CaltechAUTHORS
Published March 2013 | Version Published
Journal Article Open

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

Creators

  • 1. ROR icon University of Erlangen-Nuremberg
  • 2. ROR icon University of Geneva
  • 3. ROR icon Leibniz Institute for Astrophysics Potsdam
  • 4. ROR icon Computational Sciences (United States)
  • 5. ROR icon United States Naval Research Laboratory
  • 6. ROR icon CEA Saclay
  • 7. ROR icon Goddard Space Flight Center
  • 8. ROR icon University of Maryland, Baltimore County
  • 9. ROR icon California Institute of Technology
  • 10. ROR icon University of California, San Diego
  • 11. ROR icon University of Alicante
  • 12. ROR icon University of Tübingen

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.

Additional Information

© 2013 ESO. Article published by EDP Sciences. Received 10 September 2012. Accepted 23 November 2012. Published online 08 February 2013. We thank the referee for his/her insightful comments. We also thank the schedulers of RXTE and INTEGRAL for their role in making this campaign possible, and the International Space Science Institute in Bern, Switzerland, for their hospitality. We acknowledge funding by the Bundesministerium für Wirtschaft und Technologie under Deutsches Zentrum für Luft- und Raumfahrt grants 50OR0808, 50OR0905, and 50OR1113, and by the Deutscher Akademischer Austauschdienst. M.T.W. is supported by the US Office of Naval Research. IC acknowledges financial support from the French Space Agency CNES through CNRS. S.M.N. and J.M.T. 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. This research is in part based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic, and Poland, and with the participation of Russia and USA. We thank John E. Davis for the development of the SLxfig module, which was used to create all figures in the paper.

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Additional details

Identifiers

Eprint ID
38315
Resolver ID
CaltechAUTHORS:20130507-095614833

Funding

Bundesministerium für Wirtschaft und Technologie (BMWi)
50OR0808
Bundesministerium für Wirtschaft und Technologie (BMWi)
50OR0905
Bundesministerium für Wirtschaft und Technologie (BMWi)
50OR1113
Deutscher Akademischer Austauschdienst (DAAD)
Office of Naval Research (ONR)
Centre National d'Études Spatiales (CNES)
Centre National de la Recherche Scientifique (CNRS)
Ministerio de Ciencia, Tecnología e Innovación (MCINN)
AYA2010-15431
Ministerio de Economia y Competitividad (MEC)
AIB2010DE-00057
ESA Member States

Dates

Created
2013-05-07
Created from EPrint's datestamp field
Updated
2021-11-09
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