  Nearby galaxies host ultra-luminous X-ray sources (ULXs), whose nature
remains largely unknown. Until the discovery of the first ULX pulsar, M82 X-2,
the mechanism powering the large luminosities of most ULXs was thought to be
super-Eddington accretion onto black holes. The ULX pulsar clearly indicates
that this hypothesis is not universal, and the question arises if other ULXs
are as well powered by accretion onto neutron stars. One possibility to have
highly super-Eddington luminosity is by reducing the opacity by strong magnetic
fields as in magnetars, as proposed for M82 X-2. To study the link between ULXs
and magnetar bursts/flares, we have performed a comparative spectral study
between these classes, which both emit at similar super-Eddington luminosities
at around $L \sim 10^{40}$ erg s$^{-1}$. We find that, when their spectra are
fitted with dual thermal models, the long term spectral variations of ULXs are
similar to short term spectral variability seen during magnetar flares. In both
classes of sources the black body temperatures and radii follow similar
$R_\textrm{bb} \propto T_\textrm{bb}^{-2}$ trends, the only difference being
that the temperatures in ULXs are an order of magnitude lower and thus they
have much larger characteristic emission areas. Furthermore, the ratio between
the luminosities of the two black body components shows the same one-to-one
dependence in ULXs and magnetars. We discuss the possible origins of these
similarities, and speculate that the reason for the lack of pulsations in
"magnetar-ULXs" could be that their photospheres are extended far beyond the
neutron star surface.
