Speaker
Description
We report a detailed LHAASO study of the extended very-high-energy $\gamma$-ray source LHAASO J0635$+$0619 and investigate its possible association with the middle-aged pulsar PSR J0633$+$0632 as a pulsar halo candidate toward the Monoceros Loop region. Using WCDA data collected from 2021 March to 2025 July and KM2A data from 2019 December to 2025 July, we performed a three-dimensional maximum-likelihood analysis of the source morphology and spectrum. The source is significantly detected above 1 TeV with a statistical significance of $11.65\sigma$. Morphological tests show that an extended diffusion template provides a better description than a simple Gaussian or point-source model, with a characteristic diffusion angle of $\theta_d = 1.83 \pm 0.27^\circ$. In the same region, WCDA also reveals a previously unreported point-like source spatially consistent with the TeV binary HESS J0632$+$057, which is included in the joint modeling.
The broadband LHAASO spectrum of LHAASO J0635$+$0619 can be described by either a power law with an exponential cutoff or a log-parabola model, indicating significant spectral curvature at tens of TeV. Multiwavelength considerations favor a leptonic origin associated with PSR J0633$+$0632 rather than a hadronic scenario. The source centroid is consistent with the pulsar position, while the observed TeV extent is much larger than the known arcminute-scale X-ray pulsar wind nebula, supporting an interpretation as a $\gamma$-ray halo produced by escaped electrons diffusing in the surrounding interstellar medium.
Assuming a pulsar-halo scenario, we model the spectral and surface-brightness distributions with an electron diffusion-loss framework. The preferred model suggests a low magnetic-field environment and yields an electron cutoff energy of about 100 TeV, a reasonable conversion efficiency from pulsar spin-down power, and a diffusion coefficient $D_0 = (5.2 \pm 1.3)\times10^{27}\ \mathrm{cm^2,s^{-1}}$ at 100 TeV, comparable to those inferred for the Geminga and Monogem halos. These results support the presence of a slow-diffusion environment around PSR J0633$+$0632. If confirmed, LHAASO J0635$+$0619 would represent another important pulsar halo system and may provide new insight into particle transport around middle-aged pulsars, especially in a possible supernova-remnant environment associated with the Monoceros Loop.
