  Strongly magnetized isolated neutron stars (NSs) are categorized into two
families, according mainly to their magnetic field strength. The one with a
higher magnetic field of $10^{14}$ - $10^{15}$ G is called magnetar,
characterized with repeated short bursts, and the other is X-ray isolated
neutron star (XINS) with $10^{13}$ G. Both magnetars and XINSs show thermal
emission in X-rays, but it has been considered that the thermal spectrum of
magnetars is reproduced with a two-temperature blackbody (2BB), while that of
XINSs shows only a single-temperature blackbody (1BB) and the temperature is
lower than that of magnetars. On the basis of the magnetic field and
temperature, it is often speculated that XINSs may be old and cooled magnetars.
Here we report that all the seven known XINSs show a high-energy component in
addition to the 1BB model. Analyzing all the XMM-Newton data of the XINSs with
the highest statistics ever achieved, we find that their X-ray spectra are all
reproduced with a 2BB model, similar to magnetars. Their emission radii and
temperature ratios are also similar to those of magnetars except for two XINSs,
which show significantly smaller radii than the others. The remarkable
similarity in the X-ray spectra between XINSs and magnetars suggests that their
origins of the emission are also the same. The lower temperature in XINSs can
be explained if XINSs are older than magnetars. Therefore, these results are
the observational indication that supports the standard hypothesis on the
classification of highly-magnetized NSs.
