ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources

Aims: In light of recent discoveries of pulsating ULXs and recently introduced models, that propose neutron stars (NSs) as the central engines of ULXs, we revisit the (XMM-Newton and NuSTAR) spectra of eighteen well known ULXs, in search of indications that favor this new hypothesis. Results: We confirm the, previously noted, presence of the low energy (<6 keV) spectral rollover and argue that it could be interpreted as thermal emission. The spectra are well described by a double thermal model consisting of a "hot" (>1 keV) and a "cool" (<0.7 keV) multicolor black body (MCB). Under the assumption that the "cool" MCD emission originates in a disk truncated at the NS magnetosphere, we find that all ULXs in our sample are consistent with accretion onto a highly magnetized (B>10^12G) NS. We note a strong correlation between the strength of the magnetic field, the temperature of the "hot" thermal component and the total unabsorbed luminosity. Examination of the NuSTAR data supported this interpretation and confirmed the presence of a weak, high energy (>15 keV) tail, most likely the result of modification of the MCB emission by inverse Compton scattering. We also note that the apparent high energy tail, may simply be the result of mis-modeling of MCB emission with an atypical temperature (T) vs radius (r) gradient, using a standard MCD model with a fixed gradient of T~r^-0.75. Conclusions: We have offered a new physical interpretation for the dual-thermal spectra of ULXs. We find that the best-fit derived parameters, are in excellent agreement with recent predictions that favor super-critically accreting NSs as the engines of a large fraction of ULXs. Nevertheless, the considerable degeneracy between models and the lack of unequivocal evidence cannot rule out other equally plausible interpretations. Our results warrant further exploration of this new framework.

Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages 5 figures. This version: added citation(s), title change