Discovery of a 9.67 s Pulsar in an Ultraluminous X-Ray Source in NGC 4631 with XMM-Newton

Abstract Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P = 9.6652 ± 0.0002 s in a new ultraluminous X-ray source (ULX) in the galaxy NGC 4631. This source, dubbed as X−8, shows one of the largest spin-up rates ever observed, P ̇ = ( − 9.6 ± 0.5 ) × 1 0 − 8 s s −1 . These findings indicate that the compact object is a neutron star, and X−8 is a new member of the pulsating ULX class. The 0.3–10 keV luminosity of X−8 is ∼3.4 × 10 39 erg s −1 , and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large spin-up rate: Doppler shift from orbital motion of the neutron star and intrinsic spin-up due to accretion torque. This new ULX pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.