Gamma-ray and X-ray constraints on non-thermal processes in $\eta$ Carinae

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.

Submitted for publication to A&A

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• Eta Carinae with Fermi-LAT: Two full orbits and the third periastron
• Diffuse $\gamma$-ray emission around the massive star forming region of Carina Nebula Complex
• The high-energy emission from HD~93129A near periastron
• Inverse Compton cooling of thermal plasma in colliding-wind binaries
• Interpreting the GeV-TeV Gamma-Ray Spectra of Local Giant Molecular Clouds using GEANT4 Simulation
• Detection of very-high-energy gamma-ray emission from Eta Carinae during its 2020 periastron passage
• Probing cosmic ray escape from η Carinae
• Radio study of the colliding wind binary HD93129A near periastron and its surroundings
• Limits on the non-thermal emission of the WR-WR system Apep