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Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4

Vybornov, V. and Klochkov, D. and Gornostaev, M. and Postnov, K. and Sokolova-Lapa, E. and Staubert, R. and Pottschmidt, K. and Santangelo, A. (2017) Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4. Astronomy and Astrophysics, 601 . Art. No. A126. ISSN 0004-6361. doi:10.1051/0004-6361/201630275.

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Context. X-ray spectra of accreting pulsars are generally observed to vary with their X-ray luminosity. In particular, the hardness of the X-ray continuum is found to depend on luminosity. In a few sources, the correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the luminosity is clear. Different types (signs) of the correlation are believed to reflect different accretion modes. Aims. We analyse two NuSTAR observations of the transient accreting pulsar Cep X-4 during its 2014 outburst. Our analysis is focused on a detailed investigation of the dependence of the CRSF energy and of the spectral hardness on X-ray luminosity, especially on short timescales. Methods. To investigate the spectral changes as a function of luminosity within each of the two observations, we used the intrinsic variability of the source on the timescale of individual pulse cycles (tens of seconds), the so-called pulse-to-pulse variability. Results. We find that the NuSTAR spectrum of Cep X-4 contains two CRSFs: the fundamental line at ~30 keV and its harmonic at ~55 keV. We find for the first time that the energy of the fundamental CRSF increases and the continuum becomes harder with increasing X-ray luminosity not only between the two observations, that is, on the long timescale, but also within an individual observation, on the timescale of a few tens of seconds. We investigate these dependencies in detail including their non-linearity. We discuss a possible physical interpretation of the observed behaviour in the frame of a simple one-dimensional model of the polar emitting region with a collisionless shock formed in the infalling plasma near the neutron star surface. With this model, we are able to reproduce the observed variations of the continuum hardness ratio and of the CRSF energy with luminosity.

Item Type:Article
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URLURL TypeDescription Paper
Pottschmidt, K.0000-0002-4656-6881
Alternate Title:Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cep X-4
Additional Information:© 2017 ESO. Article published by EDP Sciences. Received 17 December 2016; Accepted 8 February 2017; Published online 19 May 2017. The research is supported by the joint DFG grant KL 2734/2-1 and Wi 1860 11-1 and the RFBR grants 14-02-91345 and 18-502-12025. We thank the anonymous referee for the useful comments and suggestions that substantially improved the manuscript. The research used the data obtained from HEASARC Online Service provided by the NASA/GSFC.
Subject Keywords:X-rays: binaries – X-rays: stars – stars: magnetic field – pulsars: individual: Cep X-4
Record Number:CaltechAUTHORS:20170222-114208640
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Official Citation:Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4. V. Vybornov, D. Klochkov, M. Gornostaev, K. Postnov, E. Sokolova-Lapa, R. Staubert, K. Pottschmidt and A. Santangelo A&A, 601 (2017) A126. DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:74468
Deposited By: Joy Painter
Deposited On:22 Feb 2017 19:48
Last Modified:11 Nov 2021 05:27

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