Optical identification of the hardest X-ray source in the ASCA large sky survey

Masayuki Akiyama, Kouji Ohta, Toru Yamada, Michael Eracleous, Jules P. Halpern, Nobunari Kashikawa, Masafumi Yagi, Wataru Kawasaki, Masaaki Sakano, Takeshi Tsuru, Yoshihiro Ueda, Tadayuki Takahashi

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


We report the optical identification of the hardest X-ray source (AX J131501 + 3141) detected in an unbiased wide-area survey in the 0.5-10 keV band, the ASCA Large Sky Survey (LSS). The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Γ = 1.5-0.6+0.7) with NH = 6-2+4 × 1022 cm-2. We have found a galaxy with R = 15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2 × 1043 ergs s-1 (2-10 keV) and MB = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured active galactic nucleus (AGN). The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

Original languageEnglish (US)
Pages (from-to)173-180
Number of pages8
JournalAstrophysical Journal
Issue number1 PART I
StatePublished - 1998

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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