Evidences of low-diffusion bubbles around Galactic pulsars

Recently, a few-degrees extended γ-ray halo in the direction of the Geminga pulsar has been detected by HAWC, Milagro and Fermi-LAT. These observations can be interpreted with positrons (e⁺) and electrons (e^-) accelerated by the Geminga pulsar wind nebula (PWN), released in a Galactic environment with a low diffusion coefficient (D₀), and inverse Compton scattering (ICS) with the interstellar radiation fields. We inspect here how the morphology of the ICS γ-ray flux depends on the energy, the pulsar age and distance, and the strength and extension of the low-diffusion bubble. In particular we show that γ-ray experiments with a peak of sensitivity at TeV energies are the most promising ones to detect ICS halos. We perform a study of the sensitivity of HAWC, HESS and the future CTA experiment finding that, with efficiencies of the order of a few %, the first two experiments should have already detected a few tens of ICS halos while the latter will increase the number of detections by a factor of 4. We then consider a sample of sources associated to PWNe and detected in the HESS Galactic plane survey and in the second HAWC catalog. We use the information available in these catalogs for the γ-ray spatial morphology and flux of these sources to inspect the value of D₀ around them and the e^± injection spectrum. All sources are detected as extended with a γ-ray emission extended about 15--80 pc. Assuming that most of the e^± accelerated by these sources have been released in the interstellar medium, the diffusion coefficient is 2--30×10²⁶ cm²/s at 1 TeV, i.e., 2 orders of magnitude smaller than the value considered to be the average in the Galaxy. These observations imply that Galactic PWNe have low-diffusion bubbles with a size of at least 80 pc.