  A detailed search for emission and absorption lines and assessing their upper
limits are performed for Suzaku data. The method utilizes a matched-filtering
approach to maximize the signal-to-noise ratio for a given energy resolution,
which could be applicable to many types of line search. We first applied it to
well-known AGN spectra that have been reported to have ultra-fast outflows, and
find that our results are consistent with previous findings at the ~3{\sigma}
level. We proceeded to search for emission and absorption features in the two
bright magnetars 4U 0142+61 and 1RXS J1708-4009, applying the filtering method
to Suzaku data. We found that neither source showed any significant indication
of line features, even using long Suzaku observations and dividing their
spectra into spin phases. The upper limits on the equivalent width of
emission/absorption lines are constrained to be a few eV at ~1 keV, and a few
hundreds of eV at ~10 keV. This strengthens previous reports that persistently
bright magnetars do not show proton cyclotron absorption features in soft
X-rays and, even if they exist, they would be broadened or much weaker than
below the detection limit of X-ray CCD.
