Discovery of a third harmonic cyclotron resonance scattering feature in the X-ray spectrum of 4U 0115+63

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(-0.35)(+0.65), 21.45(-0.38)(+0.25), and 33.56(-0.90)(+0.70) 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.