The hardest X-ray source discovered in the ASCA Large Sky Survey: Implications to the cosmic X-ray background Original Research Article

We present results of ASCA deep exposure observations and optical identifications of the hardest X-ray source discovered in the ASCA Large Sky Survey project, designated as AX J131501+3141. AX J131501+3141 exhibits a large X-ray absorption of NH = (6−2+4) × 1026H m−2. The observed X-ray flux was time variable by a factor of 30% in 0.5 year: FX (2–10keV) = (4.8−6.2)×10−16J m−2 s−1. From optical photometric and spectroscopic observations, we found one galaxy with R = 15.6mag in the X-ray error circle, whose emission line ratios are similar to those found in type 2 Seyfert galaxies. Its redshift is determined to be 0.07, hence the absorption-corrected X-ray luminosity is LX ∼ 2 × 1036J s−1 (2–10 keV). Accordingly, we conclude that AX J131501+3141 is a type 2 Seyfert galaxy. Discovery of such a low flux and highly absorbed X-ray source could have a significant impact on the origin of the cosmic X-ray background.