In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds , we made a pointed 74 ks observation of the nearby early B giant , \beta Cru ( B0.5 III ) , with the Chandra High Energy Transmission Grating Spectrometer .
We find that the X-ray spectrum is quite soft , with a dominant thermal component near 3 million K , and that the emission lines are resolved but quite narrow , with half-widths of 150 km s ^ { -1 } .
The forbidden-to-intercombination line ratios of Ne ix and Mg xi indicate that the hot plasma is distributed in the wind , rather than confined near the photosphere .
It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars , primarily because of the narrow lines , but also because of the high X-ray production efficiency .
A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data , but not very well motivated theoretically .
It is possible that magnetic channeling could explain the X-ray properties , although no field has been detected on \beta Cru .
We detected periodic variability in the hard ( h \nu > 1 keV ) X-rays , modulated on the known optical period of 4.58 hours , which is the period of the primary \beta Cephei pulsation mode for this star .
We also have detected , for the first time , an apparent companion to \beta Cru at a projected separation of 4″ .
This companion was likely never seen in optical images because of the presumed very high contrast between it and \beta Cru in the optical .
However , the brightness contrast in the X-ray is only 3:1 , which is consistent with the companion being an X-ray active low-mass pre-main-sequence star .
The companion ’ s X-ray spectrum is relatively hard and variable , as would be expected from a post T Tauri star .
The age of the \beta Cru system ( between 8 and 10 Myr ) is consistent with this interpretation which , if correct , would add \beta Cru to the roster of Lindroos binaries – B stars with low-mass pre-main-sequence companions .