X‐Ray Variability in the Young Massive Tripleθ2Orionis A

Massive stars rarely show intrinsic X-ray variability. The only O-starscredited to be intrinsically variable are theta1 Ori C due to effects frommagnetic confinement of its wind, and theta2 Ori A suspected of similaractivity. Early Chandra observations have shown that the most massive starsystem in the Orion Trapezium Cluster, theta2 Ori A, shows rapid variability ontime scales of hours. We determine X-ray fluxes and find that the star showsvery strong variability over the last 5 years. We observed a second large X-rayoutburst in November 2004 with the high resolution transmission gratingspectrometer on-board Chandra. In the low state X-ray emissivities indicatetemperatures well above 25 MK. In the high state we find an extended emissivitydistribution with high emissivities in the range from 3 MK to over 100 MK. Theoutburst event in stellar terms is one of the most powerful ever observed andthe most energetic one in the ONC with a lower total energy limit of 1.5x10^37ergs. The line diagnostics show that under the assumption that the lineemitting regions in the low states are as close as within 1 -- 2 stellar radiifrom the O-star's photosphere, whereas the hard states suggest a distance of 3-- 5 stellar radii. The two outbursts are very close to the periastron passageof the stars. We argue that the high X-ray states are possibly the result ofreconnection events from magnetic interactions of the primary and secondarystars of the spectroscopic binary. Effects from wind collisions seem unlikelyfor this system. The low state emissivity and R-ratios strengthen thepredicament that the X-ray emission is enhanced by magnetic confinement of theprimary wind. We also detect Fe fluorescence indicative of the existence ofsubstantial amounts of neutral Fe in the vicinity of the X-ray emission.Comment: 11 pages, 8 figures, accepted for publication in The Astrophysical Main Journa