2025ApJ...994L..38D

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, $\dot{P}=(-9.6\pm 0.5)\times 1{0}^{-8}$ 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 × 10³⁹ 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.