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We present an analysis of both the resolved X‐ray emission‐line profiles and the broad‐band X‐ray spectrum of the O2 If* star HD 93129A, measured with the  Chandra  High Energy Transmission Grating Spectrometer (HETGS). This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind‐shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X‐ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10 per cent of the X‐ray flux. We find results consistent with the predictions of numerical simulations of the line‐driving instability, including line broadening indicating an onset radius of X‐ray emission of several tenths of  R  * . Helium‐like forbidden‐to‐intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind‐collision interface with the star’s closest visual companion at a distance of 100 au. The broad‐band X‐ray spectrum is fitted with a dominant emission temperature of just  kT = 0.6 keV along with significant wind absorption. The broad‐band wind absorption and the line profiles provide two independent measurements of the wind mass‐loss rate:  and 6.8 +2.8  − 2.2 × 10 −6  M ⊙  yr −1 , respectively. This is the first consistent modelling of the X‐ray line‐profile shapes and broad‐band X‐ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3–4 compared to the standard Hα mass‐loss rate that assumes a smooth wind.

Chandra X‐ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass‐loss rate