. Right: SBPs for three different energy ranges scaled to start from the same point. Vertical lines correspond to the aperture sizes and have the same color scheme as in the middle panel.
• Figure 5: Exploration of the SEDs and SBPs of HESS J1813-126 in the B2D framework. Left: Angle-integrated SEDs, calculated for increasing aperture sizes, together with the same data points, upper limits, butterfly plots, and sensitivity curves as in Fig. \ref{['fig: halo_SED_2ZISD']}, using the same color scheme. Right: SBPs for three different energy ranges scaled to start from the same point. Vertical lines correspond to the aperture sizes and follow the same color scheme as in the left panel.