Anticorrelation between X-ray luminosity and pulsed fraction in the Small Magellanic Cloud pulsar SXP 1323

Abstract

We report the evidence for the anticorrelation between pulsed fraction (PF) and luminosity of the X-ray pulsar SXP 1323, found for the first time in a luminosity range 10³⁵-10³⁷ erg s^-1 from observations spanning 15 yr. The phenomenon of a decrease in X-ray PF when the source flux increases has been observed in our pipeline analysis of other X-ray pulsars in the Small Magellanic Cloud. It is expected that the luminosity under a certain value decreases as the PF decreases due to the propeller effect. Above the propeller region, an anticorrelation between the PF and the flux might occur either as a result of an increase in the unpulsed component of the total emission or a decrease of the pulsed component. Additional modes of accretion may also be possible, such as spherical accretion and a change in emission geometry. At higher mass accretion rates, the accretion disc could also extend closer to the neutron star surface, where a reduced inner radius leads to hotter inner disc emission. These modes of plasma accretion may affect the change in the beam configuration to fan-beam dominant emission.