Synchrotron cutoff in Ultraluminous X-ray sources

Abstract: The origin of spectral curvature at energies $E\simeq 10$ keV in ultraluminous X-ray sources is not well understood. In this paper, we propose a novel mechanism based on synchrotron radiation to explain this cutoff. We show that relativistic plasma can give rise to observed spectral curvature for neutron star magnetic fields due to the variation in the latitude of synchrotron radiation. We analyze the NuSTAR data of two bright pulsar ULXs, NGC 5907 ULX1 and NGC 7793 P13, and provide estimates of the physical parameters of these sources. We fit the data for synchrotron emission at various latitudes and show that the spectral cutoff in these cases can be explained for a large range of acceptable physical parameters, e.g., a semi-relativistic plasma with $\gamma \simeq 20$ for high latitudes or a highly relativistic plasma ($\gamma \simeq 10^5$) for emission close to the electron's orbital plane in a typical magnetic field of $B\simeq 10^{12} \, \rm G$. We also discuss how such an emission mechanism can be distinguished from other proposed models. A corollary to our study is that most ULXs might be neutron stars as they display such a spectral cutoff.