X-RAY EMISSION FROM MAGNETIC MASSIVE STARS

Abstract

Magnetically confined winds of early-type stars are expected to be sources of\nbright and hard X-rays. To clarify the systematics of the observed X-ray\nproperties, we have analyzed a large series of Chandra and XMM observations,\ncorresponding to all available exposures of known massive magnetic stars (over\n100 exposures covering ~60% of stars compiled in the catalog of Petit et al.\n2013). We show that the X-ray luminosity is strongly correlated with the\nstellar wind mass-loss-rate, with a power-law form that is slightly steeper\nthan linear for the majority of the less luminous, lower-Mdot B stars and\nflattens for the more luminous, higher-Mdot O stars. As the winds are\nradiatively driven, these scalings can be equivalently written as relations\nwith the bolometric luminosity. The observed X-ray luminosities, and their\ntrend with mass-loss rates, are well reproduced by new MHD models, although a\nfew overluminous stars (mostly rapidly rotating objects) exist. No relation is\nfound between other X-ray properties (plasma temperature, absorption) and\nstellar or magnetic parameters, contrary to expectations (e.g. higher\ntemperature for stronger mass-loss rate). This suggests that the main driver\nfor the plasma properties is different from the main determinant of the X-ray\nluminosity. Finally, variations of the X-ray hardnesses and luminosities, in\nphase with the stellar rotation period, are detected for some objects and they\nsuggest some temperature stratification to exist in massive stars'\nmagnetospheres.\n