ASCA observations of two ultraluminous IRAS galaxies: IRAS 15307+3252 and IRAS 20460+1925

We report on ASCA observations of two ultraluminous far-infrared galaxies, IRAS 15307+3252 and IRAS 20460+1925. An X-ray flux of 1.53 × 10^-12 erg cm^-2 s^-1 in the 2-10 keV band was detected from IRAS 20460+1925. The continuous spectrum is well represented by a power law with a photon index of 1.99^+0.18_-0.15 and excess photoelectric absorption by a column of (2.6^+0.29_-0.32) × 10²² cm^-2 in the rest frame. The spectrum also shows an iron K emission line with an equivalent width of 260⁺¹⁴⁵_-137 eV and a centroid energy of 6.36±0.08 keV in the rest frame. These spectral properties are consistent with those of Seyfert-2 galaxies, although the intrinsic 2-10 keV X-ray luminosity (corrected for absorption) of 1.20 × 10⁴⁴ erg s^-1 is high and close to that of quasars. No X-rays were detected from IRAS 15307+3252, and the upper limit to the X-ray flux is 7.2 × 10^-14 erg cm^-2 s^-1 in the 2-10 keV band. However, the upper limit on the ratio of the X-ray flux to the far-infrared flux is not inconsistent with a source containing a highly obscured quasar.