BeppoSAX Observation of 4U 1907+09: Detection of a Cyclotron Line and its Second Harmonic

We report the detection of a cyclotron absorption line and its second harmonic in the average spectrum of the high mass X-ray binary 4U1907+09 observed by the BeppoSAX satellite. The broad band spectral capability of BeppoSAX allowed a good determination of the continuum against which the two absorption features are evident at ~ 19 and ~ 39 keV. Correcting for the gravitational redshift of a ~ 1.4 Msolar neutron star, the inferred surface magnetic field strength is B_surf = 2.1 × 10¹² G