Fermi acceleration under control: $\eta$ Carinae

TL;DR

This study analyzes 7 years of Fermi LAT data on $ta$ Carinae, showing how particle acceleration and emission vary with orbital motion, and compares observations with hydrodynamic simulations to understand the system's wind collision dynamics.

Contribution

It provides the first detailed comparison of gamma-ray observations of $ta$ Carinae with hydrodynamic simulations, revealing the role of magnetic fields and wind density modifications.

Findings

Two distinct emission components identified with different energy cutoffs.

Observed variabilities match simulation predictions assuming magnetic fields of 0.4-1 kG.

Weaker high-energy and X-ray emissions during second periastron suggest wind density changes.

Abstract

We used data from the Fermi Large Area Telescope obtained during the last 7 years and spanning two passages of $\eta$ Carinae at periastron and compared them with the predictions of particle acceleration in hydrodynamic simulations. Two emission components can be distinguished. The low-energy component cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor less than 2. Short-term variability occurs at periastron. The flux of the high energy component varies by a factor 3-4 but differently during the two periastrons. The variabilities observed at low-energy, including some details of them, and these observed at high-energy during the first half of the observations, do match the prediction of the simulation, assuming a surface magnetic field in the range 0.4-1 kG. The high-energy 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 zone. Diffuse shock acceleration in the complex geometry of the wind collision zone of $\eta$ Carinae provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system.