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Wayne Coburn (CASS/UCSD)
Neutron stars exhibit some of the strongest magnetic fields known in the universe, more than 1012 Gauss (108 Tesla). If a neutron star with such a field is accreting, then the infalling material is channeled to the magnetic polar axes. Any offset in the magnetic and spin axes gives rise to X-ray pulsations. The transverse energies of the electrons trapped in these fields are quantized in Landau levels, and by resonant scattering these electrons can create harmonically spaced absorption-like spectral features (or Cyclotron Resonant Scattering Features, CRSFs) in the 2--200\,keV range. The central energy of a CRSF is given by Ec = 11.6 B12 (1 + z)-1, where z is the gravitational redshift and B12 the magnetic field in the scattering region in units of 1012\,G. Thus these features give a direct measure of the pulsar magnetic field. Additionally, the depth, profile, and harmonic content of these features are probes of the scattering geometry and X-ray emission processes at the pulsar polar caps. With its large area and broad energy range, \emph{RXTE} is an outstanding platform for the study of CRSFs. \emph{RXTE} observations have revealed new CRSFs in the well known pulsars Cen~X-3, 4U~1626-67, and 4U~0352+309, along with the first observation of more than 2 harmonics in a single source -- in the Be-binary 4U~0115+63. In addition to these sources I have studied the known CRSFs in Her~X-1, Vela~X-1, 4U~1538-52, 4U~1907+09, and GX~301-2. I will report the observed CRSF properties of several sources, including energies, widths, depths, and behavior with pulse phase. With 9 objects observed in detail, I will also discuss some class generalizations and their implications for the conditions at the pulsar magnetic polar caps. Lastly, I will present results on the searches for CRSFs in the X-ray spectra of other accreting pulsars, including null results and upper limits.