The 0.1-100 keV spectrum of LMC X-4 in the high state: Evidence for a high-energy cyclotron absorption line

We report the spectral analysis of the X-ray pulsar LMC X-4 in its high sta te, out of eclipse, observed by BeppoSAX. During this observation no cohere nt pulsations are detected. The primary continuum is well described by a po wer law with a high-energy cutoff E-cutoff similar to E-fold similar to 18 keV. The addition of a cyclotron absorption line at similar to 100 keV impr oves the fit significantly. The inferred magnetic moment is 1.1 x 10(31) G cm(3), in agreement with the value estimated, assuming that the neutron sta r is at the spin equilibrium, as has been proposed for this source. The rem aining excess at low energies can be fitted by a Comptonization of soft pho tons by moderately hot electrons (kT similar to0.9 keV) with an optical dep th tau similar to 16. The seed photons for this Comptonization are consiste nt with blackbody emission from the accretion disk at the magnetospheric ra dius. Another possibility is to fit the soft excess with blackbody and ther mal bremsstrahlung. In this case, the blackbody would originate from cold p lasma at the magnetosphere while the bremsstrahlung component would be prod uced by the strong stellar wind from the companion star, ionized by the X-r ay emission from the pulsar.