Circumstellar Matter in the Vela X-1/HD 77581 System

Abstract

Physical properties of the circumstellar matter in the X-ray binary system Vela X-1/HD 77581 are studied by observing a wide variety of the X-ray energy spectra of Vela X-1 (4U 0900–40) with the gas scintillation proportional counters aboard Tenma. The energies of the fluorescence Kα emission line and the K-shell absorption edge of iron are found to be 6.42 keV and 7.24 keV, respectively with an accuracy of 0.05 keV. The absorption edge indicates the ionization states of iron to be between Fe iv and Fe ix, and the absorption depth in comparison with the low energy absorption measure gives the iron abundance to be 3.8 × 10−5 times the hydrogen abundance. The excess of soft X-ray flux which appears temporarily on a time scale of hours is accounted for in terms of the inhomogeneous absorption by the clumped matter. The equivalent width of the fluorescence iron line is relatively large when the inhomogeneity of the surrounding matter increases. The ionization structure of iron and the site of the clumped intervening matter are discussed with reference to the photoionization of stellar wind.