Detection of a cyclotron line and its second harmonic in 4U1907+09

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 on 1997 September 27 and 28. 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 solar mass neutron star, the inferred surface magnetic field strength is B = 2.1 10E12 G that is typical for this kind of systems. We also detected an iron emission line at ~ 6.4 keV, with an equivalent width of ~ 60 eV.