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

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 N-H = (6(-2)(+4)) x 10(26) H m(-2). The observed X-ray flux was time variable by a factor of 30% in 0.5 year: F-X (2-10 keV) =(4.8-6.2) x 10(-16) J m(-2) s(-1). From optical photometric and spectroscopic observations, we found one galaxy with R = 15.6 mag 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 L-X similar to 2 x 10(36) J 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. (C) 2000 COSPAR. Published by Elsevier Science Ltd.