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tokens: XMM-Newton spectra of HD 54879 (high-resolution RGS spectra have an insufficient signal and are not useful). The X-ray spectra of magnetic hot stars are usually well described by multi-temperature thermal plasma models (e.g., Oskinova et al. 2011 ; Nazé et al. 2014 ). This is also the case for HD 54879. As a first step, we fitted the observed spectra in the 0.2–10.0 keV band with a thermal two-temperature spectral model that assumes optically thin plasma in collisional equilibrium. At this step, the absorption was modeled as originating in the cold interstellar medium ( Wilms et al. 2000 ). The resulting spectral fit is statistically significant, with a reduced . The best fit parameters are shown in Table 3 . There is no physical motivation for restricting the plasma temperature distribution to two temperatures. In fact, spectroscopic analyses show a continuous temperature distribution in hot stars ( Wojdowski Schulz 2005 ). Therefore, as a next step, we fitted a three-temperature spectral model to the observed EPIC spectra of HD 54879. Also in this case, the fit is statistically significant, with . The three-temperature model fit is shown in Fig. 10 , and the associated parameters are shown in Table 3 . The three-temperature fit reveals the hottest plasma component with a temperature of about 2 keV (20 MK). Better S/N is required to confirm the presence of this high-temperature plasma. We also tried to fit a four-temperature plasma model. However, the quality of the data is not sufficient to further constrain the temperature distribution. Fig. 10 XMM-Newton PN, MOS1, and MOS2 spectra of HD 54879 (black, red, and green curves, respectively) with error bars corresponding to 3 with the best fit three-temperature thermal model (solid lines). The model parameters are shown in Table 1. Table 3 X-ray spectral parameters derived from the XMM-Newton observations of HD 54879 assuming a three-temperature plasma model. Notes. (a) Corrected for the interstellar absorption in the 0.2–12 keV band. In the framework of the magnetically confined wind shocks (MCWS) model ( Babel Montmerle 1997 ; ud-Doula Owocki 2002 ), the wind plasma streams collide at the magnetic equator and give rise to a shock that heats the plasma. Hence, the maximum temperature follows from a Rankine-Hugoniot condition and cannot exceed a value determined by the maximum streaming velocity. Motivated by the recent discovery of a non-thermal component in the X-ray emission of the magnetic star HR 7355 ( Leto et al. 2017 ), we also fitted the observed spectra with a composite two-temperature thermal plasma and a non-thermal, power law component. The model provides a fit of similar quality to the observed spectra ( for 138 degrees of freedom). The best fit thermal plasma temperatures are [keV], [keV], and the power-law exponent is . Thus, the quality of the data is not sufficient to discriminate between three-temperature thermal and two-temperature thermal plus non-thermal component models.