Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-Ray Spectrum of 4U 0115+63
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Heindl, W. A., W. Coburn, D. E. Gruber, M. R. Pelling, R. E. Rothschild, J. Wilms, K. Pottschmidt, and R. Staubert. “Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-Ray Spectrum of 4U 0115+63.” The Astrophysical Journal 521, no. 1 (July 6, 1999): L49. https://doi.org/10.1086/312172.
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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 2 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.