  Dead time affects many of the instruments used in X-ray astronomy, by
producing a strong distortion in power density spectra. This can make it
difficult to model the aperiodic variability of the source or look for
quasi-periodic oscillations. Whereas in some instruments a simple a priori
correction for dead-time-affected power spectra is possible, this is not the
case for others such as NuSTAR, where the dead time is non-constant and long
(~2.5 ms). Bachetti et al. 2015 suggested the cospectrum obtained from light
curves of independent detectors within the same instrument as a possible way
out, but this solution has always only been a partial one: the measured rms was
still affected by dead time, because the width of the power distribution of the
cospectrum was modulated by dead time in a frequency-dependent way.
  In this Letter we suggest a new, powerful method to normalize cospectra and,
with some caveats, even power density spectra. Our approach uses the difference
of the Fourier amplitudes from two independent detectors to characterize and
filter out the effect of dead time. This method is crucially important for the
accurate modelling of periodograms derived from instruments affected by dead
time on board current missions like NuSTAR and ASTROSAT, but also future
missions such as IXPE
