The Surprising Long-Term Evolution of the ULXP NGC 7793 P13

Abstract

The ultra-luminous x-ray pulsar (ULXP) NGC 7793 P13 has been regularly monitored with XMM-Newton, NuSTAR, and Swift for the last 8 years. Here, we present the latest results of this monitoring campaign with respect to the pulse period evolution and spectral variability. We find that since the source recovered from an x-ray low state in 2020–2022 the spin-up rate has increased significantly compared with before the off-state, even though the x-ray luminosity has not shown an equivalent increase. We find that the x-ray and optical/UV flux are anti-correlated, and speculate that this variability might be driven by a large accretion disk, precessing at a super-orbital period of 7–8 years. We study the spectral behavior in the XMM-Newton and NuSTAR data, and find very little changes in the spectral shape, despite the large flux variability. This spectral consistency provides further indication that the observed flux variability is a geometric effect and not due to intrinsic changes of the accretion rate.