Last week NuSTAR performed the first of two observations coordinated with ESA's XMM-Newton observatory targeting an Ultra-Luminous X-ray source (ULX) in the nearby galaxy NGC 5907. As the name implies, ULXs have intrinsic luminosities hundreds of times higher than theoretically possible from standard accretion theories. For decades ULXs were thought to be powered by accretion onto black holes, pulling material from the atmospheres of companion stars, but exactly how these sources are able to reach such luminosities is still not well understood. Following a series of remarkable discoveries from observations by NuSTAR, we now know that some of the most luminous members of the ULX population are actually powered by accretion onto neutron stars. NGC 5907 ULX-1 stands out as the most luminous of these ULX pulsars but since emerging from the latest of its low-flux states, ULX-1 is clearly spinning too fast for an extrapolation of the current model for its long-term spin evolution. The NuSTAR and XMM-Newton observations will obtain multiple measurements of the pulsar spin period and ULX-1 luminosity to determine the current secular spin-up of the neutron star. This will provide key constraints on the strength of the magnetic field and how the current model for the evolution of neutron star spin periods should be revised.

Author: Dominic Walton (Senior Lecturer in Astrophysics, University of Hertfordshire, UK)