Self-consistent interpretations of the multiwavelength gamma-ray spectrum of LHAASO $\mathrm{J}0621+3755$

LHAASO $\mathrm{J}0621+3755$ is a TeV gamma-ray halo newly identified by LHAASO-KM2A. It is likely to be generated by electrons trapped in a slow-diffusion zone around PSR $\mathrm{J}0622+3749$ through inverse Compton scattering. However, when the gamma-ray spectrum of LHAASO-KM2A is fitted, the GeV fluxes derived by the commonly used one-zone normal diffusion model for electron propagation are significantly higher than the upper limits of Fermi-LAT. In this work, we try to solve the contradiction by adopting a more generalized propagation model, i.e., the superdiffusion model or the two-zone diffusion model. For the superdiffusion scenario, we find that a model with superdiffusion index $\alpha \lesssim 1.2$ can meet the constraints of Fermi-LAT observation. For the two-zone diffusion scenario, the size of the slow-diffusion zone is required to be smaller than $\sim 50\mathrm{pc}$, which is consistent with theoretical expectations. Future precise measurements of the Geminga halo may further distinguish between these two scenarios for the electron propagation in pulsar halos.

Figure 1

Left: best-fit SBPs to the LHAASO-KM2A data with both the normal diffusion ($\alpha =2$) and superdiffusion ($\alpha =1.5$, 1.0) models. Right: SBPs before the convolution with the PSF, corresponding to the results in the left.

Figure 2

Comparison between the gamma-ray spectra calculated with superdiffusion models and the multiwavelength observations. The theoretical spectra shown here are the integrated fluxes within 20° around the pulsar, and so do the Fermi-LAT ULs. The power-law index of the electron injection spectrum is 1.5 for all the cases.

Figure 3

The same as Fig. 2 but for the two-zone diffusion models.

Figure 4

Gamma-ray extensions as functions of energy for the superdiffusion (left) or the two-zone diffusion (right) scenario. The extension is defined as the angular size within which 68% of the gamma-ray flux is included.