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Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR

Fürst, F. and Pottschmidt, K. and Miyasaka, H. and Bhalerao, V. and Bachetti, M. and Boggs, S. E. and Christensen, F. E. and Craig, W. W. and Grinberg, V. and Hailey, C. J. and Harrison, F. A. and Kennea, J. A. and Rahoui, F. and Stern, D. and Tendulkar, S. P. and Tomsick, J. A. and Walton, D. J. and Wilms, J. and Zhang, W. W. (2015) Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR. Astrophysical Journal Letters, 806 (2). Art. No. L24. ISSN 2041-8205. doi:10.1088/2041-8205/806/2/L24.

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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) × 10^(36) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription DOIArticle Paper
Fürst, F.0000-0003-0388-0560
Pottschmidt, K.0000-0002-4656-6881
Miyasaka, H.0000-0002-8074-4186
Bhalerao, V.0000-0002-6112-7609
Bachetti, M.0000-0002-4576-9337
Boggs, S. E.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Grinberg, V.0000-0003-2538-0188
Harrison, F. A.0000-0003-2992-8024
Kennea, J. A.0000-0002-6745-4790
Rahoui, F.0000-0001-7655-4120
Stern, D.0000-0003-2686-9241
Tendulkar, S. P.0000-0003-2548-2926
Tomsick, J. A.0000-0001-5506-9855
Walton, D. J.0000-0001-5819-3552
Wilms, J.0000-0003-2065-5410
Zhang, W. W.0000-0002-1426-9698
Additional Information:© 2015 The American Astronomical Society. Received 2015 April 22; accepted 2015 May 9; published 2015 June 15. We thank the anonymous referee for valuable comments. This work was supported under NASA contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Non-Subject Keywords:accretion, accretion disks – radiation: dynamics – stars: neutron – X-rays: binaries – X-rays: individual (Cep X-4)
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2015-06
Issue or Number:2
Record Number:CaltechAUTHORS:20150615-060247495
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58227
Deposited By: Joy Painter
Deposited On:15 Jun 2015 14:51
Last Modified:10 Nov 2021 22:01

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