Spinning up and spinning down: the pulse period evolution of ultra-luminous X-ray pulsars

With the discovery that some ultra-luminous X-ray sources are powered by neutron stars and are therefore pulsating a whole new way of investigating these objects has opened up: pulsar timing and period evolution. First of all, this allows us to determine the orbital ephemeris of the system and hence learn more about its origins and evolution. Secondly, by monitoring the pulse period over a long time, we can study the amount of accreted angular momentum in relation to the observed X-ray luminosity, which gives us clues about the accretion efficiency as well as constraints on the magnetic field of the neutron star.

Here I will present pulse period evolution studies of two ultra-luminous X-ray pulsars (ULXPs), NGC 7793 P13 and NGC 5907 ULX-1. P13 has one of the strongest pulsation signals of all known ULXPs, and has been spinning up ever since its first detection. Even during a drop in the X-ray flux in 2019, the spin-up continued with the same strength, indicating continued strong accretion. I will discuss how we use these data to constrain the accretion geometry of the system.

NGC 5907 ULX-1 entered an off-state in 2019, during which the source was below the detection limit for Swift/XRT. Luckily, we obtained pulsations measurements just before and just after this off-state, showing a significant spin-down during this period. In this case therefore the drop in X-ray flux was related to a cessation of accretion, resulting in a slowing down of the neutron star. By combining the flux and period evolution, we can limit the possibility of magnetic field strengths. It is likely that NGC 5907 ULX-1 is powered by a highly magnetized neutron star, with a magnetic field around 10e13G.