  A newly formed magnetar has been proposed as the central engine of short GRBs
to explain on-going energy injection giving observed plateau phases in the
X-ray lightcurves. These rapidly spinning magnetars may be capable of emitting
pulsed emission comparable to known pulsars and magnetars. In this paper we
show that, if present, a periodic signal would be detectable during the plateau
phases observed using the Swift X-Ray Telescope recording data in Window Timing
mode. We conduct a targeted deceleration search for a periodic signal from a
newly formed magnetar in 2 Swift short GRBs and rule out any periodic signals
in the frequency band 10-285 Hz to $\approx$15-30% rms. These results
demonstrate that we would able to detect pulsations from the magnetar central
engine of short GRBs if they contribute to 15-30% of the total emission. We
consider these constraints in the context of the potential emission mechanisms.
The non-detection is consistent with the emission being reprocessed in the
surrounding environment or with the rotation axis being highly aligned with the
observing angle. As the emission may be reprocessed, the expected periodic
emission may only constitute a few percent of the total emission and be
undetectable in our observations. Applying this strategy to future observations
of the plateau phases with more sensitive X-ray telescopes may lead to the
detection of the periodic signal.
