Data Centre for Astrophysics
Astronomy Department of the University of Geneva

Eta Carinae: a very large hadron collider


Context. η Carinae is the colliding wind binary with the highest mass-loss rate in our Galaxy and the only one in which hard X-ray emission has been detected.
Aims. η Carinae is therefore a primary candidate to search for particle acceleration by probing its gamma-ray emission.
Methods. 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 non-thermal spectral energy distribution.
Results. A bright gamma-ray source is detected at the position of η Carinae. Its flux at a few 100 MeV can be modelled by an extrapolation of the hard X-ray spectrum towards higher energies. The spectral energy distribution features two distinct components. The first one extends from 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 γ ∼ 10**4 in the colliding wind region. The expected synchrotron emission is compatible with the existing upper limit to the non-thermal radio emission. 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. The ratio of the fluxes of the π0 to inverse Compton components is roughly as predicted by simulations of colliding wind binaries. This hard gamma-ray tail can only be understood if emitted close to the wind collision region. The energy transferred to the accelerated particles (∼ 5% of the collision mechanical energy) is comparable to that of the thermal X-ray emission.
Conclusions. The electron spectrum responsible for the keV to GeV emission was modelled and the observed emission above 20 GeV strongly suggests hadronic acceleration in η Carinae. These observations are thus in good agreement with the colliding wind scenario proposed for η Carinae.
From several years from now, Eta Carinae is believed to be formed by two very massive stars, probably a luminous blue variable star with the highest mass loss rate observed and a Wolf-Rayet or type O star. Both eject material (electrons, protons, ions) through dense supersonic stellar winds. Eta Carinae is actually the only colliding winds binary system detected through high energy gamma-ray. Its peculiar spectrum as observed by Fermi/LAT between 200 MeV and 100 GeV strongly suggests the presence of accelerated protons up to 10 TeV. The important amount of energy necessary to sustain the observed high energetic gamma-ray component attributed to the collisions of protons in the two winds implies a cosmic ray production rate similar to what observed in supernovae remnants. Colliding winds binary systems can therefore play an important role in the understanding of the galactic cosmic ray spectrum.