Eta Carinae: a very Large Hadron Collider

Full text: Eta Carinae is a primary candidate to search for particle acceleration by probing its gamma-ray emission. We used the first 21 months of Fermi/LAT data to extract gamma-ray (0.2-100 GeV) images, spectra and light-curves, then combined them with multi-wavelength observations to model the nonthermal spectral energy distribution. A bright gamma-ray source is detected. The spectral energy distribution features two distinct components. The first one extends over the keV to GeV energy range, and features an exponential cutoff at ∼1 GeV. It can be understood as inverse Compton scattering of ultraviolet photons by electrons accelerated up to gamma ∼ 104 in the colliding wind region. The second component is a hard gamma-ray tail detected above 20 GeV. It could be explained by π0 decay of accelerated hadrons interacting with the dense stellar wind. This hard gamma-ray tail can only be understood if emitted close to the wind collision region. The energy transferred to the accelerated hadrons (∼5% of the collision mechanical energy) is comparable to that of the thermal X-ray emission. (author)