Limits on the non-thermal emission of the WR-WR system Apep

TL;DR

This study reports the non-detection of gamma-ray emission from the WR-WR binary Apep by Fermi-LAT, constraining magnetic field conditions and challenging the use of non-thermal radio emission as an indicator of gamma-ray activity.

Contribution

It provides the first gamma-ray emission limits for Apep, a key Wolf-Rayet binary, and discusses implications for magnetic fields and emission mechanisms in colliding-wind binaries.

Findings

Apep is not a bright gamma-ray emitter.

Limits constrain magnetic field pressure in the wind-collision region.

Non-thermal radio emission does not reliably indicate gamma-ray emission.

Abstract

Colliding-wind binaries (CWBs) constitute an emerging class of $\gamma$-ray sources powered by strong, dense winds in massive stellar systems. The most powerful of them are those binaries hosting a Wolf-Rayet (WR) star. Following the recent discovery of Apep -- the closest known Galactic WR--WR binary -- we discuss the non-detection of its putative high-energy emission by the Fermi Large Area Telescope (Fermi-LAT) in this Letter. The limits reported in the GeV regime can be used to set a lower limit on the magnetic field pressure density within the shocked wind-collision region (WCR), and to exclude Apep as a bright gamma-ray emitting binary. Given that this WR--WR system is the most luminous CWB identified until now at radio wavelengths, this result proves unambiguously that non-thermal synchrotron emission is not a suitable identifier for the subset of gamma-ray emitters in this class of particle accelerators. Rather, Apep could be an interesting case of study for magnetic field amplification in shocked stellar winds.