Particle acceleration at the bow shock of runaway star LS 2355: non-thermal radio emission but no $\gamma$-ray counterpart

TL;DR

This study investigates the non-thermal radio emission from the bow shock of runaway star LS 2355, confirming its radio counterpart, ruling out gamma-ray association, and analyzing the shock's properties and surrounding environment.

Contribution

It provides the first spectrally confirmed non-thermal radio counterpart for LS 2355's bow shock and refines the star's velocity and interaction model with the ISM.

Findings

Confirmed non-thermal radio emission from LS 2355's bow shock.

Ruled out gamma-ray association with the bow shock.

Derived a reduced stellar velocity of 7.0±2.5 km/s.

Abstract

Massive stars that travel at supersonic speeds can create bow shocks as their stellar winds interact with the surrounding interstellar medium. These bow shocks - prominent sites for mechanical feedback of individual massive stars - are predominantly observed in the infrared band. Confirmed high-energy emission from stellar bow shocks has remained elusive and confirmed radio counterparts, while rising in recent years, remain rare. Here, we present an in-depth multi-wavelength exploration of the bow shock driven by LS 2355, focusing on its non-thermal properties. Using the most-recent Fermi source catalogue, we rule out its previously-proposed association with an unidentified $\gamma$-ray source. Furthermore, we use deep ASKAP observations from the Rapid ASKAP Continuum Survey and the Evolutionary Map of the Universe survey to identify a non-thermal radio counterpart: the third spectrally confirmed non-thermal bow shock counterpart after BD +43$^{\rm o}$ 3654 and BD +60$^{\rm o}$ 2522. We finally use WISE IR data and Gaia to study the surrounding ISM and update the motion of LS 2355. Specifically, we derive a substantially reduced stellar velocity, $v_* = 7.0\pm2.5$ km/s, compared to previous estimates. The observed non-thermal properties of the bow shock can be explained by an interaction between the wind of LS 2355 and a dense HII region, at a magnetic field close to the maximum magnetic field strength allowed by the compressibility of the ISM. Similar to earlier works, we find that the thermal radio emission of the shocked ISM is likely to be substantially suppressed for it to be consistent with the observed radio spectrum.