Last week, NuSTAR performed the first of two planned observations of the ultraluminous X-ray source NGC 7793 P13. Like its fellow ultraluminous source M82 X-2, P13 is a pulsar, meaning that its extraordinary luminosity comes from extreme levels of accretion onto a tiny neutron star, and its spin period decreases over time as the accreting material spins it up by adding angular momentum. Every now and again, P13 appears to fade dramatically in brightness but, unusually for this class of objects, when it returns to its usual luminosity again its rate of spin has increased further rather than slowing down, suggesting that it doesn't stop spinning up even when it is in a faint state. Additionally, this binary system has an orbital period a little over 60 days long, with a variation in the ultraviolet emission that is very slightly out of sync with it, which might indicate some unusual interaction between different parts of the accreting system. These NuSTAR observations will continue to track its changing period over time, helping to piece together the shape of this system and the reasons for its strange behavior.

Author: Hannah Earnshaw (NuSTAR Project Scientist, Caltech)