DISCOVERY OF A CYCLOTRON-RESONANCE FEATURE AT 30 KEV FROM THE TRANSIENT-X-RAY PULSAR CEPHEUS X-4

From Ginga observations of the transient X-ray pulsar Cep X-4, a spectral line feature attributable to electron cyclotron resonance was discovered at about 30 keV in the 1.2-37 keV X-ray spectrum. The detection, a fifth firm example of cyclotron resonance from X-ray pulsars, implies a surface magnetic field of about 2.6 x 10(12)(1 + z) G for this pulsar, where z is the gravitational redshift. Throughout the 66.25 s pulses, the cyclotron feature appears in absorption, with at most +/- 5% variation in the resonance center energy around the mean value of 30.5 +/- 0.4 keV. The resonance profile depends significantly on the pulse phase in such a way that it is deepest on the decay slope of the leading peak of the double-peaked pulse profile.