DISCOVERY OF A PROMINENT CYCLOTRON ABSORPTION FEATURE FROM THE TRANSIENT-X-RAY PULSAR X0331+53

A remarkable absorption feature at 28.5 keV, attributable to electron cyclotron resonance, has been discovered in the 1.9-60 keV X-ray spectrum of the recurrent transient X-ray pulsar X0331+53 (V0332+53). The observed resonance energy implies a neutron star surface magnetic field of 2.5(1 + z) × 1012 G, where z is the gravitational redshift. The detection was made with the Ginga observatory in 1989 October, during an outburst of this transient with a flux level of ∼ 0.3 Crab. The feature is very deep and has been resolved with excellent statistics. This is the fourth unambiguous detection of cyclotron resonant scattering features from X-ray pulsars, suggesting that these features are a common phenomenon among these objects. An empirical relation found between the cyclotron resonance energy and the spectral cutoff energy suggests that the magnetic field strengths of the known X-ray pulsars are clustered in a range (1-4) × 1012 G.