Bibcode
González-Martín, O.; Masegosa, J.; Márquez, I.; Guainazzi, M.
Bibliographical reference
The Astrophysical Journal, Volume 704, Issue 2, pp. 1570-1585 (2009).
Advertised on:
10
2009
Journal
Citations
69
Refereed citations
60
Description
In this paper, we study the nuclear obscuration of galaxies hosting low
ionization narrow emission regions (LINERs) based on their X-ray and
optical emission. They show column densities at soft energies (0.5-2
keV) mostly related to the diffuse emission around the active galactic
nucleus (AGN), showing a correlation with the optical extinction. Column
densities at hard energies (2-10 keV) seem to be much higher than what
would be expected from the optical extinction. They might be associated
with the inner regions of the AGN, buried at optical wavelengths. The
main result of this paper is that around 50% of our LINER sample shows
signatures of Compton-thickness according to the most common tracers:
the X-ray spectral index, F_{X}(2-10 keV)/F([O III]) ratio, and
FeKα equivalent width (EW). However, the EWs of the Compton-thick
LINERs are significantly lower than in the Compton-thick Seyferts
(sime200 eV against >=500 eV), suggesting that the 2-10 keV emission
is dominated by electron scattering of the otherwise invisible AGN, or
by emission from shocked gas associated with star formation rather than
by reflection from the inner wall of the torus. However, no clear
relation seems to exist between galaxies with optical dust lanes and
X-ray classified Compton-thick objects. This may suggest that
Compton-thick sources should be related to absorbing material located at
the very inner regions of the AGN, maybe in the putative dusty torus.
Larger black hole masses and lower Eddington ratios than Seyfert
galaxies have been found. This effect can be better attributed to LINER
nuclei being hosted by earlier morphological types than Seyfert nuclei.
However, it has to be noted that, once a proper correction to the X-ray
luminosity is applied, LINERs show Eddington ratios overlapping those of
type 2 Seyferts. We speculate with a possible scenario for LINER nuclei:
an inner obscuring matter similar to that of type 2 Seyfert, and an
external obscuring matter responsible for the optical extinction.
Compton-thick sources appear to be more common among LINERs than
Seyferts.