A BeppoSAX study of the pulsating transient X0115+63:: The first X-ray spectrum with four cyclotron harmonic features

The recurrent hard pulsating X-ray transient X0115+63 was observed with BeppoSAX on 1999 March 19, when the source was at a 2-10 keV flux level of similar to 310 mcrab. We report on the high-energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent, and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74, 24.16, 35.74, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1 : (1.9) : (2.8) : (3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for four harmonically spaced lines in the spectrum of an accreting X-ray pulsar, yielding the dearest confirmation to date of their magnetic origin.