Further studies of 1E 1740.7-2942 with ASCA

Abstract

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 years. 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 densities of hydrogen N_H is $\sim$ 1.0 x 10^{23} H cm^{-2} and that of iron N_{Fe} is $\sim$ 10^{19} Fe cm^{-2}. These large column densities indicate that 1E 1740.7-2942 is near at 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 5 x 10^{17} Fe cm^{-2}. In addition, the derived hydrogen column density is about 1/6 of that of giant molecular clouds in the line of sight. All these facts support 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.