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Published October 2012 | Published
Book Section - Chapter Open

A constant Cyclotron Line Energy in 4U 0115+634

Abstract

We present a study of RXTE and INTEGRAL spectra of the transient 3.6 s X-ray pulsar 4U 0115+634 taken during a giant outburst in 2008 March/April. The spectra can be almost equally well modeled by two different semi-empirical continuum models, modified by an Fe Kα fluorescence line, interstellar absorption, and cyclotron resonance scattering features (CRSFs) located at ∼10.7, 21.8, 35.5, 46.7, and 59.7 keV. One of these two models, the so called NPEX model, leads to an anticorrelation between the centroid energy of the fundamental CRSF E_0 and the X-ray flux F_X, in agreement with previous works. The other model, consisting of a simple exponentially cutoff power law modified by a Gaussian emission feature around 10 keV, however, leads to a constant value for E_0 for the observed fluxes and a comparatively narrow line shape. We show that the cyclotron line model component resulting from the NPEX fits rather contribute to the broadband continuum model. We conclude that the previously reported anticorrelation is probably due to an artifact of the particular modeling of the continuum.

Additional Information

Copyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-ShareAlike. This research has been funded by the Bundesministerium für Wirtschaft und Technologie under Deutsches Zentrum für Luft- und Raumfahrt grants 50OR0808, 50OR0905, 50OR1113, and 50OR1007, and by the Deutscher Akademischer Austauschdienst. MTW is supported by the US Office of Naval Research. IC acknowledges financial support from the French Space Agency CNES through CNRS. SMN and JMT acknowledge support from the Spanish Ministerio de Ciencia, Tecnología e Innovación (MCINN) through grant AYA2010-15431 and the use of the computer facilities made available through the grant AIB2010DE-00057.

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