The 155-day X-ray cycle of the very massive Wolf-Rayet star Melnick 34 in the Large Magellanic Cloud

TL;DR

This study reveals a 155-day X-ray cycle in the Wolf-Rayet star Mk 34 in the Large Magellanic Cloud, likely caused by an eccentric colliding-wind binary system with extreme luminosity and phase-related spectral changes.

Contribution

First detailed X-ray monitoring of Mk 34, establishing its 155-day cycle and linking it to a massive eccentric binary system in the LMC.

Findings

X-ray cycle of 155.1 days identified

Mk 34 is a highly luminous colliding-wind binary

Spectral changes correlate with orbital phase

Abstract

The Wolf-Rayet star Mk 34 was observed more than 50 times as part of the deep T-ReX Chandra ACIS-I X-ray imaging survey of the Tarantula Nebula in the Large Magellanic Cloud conducted between 2014 May and 2016 January. Its brightness showed one bright maximum and repeated faint minima which help define an X-ray recurrence time of $155.1\pm0.1$ days that is probably the orbital period of an eccentric binary system. The maximum immediately precedes the minimum in the folded X-ray light curve as confirmed by new Swift XRT observations. Notwithstanding its extreme median luminosity of $1.2\times10^{35}\mathrm{erg}~\mathrm{s}^{-1}$, which makes it over an order of magnitude brighter than comparable stars in the Milky Way, Mk 34 is almost certainly a colliding-wind binary system. Its spectrum shows phase-related changes of luminosity and absorption that are probably related to the orbital dynamics of two of the most massive stars known.