  The internal-plateau X-ray emission of gamma-ray bursts (GRBs) indicates that
a newly born magnetar could be the central object of some GRBs. The observed
luminosity and duration of the plateaus suggest that, for such a magnetar, a
rapid spin with a sub- or millisecond period is sometimes able to last
thousands of seconds. In this case, the conventional neutron star (NS) model
for the magnetar may be challenged, since the rapid spin of nascent NSs would
be remarkably decelerated within hundreds of seconds due to r-mode instability.
In contrast, the r-modes can be effectively suppressed in nascent strange stars
(SSs). In other words, to a certain extent, only SSs can keep nearly-constant
extremely-rapid spin for a long period of time during the early ages of the
stars. We thus propose that the sample of the GRB rapidly-spinning magnetars
can be used to test the SS hypothesis based on the distinct spin limits of NSs
and SSs.
