U 0115+63

W. A. Heindl and W. Coburn, University of California, San Diego, report: "The current outburst of the 3.6-s x-ray pulsar 4U 0115+63 (IAUC 7116) has been monitored with daily RXTE pointings since Mar. 3. The flux (2-10 keV) has increased from 5.6 x 10E-9 (Mar. 3) to 1.1 x 10E-8 erg cmE-2 sE-1 on Mar. 12. Fitting the Mar. 3 joint PCA/HEXTE spectrum reveals cyclotron absorption features at about 14 and 21 keV. Because the phase-averaged spectra are complex due to pulse-phase spectral variability, these line energies are not well-determined. A cyclotron absorption feature was first seen in this source with HEAO 1 (Wheaton et al. 1979, Nature 282, 240). In addition to the short monitoring observations, we have made two long pointings on Mar. 7.16-7.45 and 11.89-12.32 UT. The second of these spanned periastron passage (according to the ephemeris of Bildsten et al. 1997, Ap.J. Suppl. 113, 367) at Mar. 11.95. During this observation, the source exhibited large oscillations with a timescale of about 500 s. The oscillation rms deviations were about 30 percent of the mean flux. Such slow oscillations are not consistent with the beat-frequency QPO model whose expected frequencies are of order 5 Hz. The oscillations may have been present during 2500 s of the Mar. 7 observation, but at greatly reduced amplitude. Additional observations are encouraged. The RXTE/ASM real-time light curve is available at http://heasarc.gsfc.nasa.gov/xte_weather/."