Discovery of a 9.67-s pulsar in an ultraluminous X-ray source in NGC 4631 with XMM-Newton

TL;DR

A neutron star in NGC 4631 was discovered as a pulsating ultraluminous X-ray source with a 9.67-second period and an exceptionally high spin-up rate, providing new insights into super-Eddington accretion phenomena.

Contribution

First detection of a pulsating ULX in NGC 4631 with detailed timing and spectral analysis, revealing a rapidly spinning neutron star with extreme spin-up behavior.

Findings

Discovered a 9.67 s pulsar in ULX X-8 in NGC 4631.

Observed the largest spin-up rate among ULX pulsars.

X-8's luminosity and spectrum are consistent with known pulsating ULXs.

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, dP/dt = (-9.6 +/- 0.5)*1E-8 s/s. 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 ~3.4E39 erg/s, 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.