A statistical study of type II outbursts of XRPs: Brighter accreting pulsars rotate faster

X-ray pulsars (XRPs) consist of a magnetized neutron star (NS) and an optical donor star. The NS accretes matter from the donor star, producing pulsed X-ray emission. In most cases, the donor stars are Be stars, and accretion is episodic, that is, the NSs are generally X-ray dim but occasionally experience outbursts. Here, we carry out a statistical study with the X-ray monitoring data and obtain strong correlations between the spin periods of the NSs and the outburst parameters for the first time. We show that XRPs containing faster rotating NSs tend to display more violent eruptions. In addition, pulsating ultraluminous X-ray sources in nearby galaxies follow a similar relationship. We demonstrate that most of these systems are close to the spin equilibrium, and that brighter pulsars have acquired more angular momentum by accreting matter from their companion stars, resulting in faster rotating NSs.