The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE

Minimizing cyclotron line biases

¹ Dr. Karl Remeis-Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: matthias.kuehnel@sternwarte.uni-erlangen.de
² ISDC Data Center for Astrophysics, University of Geneva, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21250, USA
⁴ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁵ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, Villanueva de la Cañada, Madrid, Spain
⁶ Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁸ Massachusetts Institute of Technology, Kavli Institute for Astrophysics, Cambridge, MA 02139, USA
⁹ Department of Physics, Makerere University, PO Box 7062, Kampala, Uganda
¹⁰ Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, 39005 Santander, Spain
¹¹ Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías, University of Alicante, PO Box 99, 03080 Alicante, Spain
¹² Department of Physics and Astronomy, George Mason University, Fairfax, VA, USA
¹³ Space Science Division, Naval Research Laboratory, Washington DC 20375, USA

Received: 11 April 2019
Accepted: 13 December 2019

Abstract

We present an analysis of X-ray spectra of the high-mass X-ray binary 4U 0115+634 as observed with Suzaku and RXTE in 2011 July, during the fading phase of a giant X-ray outburst. We used a continuum model consisting of an absorbed cutoff power law and an ad hoc Gaussian emission feature centered around 8.5 keV, which we attribute to cyclotron emission. Our results are consistent with a fundamental cyclotron absorption line centered at ∼10.2 keV for all observed flux ranges. At the same time we rule out significant influence of the 8.5 kev Gaussian on the parameters of the cyclotron resonant scattering feature, which are not consistent with the cyclotron line energies or the depths of previously reported flux-dependent descriptions. We also show that some continuum models can lead to artificial line-like residuals in the analyzed spectra, which are then misinterpreted as unphysically strong cyclotron lines. Specifically, our results do not support the existence of a previously claimed additional cyclotron feature at ∼15 keV. Apart from these features, we find for the first time evidence for a He-like Fe XXV emission line at ∼6.7 keV and weak H-like Fe XXVI emission close to ∼7.0 keV.