Cyclotron absorption features observed in an X-ray pulsar

Observations of X-ray pulsars with the GINGA Large Area Counter (LAC) have been exploring a new frontier in the understanding of X-ray pulsar spectra. Amongst them, the GINGA observation of the binary X-ray pulsar 4U 1538-52 revealed a noticeable feature in its energy spectra near 20 keV, which was strongly pulse-phase-dependent. We regard this structure as cyclotron absorption caused by electrons in an intense magnetic field of the pulsar. In this case, the magnetic field strength near the surface of the neutron star in 4U 1538-52 is estimated to be 2 x 1012 Gauss. In this report, we show the results of spectral analysis of 4U 1538-52, especially concentrating on the cyclotron structure. (author)