Speaker
Description
NGC 5907 ULX-1, the most luminous ULX pulsar (peak luminosity 10⁴¹ erg/s), shows super-orbital modulation whose physical origin has implications for the magnetic field strength required to sustain super-Eddington accretion. From >10 years of Swift XRT monitoring (2014–2025), we characterise this modulation using Bayesian mixture models that properly marginalise over intermediate-flux states diluting the periodic signal in the recent high-flux epoch. We recover a tightly constrained ~78 d period with a fast-rise-exponential-decay profile that maintains phase-coherence across an extended quiescent interval (mid-2017–mid-2020; ~14 super-orbital cycles), favouring a precession mechanism anchored to the neutron star itself. We use this timing solution to assign super-orbital phases to the full archival set of XMM-Newton and NuSTAR observations, constructing phase-resolved broadband spectra that substantially extend the limited sampling available to earlier work (Fürst et al. 2017). Tracking the spectral continuum as a function of super-orbital phase offers a crucial diagnostic of the geometric beaming that is central to interpreting the extreme apparent luminosities of ULX pulsars: precession-driven changes in viewing angle modulate the degree of collimation visible to the observer, so phase-resolved spectroscopy can disentangle intrinsic luminosity variations from these viewing angle effects. For NGC 5907 ULX-1, where the apparent luminosity exceeds the neutron star Eddington limit by a factor of ~500, this approach provides a direct test of the beaming geometry invoked to reconcile the observed flux with physically plausible accretion rates.
