$\eta$ Carinae: particle acceleration and multi-messenger aspects

TL;DR

This paper discusses how the colliding stellar winds in $ta$ Carinae accelerate particles to relativistic energies, producing variable non-thermal X-ray and gamma-ray emissions, and explores implications for multi-messenger astrophysics and future observations.

Contribution

It provides new insights into particle acceleration mechanisms in $ta$ Carinae through multi-wavelength observations and simulations, highlighting the role of magnetic fields and wind density modifications.

Findings

Orbital variability of gamma-ray emission matches simulation predictions.

Weaker high-energy and thermal X-ray emissions observed in 2014 suggest wind density changes.

Future MeV instruments and CTA will further constrain acceleration models.

Abstract

$\eta$ Carinae is composed of two very massive stars orbiting each other in 5.5 years. The primary star features the densest known stellar wind, colliding with that expelled by its companion. The wind collision region dissipates energy and accelerate particles up to relativistic energies, producing non thermal X- and $\gamma$-ray emission detected by Beppo-SAX, INTEGRAL, Swift, Suzaku, Agile, Fermi and H.E.S.S.. The orbital variability of the system provides key diagnostic on the physics involved and on the emission mechanisms. The low-energy component, which cuts off below 10 GeV and varies by a factor < 2 along the orbit, is likely of inverse Compton origin. The high energy component varies by larger factors and differently during the two periastrons observed by Fermi. These variations match the predictions of simula- tions assuming a magnetic field in the range 0.4-1 kG at the surface of the primary star. The high-energy component and the thermal X-ray emission were weaker than expected around the 2014 periastron suggesting a modification of the inner wind density. Diffuse shock acceleration in the complex geometry of the wind collision zone provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. A future instrument sensitive in the MeV energy range could discriminate between lepto-hadronic and hadronic models for the gamma-ray emission. At higher energies, the Cherenkov Telescope Array will distinguish orbital modulations of the high-energy component from these of ultraviolet-TeV photo absorption providing a wealth of information constraining acceleration physics in more extreme conditions than found in SNR.