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 RossiX-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range showsCRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. Withcentroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, theCRSFs are not harmonically spaced. Strong variability of the continuum andCRSFs with pulse phase indicate a complex emission geometry near the neutronstar polar cap. In addition, one RXTE observation, which spanned periastronpassage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This isslower by two orders of magnitude than the beat-frequency QPO expected in thissystem and slower by a factor of more than 5 compared with other QPOs seen inaccreting X-ray pulsars.Comment: To appear in The Astrophysical Journal Letters. 4 pages, 5 figures. Uses "emulateapj.sty". Revised version includes new figures and additions to the analysi