2022MNRAS.515.4130C

We reassess the historical L_X/L_Bol relation for early-type stars from a comparison between T-ReX, the Chandra ACIS X-ray survey of the Tarantula Nebula in the Large Magellanic Cloud (LMC), and contemporary spectroscopic analysis of massive stars obtained primarily from VLT/FLAMES, VLT/MUSE, and HST/STIS surveys. For 107 sources in common (some host to multiple stars), the majority of which are bolometrically luminous (40 per cent exceed 10⁶L_⊙), we find an average log L_X/L_Bol = -6.90 ± 0.65. Excluding extreme systems Mk 34 (WN5h+WN5h), R140a (WC4+WN6+), and VFTS 399 (O9 IIIn+?), plus four WR sources with anomalously hard X-ray components (R130, R134, R135, Mk 53) and 10 multiple sources within the spatially crowded core of R136a, log L_X/L_Bol = -7.00 ± 0.49, in good agreement with Galactic OB stars. No difference is found between single and binary systems, nor between O, Of/WN, and WR stars, although there does appear to be a trend towards harder X-ray emission from O dwarfs, through O (super)giants, Of/WN stars, and WR stars. The majority of known OB stars in the Tarantula are not detected in the T-ReX point source catalogue, so we have derived upper limits for all undetected OB stars for which log L_Bol/L_⊙ ≥ 5.0. A survival analysis using detected and upper limit log L_X/L_Bol values indicates no significant difference between luminous O stars in the LMC and the Carina Nebula. This analysis suggests that metallicity does not strongly influence L_X/L_Bol. Plasma temperatures for single, luminous O stars in the Tarantula ($\overline{kT_{m}}=1.0$ keV) are higher than counterparts in Carina ($\overline{kT_{m}}=0.5$ keV).