  Most young neutron stars belonging to the class of Central Compact Objects in
supernova remnants (CCOs) do not have known periodicities. We investigated
seven such CCOs to understand the common reasons for the absence of detected
pulsations. Making use of XMM-Newton, Chandra, and NICER observations, we
perform a systematic timing and spectral analysis to derive updated sensitivity
limits for both periodic signals and multi-temperature spectral components that
could be associated with radiation from hotspots on the neutron star surface.
Based on these limits, we then investigated for each target the allowed viewing
geometry that could explain the lack of pulsations. We estimate it is unlikely
($< 10^{-6}$) to attribute that we do not see pulsations to an unfavorable
viewing geometry for five considered sources. Alternatively, the carbon
atmosphere model, which assumes homogeneous temperature distribution on the
surface, describes the spectra equally well and provides a reasonable
interpretation for the absence of detected periodicities within current limits.
The unusual properties of CCOs with respect to other young neutron stars could
suggest a different evolutionary path, as that proposed for sources
experiencing episodes of significant fallback accretion after the supernova
event.
