Comparison of this works constraints to selected literature values. The best-fit line energy varies between ~ 3.4−3.6 keV. This work: 95% confidence interval (CI) for Chandra ACIS-I (bottom) and ACIS-S (middle) stacked spectra and for the error-weighted average of all individual limits (top). Bulbul et al. (2014): 68% CI for the full cluster sample using the XMM-Newton MOS detectors. Boyarsky et al. (2014): 68% CI (statistical only) for the M 31 XMM-Newton data. Anderson et al. (2015): 68% CI for negative residual in stacked XMM-Newton data of galaxy halos. Jeltema+2015a (see Jeltema & Profumo 2015): range of values obtained for different mass models of the Galactic centre with XMM-Newton observations. Jeltema+2015b (see Jeltema & Profumo 2016): 95% CI upper limit from a deep observation of the Draco dwarf galaxy with XMM-Newton. Ruchayskiy et al. (2016): range of values obtained for different mass models of the Draco dwarf galaxy with the XMM-Newton PN instrument. Boyarsky et al. (2015): best-fit range for observations of the Galactic centre with the XMM-Newton MOS instrument. Iakubovskyi et al. (2015): range of best-fit values for a sample of galaxy clusters observed with XMM-Newton. Negative mixing angles are unphysical but result from Eq. (1) when allowing negative flux. The plot only shows selected constraints from the listed publications.