Explaining the Asymmetric Line Profile in Cepheus X-4 with Spectral Variation Across Pulse Phase
2018
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.
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