On the Spin Period of the Neutron Star in the Ultraluminous X-Ray Source M51 ULX-8

Abstract

The recent discovery of periodic pulsations from several members of the ultraluminous X-ray source (ULX) family in nearby galaxies as well as in our own galaxy unveiled the nature of the accreting compact object. Neutron stars rather than black holes are currently believed to power a substantial number of ULXs whether or not pulsations are observed. The detection of cyclotron absorption lines in the X-ray spectrum of a ULX provides an alternative way to identify the compact object as a neutron star. Among the non-pulsating ULXs, the presence of a cyclotron resonance scattering feature (CRSF) in the spectrum of M51 ULX-8 has been reported. In the present work, the magnetic field strength on the surface of the neutron star in M51 ULX-8 is inferred from the energy of the observed CRSF to estimate the beaming fraction in X-ray emission and more importantly the observable range for the elusive neutron-star spin period to be hopefully discovered by the forthcoming space missions in the near future.