On the Mass and Magnetic Field of the Neutron Star in the Ultraluminous X-Ray Source NGC 300 ULX1

Abstract

The accreting compact objects in most of ultraluminous X-ray sources (ULXs) are likely to be neutron stars rather than black holes as suggested by the recent detection of periodic pulsations from some of these sources located in neighboring galaxies and one ULX that has hitherto been discovered in our own galaxy. As a member of the ULX family, NGC 300 ULX1 is a new pulsating ULX (PULX) spinning up at substantially high rates compared with other PULXs. In this paper, the strength of the magnetic field on the surface of the neutron star is inferred from the energy of the cyclotron absorption line detected in the pulsed X-ray spectrum of NGC 300 ULX1 and the plausible ranges for the neutron-star mass and beaming fraction are estimated using the observed spin period and period derivative of the pulsar and the measured X-ray flux of the source. Our analysis favors proton cyclotron resonance scattering as a viable mechanism to account for both the observed cyclotron energy and high spin-up rates provided that the absorption line is generated close to the surface of the neutron star.