Investigating $γ$-ray halos around three HAWC bright sources in Fermi-LAT data

Abstract: Numerous extended sources around Galactic pulsars have shown significant $\gamma$-ray emission from GeV to TeV energies, revealing hundreds of TeV energy electrons scattering off of the underlying photon fields through inverse Compton scattering (ICS). HAWC TeV gamma-ray observations of few-degree extended emission around the pulsars Geminga and Monogem, and LAT GeV emission around Geminga, suggest that systems older than 100 kyr have multi-TeV $e^\pm$ propagating beyond the SNR-PWN system into the interstellar medium. Following the discovery of few $\gamma$-ray sources by HAWC at energies E$>100$~TeV, we investigate the presence of an extended $\gamma$-ray emission in Fermi-LAT data around the three brightest sources detected by HAWC up to 100 TeV. We find an extended emission of $\theta_{68} = 1.00^{+0.05}_{-0.07}$ deg around eHWC J1825-134 and $\theta_{68} = 0.71\pm0.10$ deg eHWC J1907+063. The analysis with ICS templates on Fermi-LAT data point to diffusion coefficient values which are significantly lower than the average Galactic one. When studied along with HAWC data, the $\gamma$-ray Fermi-LAT data provide invaluable insight into the very high-energy electron and positron parent populations.