Cosmic Rays in the Orion-Eridanus Superbubble
2019 July 22
The Orion-Eridanus superbubble, formed by the supernovae and supersonic winds of massive stars in the Orion OB associations, has likely fostered cosmic-ray production. The large level of magnetohydrodynamic turbulence in the bubble can also locally alter the diffusion of cosmic rays in space and in momentum. We have studied the cosmic-ray spectrum inside the superbubble by measuring the γ-ray emissivity of gas clouds located in the bubble and along its rim. We have compared these spectra with the average found in other clouds in the solar neighbourhood. To this aim, we have used ten years of data from the Fermi Large Area Telescope (LAT) between 250 MeV and 63 GeV. We have modelled the γ radiation from hadronic interactions using multi-wavelength tracers for the hydrogen column densities in the different gas phases, and using the gas distribution in space and velocity to separate the superbubble medium from its foregrounds and backgrounds. The model includes other ancillary components such as inverse-Compton emission, point sources, and solar and lunar emissions. Despite the stellar activity in the superbubble, we find the gas emissivity spectrum to be consistent with the average spectrum measured in the local interstellar medium (ISM). Yet, a small cirrus cloud lying just outside the superbubble exhibits a significantly lower cosmic-ray flux. We will discuss this change in the light of the magnetic-field geometry inferred from the Planck data on dust polarisation.