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Iron K line Variability in the Low-Luminosity AGN NGC 4579
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To appear in The Astrophysical Journal (Letters).
Iron K line Variability in the Low-Luminosity AGN NGC 4579
Yuichi Terashima1, Luis C. Ho2, Andrew F. Ptak3, Tahir Yaqoob1,4, Hideyo Kunieda5, Kazutami
Misaki5, and Peter J. Serlemitsos1
ABSTRACT
We present results of new ASCA observations of the low-luminosity AGN (LLAGN)
NGC 4579 obtained on 1998 December 18 and 28, and we report on detection of vari-
ability of an iron K emission line. The X-ray luminosities in the 2–10 keV band for the
two observations are nearly identical (LX ≈ 2×10
41 ergs s?1), but they are ～35% larger
than that measured in 1995 July by Terashima et al. An Fe K emission line is detected
at 6.39± 0.09 keV (source rest frame) which is lower than the line energy 6.73+0.13
?0.12 keV
in the 1995 observation. If we fit the Fe lines with a blend of two Gaussians centered at
6.39 keV and 6.73 keV, the intensity of the 6.7 keV line decreases, while the intensity of
the 6.4 keV line increases, within an interval of 3.5 yr. This variability rules out thermal
plasmas in the host galaxy as the origin of the ionized Fe line in this LLAGN. The detec-
tion and variability of the 6.4 keV line indicates that cold matter subtends a large solid
angle viewed from the nucleus and that it is located within ～ 1 pc from the nucleus. It
could be identified with an optically thick standard accretion disk. If this is the case, a
standard accretion disk is present at the Eddington ratio of LBol/LEddington ～ 2×10
?3.
A broad disk-line profile is not clearly seen and the structure of the innermost part of
accretion disk remains unclear.
Subject headings: galaxies: active — galaxies: individual(NGC 4579) — galaxies: nuclei
— galaxies: Seyfert — X-rays: galaxies
1. Introduction
Fe line emission in AGNs is an X-ray spectral feature produced through reprocessing by the
matter surrounding the nucleus, and it can be used as a probe of the matter in the vicinity o
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