DISTORTED CYCLOTRON LINE PROFILE IN CEP X-4 AS OBSERVED BY NuSTAR
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Author/Creator ORCID
Date
2015-06-15
Type of Work
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Citation of Original Publication
Fürst, F., K. Pottschmidt, H. Miyasaka, V. Bhalerao, M. Bachetti, S. E. Boggs, F. E. Christensen, et al. “Distorted Cyclotron Line Profile in CEP X-4 as Observed by NuSTAR.” The Astrophysical Journal Letters 806, no. 2 (June 2015): L24. https://doi.org/10.1088/2041-8205/806/2/L24.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
Public Domain Mark 1.0
Public Domain Mark 1.0
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Abstract
We present spectral analysis of Nuclear Spectroscopic Telescope Array and Swift observations of Cep X-4 during its outburst in 2014. We observed the source once during the peak of the outburst and once during the decay, finding good agreement in the spectral shape between the observations. We describe the continuum using a power law with a Fermi–Dirac cutoff at high energies. Cep X-4 has a very strong cyclotron resonant scattering feature (CRSF) around 30 keV. A simple absorption-like line with a Gaussian optical depth or a pseudo-Lorentzian profile both fail to describe the shape of the CRSF accurately, leaving significant deviations at the red side of the line. We characterize this asymmetry with a second absorption feature around 19 keV. The line energy of the CRSF, which is not influenced by the addition of this feature, shows a small but significant positive luminosity dependence. With luminosities between (1–6) × 10³⁶ erg s⁻¹, Cep X-4 is below the theoretical limit where such a correlation is expected. This behavior is similar to Vela X-1 and we discuss parallels between the two systems.