X-ray emission from magnetic massive stars

Magnetically confined winds of early-type stars are expecte d 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-Newton 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 st ellar wind mass-loss-rate, with a power-law form that is slightly steeper than linear for the majority of the l ess luminous, lower- ˙ M B stars and flattens for the more luminous, higher- ˙ M 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 betwe en other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary t o expectations (e.g. higher temperature for stronger mass-loss rate). This suggests that the main drive r 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 s ome objects and they suggest some temperature stratification to exist in massive stars’ magnetospheres.