Chandra observations and optical identification of hard X-ray sources discovered with ASCA

Shin Watanabe, Masayuki Akiyama, Yoshihiro Ueda, Kouji Ohta, Richard Mushotzky, Tadayuki Takahashi, Toru Yamada

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6 Citations (Scopus)


We present the first results of Chandra and optical follow-up observations of hard X-ray sources detected in the ASCA Medium Sensitivity Survey (AMSS). Optical identifications were made for five objects. Three of them show either weak or absent optical narrow emission lines, and are at low redshift 〈z〉 ∼ 0.06. One of them is a broad line object at z = 0.910 and one is a z = 0.460 object with only narrow lines. All of the narrow-line objects show strong evidence of absorption in their X-ray spectra. Their line ratios are consistent with a Seyfert II/LINER identification, as are the line widths. The three low-redshift objects have the colors of normal galaxies, and apparently the light is dominated by stars. This could be due to extinction of the underlying nuclear continuum by the same matter that absorbs X-rays and/or due to dilution of the central source by starlight. These results suggest that X-ray sources that appear as "normal" galaxies in optical and near-IR bands significantly contribute to the hard X-ray background. This population of objects has a high space density, and probably dominates the entire population of active galaxies.

Original languageEnglish
Pages (from-to)683-694
Number of pages12
JournalPublication of the Astronomical Society of Japan
Issue number5
Publication statusPublished - 2002


  • Galaxies: active
  • Surveys
  • X-rays: galaxies
  • X-rays: general


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