Speaker
Description
Ultraluminous X-ray sources challenge our understanding of extreme accretion regimes, particularly ULX pulsars like NGC5907 ULX1. This remarkable system exceeds the neutron star Eddington limit by a factor of ~500 (L_X,peak ~10^41 erg/s) and exhibits a ~78-day super-orbital periodicity, providing a test case - as the most luminous ULX pulsar - to differentiate between competing descriptions of super-Eddington accretion.
We present results from Swift XRT monitoring data (2020-2023) that confirm the persistence of the ~78-day modulation in the latest high-flux observational epoch. An extended low-flux off-state separates the recent 2020-2023 high-flux activity from the 2014-2016 high-flux epoch, providing a crucial opportunity to explore phase coherence between these distinct observational epochs. Since the magnetic precession scenario allows for total phase coherence, whereas the Lense-Thirring precession model predicts a likely phase discontinuity following a significant change in accretion rate, we explore using the measurement of phase coherence as a test to determine the driving mechanism for the super-orbital modulation in NGC5907 ULX1.
Distinguishing between models for the origin of super-orbital modulation may provide independent constraints on the magnetic field strength of the ULX pulsar. The neutron star's magnetic field introduces additional structure that influences both the overall spectral characteristics as well as the long-term periodic behaviour. Our study of NGC5907 ULX1 contributes to the ongoing discussion on the role of the magnetic field in producing the extreme luminosities observed in the ULX population.
