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Preprint C/99


Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-ray Spectrum of 4U 0115+63

W. A. Heindl (1), W. Coburn (1), D. E. Gruber (1), M. R. Pelling (1), R. E. Rothschild (1), J. Wilms (2), K. Pottschmidt (2), R. Staubert (2)

(1) Center for Astrophysics and Space Sciences, Code 0424, University of California, San Diego, La Jolla, CA 92093
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

1999, Astrophysical Journal Letters, 521, L49

Abstract. We have discovered a third harmonic cyclotron resonance scattering feature (CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi X-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows CRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. With centroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, the CRSFs are not harmonically spaced. Strong variability of the continuum and CRSFs with pulse phase indicate a complex emission geometry near the neutron star polar cap. In addition, one RXTE observation, which spanned periastron passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is slower by two orders of magnitude than the beat-frequency QPO expected in this system and slower by a factor of more than 5 compared with other QPOs seen in accreting X-ray pulsars.

Key words: stars: individual (4U 0115+63) - stars: neutron - X-rays: stars

Paper (69k gzip'ed Postscript including figures)


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