The broad iron K emission line in the Seyfert 2 galaxy IRAS 18325 - 5926

K. Iwasawa, A. C. Fabian, R. F. Mushotzky, W. N. Brandt, H. Awaki, H. Kunieda

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Abstract

We report the detection of a very broad iron Kα emission line in the ASCA X-ray spectrum of the Seyfert 2 galaxy IRAS 18325 - 5926. The line is strong, with an equivalent width EW=500-800 eV, and its profile is peaked at 6.9 keV and skewed down to 4 keV. The energy of the blue peak of the line is significantly higher than the rest-energy of 6.4 keV seen in many active galaxies, which is expected from an iron Ka line arising in cold matter. If the breadth and shift of the line energy are caused by Doppler and relativistic effects in a cold accretion disc about a black hole, then the disc has an intermediate inclination, i = 40°-50°. A plausible interpretation for the large equivalent width is a supersolar abundance of iron; this also accounts for the lack of a significant reflection hump. The line profile can also be explained by emission from highly ionized matter very close to the central object. However, the lack of a deep iron K absorption edge in the Ginga spectrum is inconsistent with this interpretation. We verify that fast X-ray variability on a time-scale of 104 s is common in this object, using ASCA and ROSAT observations which are partially simultaneous. A study of spectral variability using combined ASCA and ROSAT PSPC data reveals that the X-ray flux change occurred mainly above 1 keV, and suggests a stable, or at least less-variable, component below 1 keV. It should lie outside the nuclear obscuration region.

Original languageEnglish (US)
Pages (from-to)837-846
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Volume279
Issue number3
DOIs
StatePublished - 1996

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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