Discovery of a cyclotron resonance scattering feature in the X-ray spectrum of XTE J1946+274

Observations made with the Rossi X-Ray Timing Explorer during the course of the 1998 September-November outburst of the transient accreting pulsar XTE J1946+274 reveal a cyclotron resonance scattering feature (or cyclotron line) in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2(-0.7)(+0.5) keV, which implies a magnetic field strength of 3.1(1 + z) x 10(12) G, where z is the gravitational redshift of the scattering region. The optical depth, tau = 0.33(-0.06)(+0.07), and width, sigma = 3.37(-0.75)(+0.92) keV, are typical of known cyclotron lines in other pulsars. This discovery makes XTE J1946+274 one of 13 pulsars with securely detected cyclotron lines resulting in direct magnetic field measurements.