Bibcode
Hernández-García, L.; González-Martín, O.; Masegosa, J.; Márquez, I.
Bibliographical reference
Astronomy and Astrophysics, Volume 569, id.A26, 46 pp.
Advertised on:
9
2014
Journal
Citations
29
Refereed citations
28
Description
Context. Variability is a general property of active galactic nuclei
(AGN). The way in which these changes occur at X-rays is not yet clearly
understood. In the particular case of low-ionization nuclear emission
line region (LINER) nuclei, variations on the timescales from months to
years have been found for some objects, but the main driver of these
changes is still debated. Aims: The main purpose of this work is
to investigate the X-ray variability in LINERs, including the main
driver of these variations, and to search for possible differences
between type 1 and 2 objects. Methods: We examined the 18 LINERs
in the Palomar sample with data retrieved from the Chandra and/or
XMM-Newton archives that correspond to observations gathered at
different epochs. All the spectra for the same object were fitted
simultaneously to study long-term variations. The nature of the
variability patterns were studied by allowing different parameters to
vary during the spectral fit. Whenever possible, short-term variations
from the analysis of the light curves and long-term UV variability were
studied. Results: Short-term variations are not reported in
X-rays. Three LINERs are classified as non-AGN candidates in X-rays, all
of them are Compton-thick candidates; none of them show variations at
these frequencies, and two of them vary in the UV. Long-term X-ray
variations were analyzed in 12 out of 15 AGN candidates; about half of
them showed variability (7 out of the 12). At UV frequencies, most of
the AGN candidates with available data are variable (five out of six).
Thus, 13 AGN candidates are analyzed at UV and/or X-rays, ten of which
are variable at least in one energy band. None of the three objects that
do not vary in X-rays have available UV data. This means that
variability on long-timescales is very common in LINERs. These X-ray
variations are mainly driven by changes in the nuclear power, while
changes in absorptions are found only for NGC 1052. We do not find any
difference between type 1 and 2 LINERs, neither in the number of
variable cases (three out of five type 1 and four out of seven type 2
LINERs), nor in the nature of the variability pattern. We find
indications of an anticorrelation between the slope of the power law,
Γ, and the Eddington ratio. Conclusions: LINERs are
definitely variable sources irrespective of whether they are classified
as optical type 1 or 2. Their BH masses, accretion rates, and
variability timescales place them in the same plane as more powerful AGN
at X-rays. However, our results suggest that the accretion mechanism in
LINERs may be different. UV variations of some type 2 LINERs were found,
this could support the hypothesis of a torus that disappears at low
luminosities.
Appendices are available in electronic form at http://www.aanda.org
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