Merged simulations with 10 times the exposure of original X-ray spectra and an additional Gaussian line at 3.55 keV (ACIS-I top and ACIS-S bottom) with residuals of different fitted models and the effective area (ARF and RMF combined). Fitted XSPEC models: apec+apec+zgauss with best-fit (Bf), upper and lower confidence values (99.7 per cent) of the Gaussian flux in counts cm^-2 s^-1. The annotations show the best-fit value and the confidence interval obtained using MCMC. Residuals are shown for the fit with upper (blue) and lower (red) confidence limit of the Gaussian flux.