Circumstellar Disc and X-ray Variability in the Be/X-ray Binary SXP 5.05 During its 2024 Outburst

Abstract

Be/X-ray binaries provide a unique opportunity to study the interaction between neutron stars and circumstellar discs. SXP 5.05 is a particularly rare system, exhibiting eclipse-like X-ray variability attributed to obscuration by the Be star disc rather than a simple stellar eclipse. Motivated by its unusual geometry and the well-studied 2013 outburst, we present a multiwavelength analysis of its 2024 outburst using NICER X-ray observations and long-term optical monitoring from OGLE. The X-ray light curve shows a declining outburst with lower peak intensity and shorter duration compared to 2013, indicating a reduced accretion episode. The spectral evolution, characterized through hardness ratios, reveals a transition from a soft, high-intensity state to a harder, low-intensity state. Coherent pulsations near 5.05 s are detected throughout the observations, with properties consistent with previous measurements. The optical light curves show a reduced variability amplitude relative to 2013, possibly from a less extended or less dense circumstellar disc. Orbital-phase-folded optical profiles reveal a persistent, phase-locked dip structure, indicating a stable non-axisymmetric disc component that evolves across outburst phases. Together, these results support a picture in which the observed variability is driven by changes in disc structure and viewing geometry. SXP 5.05 thus remains a key system for probing the time-dependent properties of Be star discs through combined X-ray and optical observations.