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W.A. Heindl, W. Coburn, D.E. Gruber, M.R. Pelling, R.E. Rothschild (UCSD), P. Kretschmar, I. Kreykenbohm, K. Pottschmidt, R. Staubert, J. Wilms (IAAT)
We made four long RXTE observations of the transient Be X-ray binary pulsar 4U 0115+63 during its 1999 outburst. Analysis of one of these observations revealed as many as 5 cyclotron line harmonics in the range 13 -- 62 keV which vary in energy as a function of pulse phase. The presence of multiple harmonics is problematic for models of the line formation, as it is expected that the cross sections for the high harmonics should be rapidly falling. Furthermore, the CRSFs are apparently not harmonically spaced as expected in the simplest theory (although this may be due to inadequate fitting of the complex shape of the fundamental). We report the phase-resolved spectral analyses of all four RXTE observations. These multiple observations allow us to search for secular changes in the line parameters through the outburst. Furthermore, we compare the observed spectra to Monte-Carlo models of cyclotron line formation at the neutron star polar cap.