1999ApJ...520..316S

We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained with five pointing observations in a time span of 3.5 yr. The X-ray spectrum for each period is well fitted with a single power law absorbed by a high column of gas. The X-ray flux changes by a factor of 2 from period to period, but the other spectral parameters show no significant change. The photon index is flat with Γ=0.9-1.3. The column density of hydrogen N_H is ∼1.0x10²³ H.cm^–2 and that of iron N_Fe is ∼10¹⁹ Fe.cm^–2. These large column densities indicate that 1E 1740.7-2942 is near the Galactic center. The column density ratio leads the iron abundance to be 2 times larger than the other elements in a unit of the solar ratio. The equivalent width of the Kα line from a neutral iron is less than 15 eV in 90% confidence. This indicates that the iron column density within several parsecs from 1E 1740.7-2942 is less than 5x10¹⁷ Fe.cm^–2. In addition, the derived hydrogen column density is about one-sixth of that of giant molecular clouds in the line of sight. All these facts support the fact that 1E 1740.7-2942 is not in a molecular cloud, but possibly in front of it; the X-rays are not powered by accretion from a molecular cloud, but from a companion star like ordinary X-ray binaries.