Discovery of a Cyclotron Resonance Scattering Feature in the X-Ray Spectrum of XTE J1946+274

Abstract

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+0.5-0.7 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+0.07-0.06, and width, σ=3.37+0.92-0.75 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.