TheBeppoSAX0.1–100 keV Spectrum of the X‐Ray Pulsar 4U 1538−52

We report the results of temporal and spectral analysis performed on theX-ray pulsar 4U 1538-52 observed by BeppoSAX. We obtained a new estimate of thespin period of the neutron star P=528.24 \pm 0.01 s (corrected for the orbitalmotion of the X-ray source): the source is still in the spin-up state, as since1988. The pulse profile is double peaked, although significant variations ofthe relative intensity of the peaks with energy are present. The broad band(0.12-100 keV) out-of-eclipse spectrum is well described by an absorbed powerlaw modified by a high energy cutoff at \sim 16 keV (e-folding energy \sim 10keV) plus an iron emission line at \sim 6.4 keV. A cyclotron line at \sim 21keV is present. The width of the line is consistent with thermal Dopplerbroadening at the temperature of the exponential cutoff. We searched for thepresence of the second harmonic, previously reported for this source. We foundno evidence of lines at \sim 42 keV, although an absorption feature at 51 keVseems to be present (at 99% confidence level). A soft excess, modelled by ablackbody with a temperature of \sim 0.08 keV could be present, probablyemitted by the matter at the magnetosphere. We also performed a spectralanalysis during the X-ray eclipse. The spectral evolution during the eclipsecan be well described by a progressive covering of the primary Comptonizationspectrum that is scattered into the line of sight. During the deep eclipse thisspectrum also softens, suggesting that the dust-scattered component becomesimportant. An alternative, more complex model, with an emission iron line andscattered components (as the one that has been used to fit the eclipse ofCentaurus X-3), also gives a good fit of the deep-eclipse data.Comment: 12 pages, including 6 figures. To appear in Ap