  We present broadband X-ray analyses of a sample of bright ultraluminous X-ray
sources with the goal of investigating the spectral similarity of this
population to the known ULX pulsars, M82 X-2, NGC7793 P13 and NGC5907 ULX. We
perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR dataset of
NGC5907 ULX, finding that the pulsed emission from the accretion column in this
source exhibits a similar spectral shape to that seen in both M82 X-2 and
NGC7793 P13, and that this is responsible for the excess emission observed at
the highest energies when the spectra are fit with accretion disk models. We
then demonstrate that similar 'hard' excesses are seen in all the ULXs in the
broadband sample. Finally, for the ULXs where the nature of the accretor is
currently unknown, we test whether the hard excesses are all consistent with
being produced by an accretion column similar to those present in M82 X-2,
NGC7793 P13 and NGC5907 ULX. Based on the average shape of the pulsed emission,
we find that in all cases a similar accretion column can successfully reproduce
the observed data, consistent with the hypothesis that this ULX sample may be
dominated by neutron star accretors. Compared to the known pulsar ULXs, our
spectral fits for the remaining ULXs suggest that the non-pulsed emission from
the accretion flow beyond the magnetosphere makes a stronger relative
contribution than the component associated with the accretion column. If these
sources do also contain neutron star accretors, this may help to explain the
lack of detected pulsations.
