| Abstract : | Almost all ULXs have shown a unique spectral curvature at around 10 keV, confirmed by the broadband X-ray data from modern observatories. The origin of such a cutoff needs to be better understood. We propose a new theoretical model which can explain this X-ray spectral cutoff with the underlying physics of synchrotron radiation. Relativistic plasma in the presence of a typical neutron star magnetic field can work as the origin of spectral cutoff due to the varying latitude of radiation. A semi-relativistic plasma with high latitude angle or a highly relativistic plasma with an emission close to the orbital plane of the electron can adequately explain this curvature. We show the congruence of the theoretical model with NuSTAR data for two known pulsar ULX systems, namely, NGC 5907 ULX1 and NGC 7793 P13. As a corollary of our study, it can be understood that almost all ULXs that show such spectral cutoff are neutron star systems. |
