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(-2)(+4) x 10(22) H cm(-2), with a photon index Gamma = 1.5(-0.6)(+0.7). The 2-10 keV flux was about 5 x 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.