Signature of a cool column on CCD4 of RGS1 (1 Ms Mrk 421 data summed in detector space in a plot of pulse-height (Energy) versus dispersion axis. Top curve is the second order of the dispersed spectrum, while bottom curve shows first order. A “cool” column can be recognized around RAWX = 50, or about 24.5 Å in first order. This column extends into second order at the same position, but at half the wavelength. Top scale is the approximate wavelength of the first order only.