Resolving diffusion signatures in distant pulsar halos with current and future experiments

Abstract

Pulsar halos provide a unique probe of cosmic-ray propagation in the vicinity of pulsars and have important implications for our understanding of particle diffusion in the interstellar medium. However, the number of firmly identified pulsar halos remains limited. One of the main challenges is the difficulty in unambiguously confirming halo candidates through precise morphological measurements with current $γ$-ray observations. In this work, we investigate the prospects for identifying pulsar halo candidates through morphological discrimination using simulations of two advanced $γ$-ray experiments: LHAASO-KM2A and the Cherenkov Telescope Array (CTA). Using mock observations with realistic instrumental responses, we assess the ability of each experiment to distinguish diffusion-based halo morphologies from alternative simplified spatial models. Our results show that both increased photon statistics and improved angular resolution significantly enhance the power of morphological discrimination. In particular, CTA benefits from its superior angular resolution, while LHAASO-KM2A gains sensitivity from its large effective area at the highest energies. These results indicate that future $γ$-ray observations have the potential to expand the sample of pulsar halos and provide further insights into cosmic-ray transport around pulsars.

Figures (2)

• Figure 1: Radial $\gamma$-ray brightness profile of a pulsar halo with the Geminga parameters. The black points represent mock profile data of CTA-North ($50$ h) generated from the diffusion model. The red, blue, and green curves show the best-fit diffusion, Gaussian, and disk models, respectively.
• Figure 2: Discrimination curves for pulsar halos achievable with CTA-North and LHAASO-KM2A. We use $\dot{E}/\dot{E}_G$ instead of $\dot{E}$ as the vertical axis, where $\dot{E}_G$ denotes the luminosity of the reference source, Geminga. Sources located above the curves can be morphologically distinguished as diffusion halos. Filled circles denote known pulsar halos or candidates, while empty ones denote other pulsar-associated sources in the LHAASO catalog. The competing spatial models and statistical decision methods used in the four panels, from top left to bottom right, are as follows: Gaussian model with AIC, disk model with AIC, Gaussian model with $\chi^2$ test, and disk model with $\chi^2$ test.