Spectroscopic comparison between ultraluminous X-ray sources and magnetar bursts

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.