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arXiv · 1810.07724

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

Yash Bhargava·Varun Bhalerao·Ralf Ballhausen·Felix Fürst·Katja Pottschmidt·John A. Tomsick·Joern Wilms
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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.

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Yash Bhargava, Varun Bhalerao, Ralf Ballhausen, Felix Fürst, Katja Pottschmidt, John A. Tomsick, Joern Wilms. 2018-10-17. Explaining the asymmetric line profile in Cepheus X-4 with spectral variation across pulse phase. https://doi.org/10.1093/mnras%2Fsty2823

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