Orbital- and spin-phase variability in the X-ray emission from the accreting pulsar Cen X-3

TL;DR

This study analyzes NuSTAR data of Cen X-3 to understand how its X-ray emission varies with orbital and spin phases, revealing absorption effects and magnetic field influences on spectral features.

Contribution

It provides the first detailed analysis of orbital- and spin-phase spectral variability in Cen X-3 using NuSTAR, highlighting the roles of stellar wind absorption and magnetic field effects.

Findings

Low energy spectral variability driven by stellar wind absorption.

High energy flux variations linked to accretion column Comptonization.

CRSF properties correlate with spin-phase flux.

Abstract

We analyzed 39 ks NuSTAR observation data of the high mass X-ray binary Cen X-3 in order to investigate the orbital- and spin-phase spectral variability. The observation covers the orbital phase of $\Phi=0.199$-$0.414$ of the source, where $\Phi=0$ corresponds to the mid-eclipse. The orbital-phase-resolved spectroscopy revealed that low energy photons are more dominant for the spectral fluctuation, and a large part of the variability can be explained in terms of absorption by clumps of stellar wind. The spin-phase-resolved spectroscopy together with energy-resolved pulse profiles, on the other hand, presented large flux variations in high energy bands, which suggests that the origin of the variability is the different efficiency of Comptonization inside the accretion column. The energy band which includes Fe emission lines or cyclotron resonance scattering feature (CRSF) shows distinct variability compared to the nearby bands. The Fe lines show low variability along the spin phase, which indicates that the emission regions are apart from the neutron star. The central energy and strength of the CRSF are both positively correlated with the spin-phase-resolved flux, which suggests that the emitted photons face stronger magnetic fields and deeper absorption when they come from high-flux regions. We also examined the independence of the orbital- and spin-phase variability. They showed no correlation with each other and were highly independent, which implies the accretion stream is stable during the observation.