2023/10/25 by Kun Fang, Fang, Kun
Physics and Astronomy · Earth and Planetary Sciences · #Astrophysics and Cosmic Phenomena #Geophysics and Gravity Measurements #Pulsars and Gravitational Waves Research
paper · pdf · doi:10.48550/arxiv.2310.16594
Measuring the electron diffusion coefficient is the most straightforward task in the study of gamma-ray pulsar halos. The updated measurements of the spatial morphology and spectrum of the Geminga halo by the High-Altitude Water Cherenkov (HAWC) experiment enable us to constrain parameters beyond the diffusion coefficient, including the size of the slow-diffusion zone and the electron injection spectrum from the pulsar wind nebulae (PWNe). Based on the two-zone diffusion model, we find that the slow-diffusion zone size (r_*) around Geminga is within the range of 30-70~pc. The lower boundary of this range is determined by the goodness of fit of the model to the one-dimensional morphology of the Geminga halo. The upper limit is derived from fitting the gamma-ray spectrum of the Geminga halo, along with the expectations for the power-law index of the injection spectrum based on simulations and PWNe observations, i.e., p\gtrsim1. With r_* set at its lower limit of 30~pc, we obtain the maximum p permitted by the HAWC spectrum measurement, with an upper limit of 2.17 at a 3σ significance. Moreover, we find that when r_*=30~pc and p=2.17, the predicted positron spectrum generated by Geminga at Earth coincides with the AMS-02 measurement in the 50-500~GeV range.