No anticorrelation between cyclotron line energy and X-ray flux in 4U 0115+634

¹ Dr. Karl Remeis-Observatory & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: Sebastian.Mueller@sternwarte.uni-erlangen.de
² ISDC Data Center for Astrophysics, University of Geneva, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ School of Physics, Astronomy, and Computational Sciences, MS 5C3, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁵ Space Science Division, Naval Research Laboratory, Code 7655, 4555 Overlook Ave., S.W., Washington, DC 20375, USA
⁶ CEA Saclay, DSM/IRFU/SAp-UMR AIM (7158) CNRS/CEA/Université Paris 7, Diderot, 91191 Gif-sur-Yvette, France
⁷ CRESST and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
⁸ Space Radiation Laboratory, 290-17 Cahill, Caltech, 1200 East California Blvd., Pasadena, CA 91125, USA
⁹ Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
¹⁰ Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías, University of Alicante, PO Box 99, 03080 Alicante, Spain
¹¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany

Received: 10 September 2012
Accepted: 23 November 2012

Abstract

We report on an outburst of the high mass X-ray binary 4U 0115+634 with a pulse period of 3.6 s in 2008 March/April as observed with RXTE and INTEGRAL. During the outburst the neutron star’s luminosity varied by a factor of 10 in the 3–50 keV band. In agreement with earlier work we find evidence of five cyclotron resonance scattering features at  ~10.7, 21.8, 35.5, 46.7, and 59.7 keV. Previous work had found an anticorrelation between the fundamental cyclotron line energy and the X-ray flux. We show that this apparent anticorrelation is probably due to the unphysical interplay of parameters of the cyclotron line with the continuum models used previously, e.g., the negative and positive exponent power law (NPEX). For this model, we show that cyclotron line modeling erroneously leads to describing part of the exponential cutoff and the continuum variability, and not the cyclotron lines. When the X-ray continuum is modeled with a simple exponentially cutoff power law modified by a Gaussian emission feature around 10 keV, the correlation between the line energy and the flux vanishes, and the line parameters remain virtually constant over the outburst. We therefore conclude that the previously reported anticorrelation is an artifact of the assumptions adopted in the modeling of the continuum.