Explaining the asymmetric line profile in Cepheus X-4 with spectral variation across pulse phase

TL;DR

This study analyzes the asymmetric cyclotron line in Cepheus X-4's spectrum by phase-resolved spectral analysis, revealing that the asymmetry can be explained by phase-dependent variations in symmetric line profiles.

Contribution

It demonstrates that phase-resolved analysis with symmetric lines can account for the observed asymmetry in phase-averaged spectra of Cepheus X-4.

Findings

Pulse phase-resolved spectra fit by symmetric cyclotron features

Parameters vary strongly with pulse phase

Combined phase-resolved spectra reproduce phase-averaged asymmetry

Abstract

The high mass X-ray binary Cep X-4, during its 2014 outburst, showed evidence for an asymmetric cyclotron line in its hard X-ray spectrum. The 2014 spectrum provides one of the clearest cases of an asymmetric line profile among all studied sources with Cyclotron Resonance Scattering Features (CRSF). We present a phase-resolved analysis of NuSTAR and Suzaku data taken at the peak and during the decline phases of this outburst. We find that the pulse-phased resolved spectra are well-fit by a single, symmetric cyclotron feature. The fit parameters vary strongly with pulse phase: most notably the central energy and depth of the cyclotron feature, the slope of the power-law component, and the absorbing column density. We synthesise a phase averaged spectrum using the best-fit parameters for these individual pulse phases, and find that this combined model spectrum has a similar asymmetry in the cyclotron features as discovered in phase-averaged data. We conclude that the pulse phase resolved analysis with simple symmetric line profiles when combined can explain the asymmetry detected in the phase-averaged data.