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Abstract

The binary system $\eta$ Carinae is a unique laboratory in which to study
particle acceleration to high energies under a wide range of conditions,
including extremely high densities around periastron. To date, no consensus has
emerged as to the origin of the GeV $\gamma$-ray emission in this important
system. With a re-analysis of the full Fermi-LAT dataset for $\eta$ Carinae we
show that the spectrum is consistent with a pion decay origin. A single
population leptonic model connecting the X-ray to $\gamma$-ray emission can be
ruled out. Here, we revisit the physical model of Ohm et al. (2015), based on
two acceleration zones associated to the termination shocks in the winds of
both stars. We conclude that inverse-Compton emission from in-situ accelerated
electrons dominates the hard X-ray emission detected with NuSTAR at all phases
away from periastron, and pion-decay from shock accelerated protons is the
source of the $\gamma$-ray emission. Very close to periastron there is a
pronounced dip in the hard X-ray emission, concomitant with the repeated
disappearance of the thermal X-ray emission, which we interpret as being due to
the suppression of significant electron acceleration in the system. Within our
model, the residual emission seen by NuSTAR at this phase can be accounted for
with secondary electrons produced in interactions of accelerated protons, in
agreement with the variation in pion-decay $\gamma$-ray emission. Future
observations with H.E.S.S., CTA and NuSTAR should confirm or refute this
scenario.