The two-component X-ray broadband spectrum of X Persei observed by BeppoSAX

We report temporal and broadband (0.1-200 keV) spectral analysis of the Be/X-ray binary X Persei observed by the Narrow Field Instruments (NFI) on board the BeppoSAX satellite. The source luminosity is similar to 1.2 x 10(34) ergs s(-1) in the energy range 0.1-10 keV and similar to 2.4 x 10(34) ergs s(-1) in the range 0.1-200 keV. The source shows pulsations from 0.1 keV up to 80 keV. No variations of the pulse profile with energy are visible. The barycentric pulse period is 837.376 +/- 0.026 s, in agreement with the secular spin-down observed since 1978. The 0.1-10 keV energy spectrum can be well fitted by a power law plus high-energy cutoff, in agreement with previous observations, although at higher energies a hard excess is visible. The whole energy range spectrum can be fitted by adding to the previous model another power law with a high-energy cutoff and a low-energy turnover. A possible interpretation of this two-component spectrum could be given in terms of a high-energy component produced by a nonthermal and partially Comptonized cyclotron emission, and a soft component produced by radiation partially thermalized by the atmosphere. In this model, the second high-energy cutoff (similar to 65 keV) gives an estimate of the cyclotron energy and the magnetic field strength. The value obtained, B similar to 5.6 x 10(12) G, is in agreement with the estimate of the accretion torque theory. The spectrum can also be fitted by adding to the low-energy component a high-temperature thermal bremsstrahlung.