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Distorted Cyclotron Line Profile in Cep X-4 as Observed by NuSTAR

Journal
Date Issued
2015
Author(s)
Fürst, F.
•
Pottschmidt, K.
•
Miyasaka, H.
•
Bhalerao, V.
•
•
Boggs, S. E.
•
Christensen, F. E.
•
Craig, W. W.
•
Grinberg, V.
•
Hailey, C. J.
•
Harrison, F. A.
•
Kennea, J. A.
•
Rahoui, F.
•
Stern, D.
•
Tendulkar, S. P.
•
Tomsick, J. A.
•
Walton, D. J.
•
Wilms, J.
•
Zhang, W. W.
DOI
Description
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).
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.
Volume
806
Issue
2
Start page
L24
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2015ApJ...806L..24F
Rights
open.access
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