2022ApJ...935...65Y

The particle diffusion coefficients of TeV pulsar halos observed so far are inferred to be significantly smaller than the typical value of the interstellar medium (ISM). The anisotropic diffusion model ascribes the slow diffusion to the cross-field diffusion, assuming sub-Alfvénic turbulence in the ISM around the pulsar if the viewing angle between the observer's line-of-sight (LOS) to the pulsar and the local mean field direction is small. In general, the TeV halo's morphology under this model highly depends on the viewing angle, and an elongated, asymmetric morphology is predicted if the LOS is not approximately aligned with the local mean field direction. While the specific requirement of a small viewing angle is supposedly established only for a small fraction of TeV halos, TeV halos with apparent asymmetric morphology have not been detected. In this paper, we will study the expectation of TeV halos measured by the TeV-PeV gamma-ray detector LHAASO in the framework of anisotropic diffusion model, with a particular focus on the influence of the viewing angle on the detectability. We show that a TeV halo is more detectable with a smaller viewing angle, and this selection effect may explain why the morphologies of all three detected TeV halos so far are consistent with being spherical. We also demonstrate that LHAASO is capable of detecting asymmetric TeV halos after several years of operation with reasonable source parameters. This can serve as a critical test of the anisotropic diffusion model.