Explaining the asymmetric line profile in Cepheus X-4 with spectral variation across pulse phase. Issue 3 (22nd October 2018)

• Record Type: Journal Article
• Authors: Bhargava, Yash; Bhalerao, Varun; Ballhausen, Ralf; Fürst, Felix; Pottschmidt, Katja; Tomsick, John A; Wilms, Joern
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 482:Issue 3(2019)
• Page Start: 2902
• Page End: 2912
• Publication Date: 2018-10-22
• Subjects: accretion, accretion discs -- stars: neutron -- X-rays: binaries -- X-rays: individual (Cep X-4); Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty2823
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: The high mass X-ray binary Cepheus 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-fitting 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.