The Hardest X-Ray Source in the ASCA Large Sky Survey: Discovery of a New Type 2 Seyfert

We present results of the ASCA deep-exposure observations of the hardest X-ray source discovered in the ASCA Large Sky Survey (LSS) project, designated as AX J131501+3141. We extract its accurate X-ray spectrum, taking into account the contamination from a nearby soft source (AX J131502+3142) separated from it by only 1'. AX J131501+3141 exhibits a large absorption of N H =6+ 4−2×1022 H cm-2, with a photon index Γ=1.5−0.6+0.7. The 2-10 keV flux was about 5 × 10-13 ergs cm-2 s-1, and was time variable by a factor of 30% in 0.5 yr. From the highly absorbed X-ray spectrum and the time variability, as well as the results of the optical follow-up observations, 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 our understanding of the origin of the cosmic X-ray background.