Phase dependence of the spectral properties of the X-ray binary 4U0115+63: Observations with the Beppo-Sax satellite

The phase dependence of the spectral properties of the X-ray source 4U0115+63 is analzed based on X-ray observations obtained with the Beppo-Sax observatory. The spectrum contains a feature that is interpreted as a cyclotron absorption line. Deep lines of the first three orders were always observed, while the fourth-order line was detected only at certain phases. The parameters of the cyclotron feature depend strongly on the phase, while the remaining spectrum varies only relatively weakly with phase. The cyclotron lines of various orders are appreciably non-equidistant, and the second-order line is nearly always deeper than the first-order line. Possible physical conditions in the active region of the source that could give rise to the observed spectrum are discussed.