ULX spectra revisited: observational evidence of accreting envelopes around magnetized Neutron stars

Ultra luminous X-ray (ULX) sources are among the most intriguing binary systems, that have long been thought to host the elusive intermediate mass black holes. Remarkably, within the last years there has been undisputed evidence that at least a few of these systems are powered by accreting neutron stars (NS) that are rotating with spin periods close to 1 s. In light of these recent discoveries and recently introduced models placing neutron stars as the engines of ULXs, we revisit the spectra of eighteen well-known ULXs, in search of indications that favor or reject this hypothesis. We find that the notable (>6keV) spectral curvature observed in most ULXs, is commensurate with the Wien tail of a hot (T>1keV) multicolor black-body component and confirm that a double thermal model (comprised of a ”cool” and ”hot” thermal component) with the addition of a faint non-thermal tail describes all ULX spectra in our list. More importantly, we offer a new physical interpretation for the dual thermal spectrum, where it is the result of accretion onto high magnetized NSs rather than black holes, in agreement with theoretical predictions. We estimate the magnetic-field strength and demonstrate that it correlates strongly with the source luminosity and the temperature of the hot component. We also discuss the application of our model on the most recent pulsating ULX "NGC 300 ULX1", casting doubts on the claimed presence of a cyclotron scattering feature in its spectrum. Our findings offer an additional and compelling argument in favor of NSs as prime candidates for powering ULXs, as has been also postulated by theory.