X-ray emission from magnetic massive stars

TL;DR

This study analyzes X-ray emissions from over 100 magnetic massive stars, revealing a strong correlation between X-ray luminosity and stellar wind mass-loss rate, and comparing observations with advanced MHD models.

Contribution

It provides the first comprehensive analysis of X-ray properties across a large sample of magnetic massive stars, establishing key scaling relations and testing theoretical models.

Findings

X-ray luminosity correlates with wind mass-loss rate, following a power-law.

Most observed X-ray properties are well reproduced by new MHD models.

Some rapidly rotating stars are overluminous in X-rays.

Abstract

Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ~60% of stars compiled in the catalog of Petit et al. 2013). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss-rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower-Mdot B stars and flattens for the more luminous, higher-Mdot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g. higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest some temperature stratification to exist in massive stars' magnetospheres.