Geminga and Monogem in the CTAO Era: Probing TeV Halos and Cosmic-Ray Transport

Abstract

TeV halos around Geminga and Monogem (B0656+14) reveal regions of strongly suppressed diffusion near middle-aged pulsars. Determining the sizes and magnetic-field strengths of these slow-diffusion zones is important for modelling lepton transport and assessing their contribution to the local positron excess. The Cherenkov Telescope Array Observatory (CTAO) will provide arcminute-scale imaging of extended gamma-ray emission. We construct two-zone diffusion models for Geminga and Monogem with GALPROP v57, injecting electrons and positrons according to the pulsar spin-down history and following their propagation through an evolving slow-diffusion bubble and the ambient interstellar medium. Calibrating to HAWC and Fermi-LAT measurements, we forward-fold the models through official CTAO instrument response functions, including realistic diffuse and instrumental backgrounds. For 50 h observations, both halos are detected with high significance ($\sim 13$--$30,\sigma$) from either site. Under optimistic background assumptions, CTAO can distinguish changes of $\Delta\gamma_1 \simeq 0.2$ in the high-energy injection index and $\Delta B \simeq 2,\mu{\rm G}$ in magnetic-field strength. For Monogem, compact bubbles with $R_{\rm SDZ}\approx30,{\rm pc}$ can be distinguished from extended ($\gtrsim50,{\rm pc}$) cases, although very large bubbles remain degenerate because of the finite field of view. Using an alternative Galactic diffuse template in the fit changes detection significances and parameter sensitivities by $\lesssim10%$, indicating robustness against plausible diffuse-background mismodelling.