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 keV, which implies a magnetic field strength of 3.1(1 + z) × 1012 G, where z is the gravitational redshift of the scattering region. The optical depth, τ = 0.33, and width, σ = 3.37 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.