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[22.06] BeppoSAX Observation of LMC X-4: Evidence for a High Energy Cyclotron Line

A. La Barbera (Dipartimento di Scienze Fisiche ed astronomiche di Palermo), L. Burderi (Osservatorio Astronomico di Roma), T. Di Salvo, R. Iaria, N. Robba (Dipartimento di Scienze Fisiche ed astronomiche di Palermo)

We report the spectral analysis of the X-ray pulsar LMC X--4 in its high state observed by BeppoSAX. During this observation no coherent pulsations are detected. The spectrum of LMC--4 is complex. The primary continuum is well described by a power law with a high energy cutoff (E_{\rm cutoff} ~E_{\rm fold} ~18 keV). The addition of a cyclotron absorption line at ~100 keV improves the fit significantly. The inferred magnetic moment is ~1.1 \times 10³¹ Gauss cm³, in agreement whit 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 ~0.9), with an optical depth \tau ~16. The seed photons for this Comptonization are consistent with black body emission from the accretion disk at the magnetospheric radius.

The author(s) of this abstract have provided an email address for comments about the abstract: nino@gifco.fisica.unipa.it