Broadening of Nearly Neutral Iron Emission Line of GX 301−2 Observed withASCA

Abstract

We present results from detailed analyses of the ASCA data of the X-ray pulsar GX 301-2 during apastron, periastron, and intermediate orbital phases. We find that the iron Kα emission line possesses a nonzero energy width of 40-80 eV in Gaussian σ during all of the three orbital phases. The observed range in the width is far larger than that expected from velocity shifts in the stellar wind. Quantitative evaluation of the width indicates that the iron Kα line of GX 301-2 is emitted from a region within ~1010 cm from the neutron star via fluorescence. In addition, we have also discovered a broad wing with σ 135 eV in the profile of the iron Kα line during the periastron phase. Since the wing profile is modulated according to the spin phase of the pulsar, we conclude that the wing is composed of blue- and redshifted 6.4 keV iron lines emanating from the bipolar accretion columns. We find that, within the accretion column, the He II ionization front is formed at ~107 cm from the pulsar, outside of which iron can be kept in a nearly neutral ionization state. The observed equivalent width of the broad wing component (~100 eV) can be explained by emission in the accretion columns outside the He II ionization front.