Speaker
Description
The discoveries of coherent X-ray pulsations from several known ultraluminous X-ray sources (ULXs) have provided conclusive proof that some ULXs host neutron stars (NSs) accreting at rates far exceeding their nominal Eddington limits, and raised questions concerning the overall contribution of NS accretors to the broader population of ULXs and the physics of super-Eddington accretion that drives them to radiate at such extreme luminosities. In this talk, I will describe a two-stage approach to X-ray pulsation searches, designed to reveal marginal or transient signals, thereby increasing the number of NS ULXs to study. This method is novel in its consideration of signal reproducibility and its handling of the intrinsic randomisation employed by XMM-Newton's Science Analysis Software. I will present a new candidate pulsating ultraluminous X-ray source (PULX) in NGC 5128 (Centaurus A), identified through this search procedure. The candidate, 4XMM J132542.2--425943 (hereafter 4XMM J1325), is a transient source, identifiable as a clear X-ray point source for ~8 months in 2014, during its only major recorded outburst. During this outburst, we find coherent X-ray pulsations at a frequency of 1.27 Hz in one XMM-Newton observation, at marginal significance. The long-term light curve of 4XMM J1325 resembles those of the class of pulsating transients with outbursts that reach the ultraluminous regime, such as the Galactic PULX, Swift J0243.6+6124. However, 4XMM J1325 displays a soft X-ray spectrum, making this source an outlier from the existing population of PULXs and raises the important question: are PULXs synonymous with a hard spectral state? I will address this question within the context of verifying the marginal pulsation signal from 4XMM J1325 and describe how our two-stage search procedure mitigates against widespread false-positive and/or false-negative pulsation detections, thereby preventing erroneous reports of PULX candidates.