X-Ray Spectroscopy of Cen X-3 with RXTE

Abstract

The high mass X-ray binary pulsar Centaurus X-3 was observed outside of eclipse by RXTE between September 10 and September 27 1996, with pointings during five different binary orbits. We find a broad dip between 20 and 30 keV in both the PCA and HEXTE spectra. We interpret this as a cyclotron scattering resonance feature.
    

Introduction

Centaurus X-3 is an X-ray binary pulsar with a pulse period of 4.8 s. The neutron star orbits its O-type companion every 2.09 days. Gradual X-Ray eclipses occur between orbital phases -0.2 and 0.2 (with phase 0 at mid-eclipse). The X-ray spectrum of Cen X-3 in the 1 to 10 keV range can be modeled by a power law with iron K emission and interstellar absorption. The fluorescent iron emission feature between 6 and 7 keV has been found to pulsate with an amplitude ~ 50% of the mean intensity (Day et al. 1993). The iron line has been resolved into components at 6.4, 6.7, and 6.9 keV (Audley et al. 1996; Ebisawa et al. 1996). The 6.4 keV line is due to fluorescence of iron in low ionization stages while the others originate in the extended stellar wind of the companion (Nagase et al. 1992).
     Above about 10 keV the spectrum falls off faster. This high energy roll off may be approximated by the factor exp(-[(E-E_co)/(E_f)]) (White, Swank & Holt 1983) with a cut off energy, E_co of 10-15 keV and an e-folding energy E_f of ~ 8 keV. Nagase et al. (1992) obtained a better fit with a roll off of the form exp(-[(t(WE/E_c)²)/((E-E_c)²+W²)]). This form is similar to that of opacity due to cyclotron scattering where E_c is the cyclotron resonance energy, t is the optical depth, and W is the width of the resonance. They estimated a resonance energy of 30±2 keV from the shape of the roll off. This is at the upper end of Ginga's bandpass so the identification of the roll off with cyclotron scattering opacity is not conclusive.
    

The Cyclotron Scattering Resonance Feature (CSRF)

     The best-fit simple continuum model is a Sunyaev-Titarchuk Comptonization model. There is a broad shallow feature in both the HEXTE and PCA spectra. This can be fitted satisfactorily with a CSRF. The best-fit cyclotron line parameters are:

This implies a magnetic field of

Audley et al (1996) used the beat frequency mass accretion model of the 40 mHz quasi-periodic oscillations to predict a surface magnetic field of about 3×10¹² G. If the scattering region extends far above the neutron star surface the field derived from a CSRF would be lower than this. We have not found any significant intensity or pulse-phase dependence in the cyclotron line parameters.

References

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     Day, C. S. R., Nagase, F., Asai, K., & Takeshima, T. 1993, ApJ, 408, 656

     Ebisawa, K, Day, C. S. R., Kallman, T. R., Nagase, F., Kotani, T., Kawashima, K., Kitamoto, S., and Woo, J. W. 1996, PASJ, 48, 425

     Mihara, T. 1995, Ph.D. Dissertation, University of Tokyo

     Nagase, F., Corbet, R. H. D., Day, C. S. R., Inoue, H., Takeshima, T., Yoshida, K., & Mihara, T. 1992, ApJ, 396, 147

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