Abstract
Iron lines in X-ray pulsars are characterized by an emission line at an energy of 6.4 keV and an absorption edge at 7.1 keV. The 6.4-keV emission line is attributed to the fluorescence Kα line due to reprocessed X-rays by cold circumstellar matter surrounding the binary system. Detailed features of iron lines have been measured so far mainly with the use of gas scintillation proportional counters on board the Tenma and EXOSAT satellites. The line properties are powerful tools to investigate the circumstellar matter in these binary systems, such as the accretion disk, the stellar wind, and the companion star atmosphere. Recent observations with Ginga revealed previously unknown complex features in the spectra of several X-ray pulsars, such as Cen X-3, Her X-1, and Vela X-1, especially in their low luminosity states. The results indicate that the temperature distribution and the ionization structure of the circumstellar matter are complicated and that at least two zones of cold matter and hot plasma are required to interpret the spectra observed in the low intensity states of Cen X-3 and Her X-1.
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