Fermi Gamma-ray Space Telescope

Cosmic rays in the Orion-Eridanus superbubble

Theo Joubaud
(Isabelle Grenier, Jean-Marc Casandjan)


The Orion-Eridanus superbubble, formed by the supernovae and supersonic winds of Orion's massive stars, has likely fostered cosmic-ray production. The large level of magnetohydrodynamic turbulence in the bubble can also affect the diffusion properties of the cosmic rays. We have probed the cosmic-ray flux inside the superbubble by comparing the gamma-ray emission produced in the gas clouds lying inside it with the average emissivity measured in other clouds in the solar neighbourhood. To this aim, we have used nine years of Fermi LAT data above 250 MeV, the spatial and spectral distributions of which have been modelled in terms of interstellar emission borne in the different gas phases (atomic, dark neutral, molecular, and ionized). The model includes other ancillary components such as inverse-Compton emission, point sources, solar and lunar emissions. The gas phases are traced by radio HI and CO emission lines, the ionized gas by H-alpha optical recombination lines, and the dark neutral medium from the coupled analysis of the gamma-ray data and the dust optical depth derived from the Planck and IRAS observations. We find that the emissivity spectrum is consistent with the average spectrum measured in nearby clouds located outside the superbubble, but within the Gould Belt. This uniformity calls for a detailed assessment of the recent supernova rate and of the energetics of massive stellar winds in the superbubble in order to estimate the epoch and rate of cosmic-ray production and to constrain the propagation conditions that can lead to such uniformity.