Pulsator-like spectra from Ultraluminous X-ray sources and the search
  for more ultraluminous pulsars
  Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects
whose luminosity exceeds the Eddington limit for a 10 Msun black hole (BH).
Their properties have been widely interpreted in terms of accreting
stellar-mass or intermediate-mass BHs. However at least three neutron stars
(NSs) have been recently identified in ULXs through the discovery of periodic
pulsations. Motivated by these findings we studied the spectral properties of a
sample of bright ULXs using a simple continuum model which was extensively used
to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that
such a model, consisting of a power-law with a high-energy exponential cut-off,
fits very well most of the ULX spectra analyzed here, at a level comparable to
that of models involving an accreting BH. On these grounds alone we suggest
that other non-pulsating ULXs may host NSs. We found also that above 2 keV the
spectrum of known pulsating ULXs is harder than that of the majority of the
other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra
of comparable hardness. We thus suggest that these two ULXs may host an
accreting NS and encourage searches for periodic pulsations in the flux.
