The 0.1–100 keV Spectrum of LMC X‐4 in the High State: Evidence for a High‐Energy Cyclotron Absorption Line

Abstract

We report the spectral analysis of the X-ray pulsar LMC X-4 in its high state, out of eclipse, observed by BeppoSAX. During this observation no coherent pulsations are detected. The primary continuum is well described by a power 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 improves 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 star is at the spin equilibrium, as has been proposed for this source. The remaining excess at low energies can be fitted by a Comptonization of soft photons by moderately hot electrons (kT similar to0.9 keV) with an optical depth tau similar to 16. The seed photons for this Comptonization are consistent with blackbody emission from the accretion disk at the magnetospheric radius. Another possibility is to fit the soft excess with blackbody and thermal bremsstrahlung. In this case, the blackbody would originate from cold plasma at the magnetosphere while the bremsstrahlung component would be produced by the strong stellar wind from the companion star, ionized by the X-ray emission from the pulsar.