Fermi acceleration along the orbit of η Carinae

Abstract

The η Carinae binary system hosts the most massive stars with the highest known mass-loss rate. Its dense wind encounters the faster wind expelled by the companion, dissipating mechanical energy in the shock, accelerating particles up to relativistic energies and producing high-energy (HE) γ-rays. We used the first 7-year data of the Fermi LAT which span two passages of η Carinae at periastron. We extracted low and HE light curves and spectra in different orbital phase bins using the new PASS8 pipeline. We used particle acceleration in hydrodynamic simulations of the system in a multi-cell geometry and compared the prediction with the observations. The γ-ray emission location is compatible with η Carinae. Two emission components are distinguished. The low-energy (LE) one cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor < 2. Short-term variability occurs at periastron. The HE component flux varies by a factor 3-4 but differently during the two periastrons. The variabilities observed at LE and HE during the first half of the observations, match the prediction of the simulation, assuming a surface magnetic field of 500 G. The HE component and the thermal X-ray emission were weaker than expected around the second periastron suggesting a modification of the wind density in the inner wind collision region (WCR). Diffuse shock acceleration in the WCR provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. Further observations are required to explain the periastron-to-periastron HE variability and to associate it firmly with hadronic origin. η Carinae is a pevatron at periastron. Its $\nu$ flux can be detected by IceCube after many years of observations. Orbital modulations of the HE component can be distinguished from those of photo absorption by CTA.

arXiv comment11 pages, 8 figures

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