Limiting Window ∼1 Ms Chandra spectrum with constraints on an additional 3.55 keV Gaussian line on top of a standard two-temperature apec model. Uncertainties are given at 99.7 percent confidence range (∼3σ). Residuals are given as a percentage deviation from the model. Red residuals are for the model with the minimum allowed flux and blue for the model with maximum flux in the additional Gaussian line.