Variations in the X-ray flux of HD 167971. Left panel: X-ray flux of HD 167971 as a function of the orbital phase of the tertiary component (Le Bouquin et al. 2017). Phase ϕ_outer = 0 corresponds to periastron passage. The top four panels correspond to the observed X-ray flux in four different energy bands: soft (0.5–1.0 ke V), medium (1.0–2.0 ke V), hard (2.0–4.0 ke V), and total (0.5–10 ke V). Violet filled symbols indicate XMM-Newton data, and open red circles stand for ROSAT observations. The bottom panel yields the flux in the 0.5–10 keV band corrected for absorption by the ISM. Right panel: same but as a function of the orbital phase of the inner eclipsing binary (Ibanoglu et al. 2013). Here ϕ_inner = 0 corresponds to the primary minimum of the eclipsing binary. Because the ROSAT-HRI observation spanned nearly one month (i.e. more than eight orbital cycles) it is not shown here.