NATURE OF THE X-RAY EMISSION IN LINERS
Omaira González Martín, Josefa Masegosa, Isabel Márquez, *Deborah Dultzin-Hacyan, Martín A. Guerrero
Instituto de Astrofísica de Andalucía (C.S.I.C.), Granada (SPAIN)
* Instituto de Astronomía, UNAM, México D.F. (MEXICO)
MOTIVATION
There is an ongoing strong debate on the origin of the energy source in LINERs. Two
alternatives have been explored: that the ionizing energy source is a low luminosity
AGN, or that this energy is of thermal origin from massive star formation and/or
from shock heating mechanisms resulting from the massive stars evolution. The
search of a compact X ray nucleus in LINERs is indeed one of the most convincing
evidences about their AGN nature. Chandra's excellent resolution allows an
investigation of the X-ray nuclear properties of these galaxies.
Figure 1.
Composite
Figure
11. Composite
colourmaps
mapsof
of the
the hard
hard
colour
and
soft
bands
forgalaxies
galaxies
and soft bands for
representativesof
of the
the four
four
representatives
cathegories:
cathegories:
NGC4203 (top
(top left),
left),
NGC4203
NGC4374 (top
(top right),
right),
NGC4374
NGC3077
(bottom
left)and
and
NGC3077 (bottom left)
NGC4696
(bottom
right).
NGC4696 (bottom right).
THE SAMPLE
We have searched in the Chandra Archive for all the galaxies from the compilation by Carrillo et al (1999). This
amounts to 65 out of 476 LINERs for which ACIS-S imaging data are available. In this paper we report the
preliminary results obtained for 52 of them. Previous detailed investigations have been reported for 21 galaxies
included in our sample. 12 of them present a compact nucleus and a hard (2-8 kev) power law X-ray spectrum,
which seems to be a clear hint on the AGN nature of these galaxies. For the remaining objects no clear nucleus
can be identified and they harbor instead a fair amount of discrete X-ray sources in and around the X-ray
diffuse emission. Here we report on the soft (0.3-2 kev) and hard (2-6 kev) data for the 52 systems paying special
attention to their morphologies and the contrast between the nuclear and the surrounding nebulosities.
1.Nuclear Morphology
We have investigated the nuclear morphology in the 2-6 kev band and have divided the sample into 4 categories
(see Fig1):
-A clearly identified nuclear source embedded in a very extended diffuse nebulosity . In this class we have
classified 30 out of the 52 galaxies,
objects.
galaxies, which represents 58% of the objects.
- Multinuclear objects have been found in 6 galaxies.
galaxies.
- An irregular gas distribution with quite a few number of compact discrete sources has been identified in 9
objects.
objects.
- Finally the remaining galaxies show mainly a large diffuse nebulosity.
nebulosity.
In some cases the reality of this pure diffuse nebulosities can be atributed to the small exposure times.
2.Haloes
We found that the contribution of the diffuse
radiation to the total luminosities is larger than
40% in most of the objects (39 for the high
energies and 43 for the soft band).
An investigation of the main contribution to the
diffuse radiation leads to the conclussion that such
contribution is not larger than 20%. Larger values
come mainly from the discrete compact sources in
the surroundings of the nuclear region.
Figure 2.
Figure
22.
Histogramsof
of the
the percentage
percentage of
of diffuse
diffuse radiation
radiation to
to the
the soft
soft
Histograms
and
hard
X-ray bands.
bands.
and hard X-ray
The nature of such sources needs to be
investigated. Previous studies (e.g. Eracleous et al
2003) point to Xray binaries and SNR in NGC
4736 and NGC 4579.
3. Computation of the X-Rays Luminosity, LX
In a first step, X-ray luminosities in the hard band have been calculated following Ho et
al(2001), who assume a power law with a spectral index of 1.8 for the spectral energy
distribution. Since most of our objects show a clear compact nuclear source, large departures
from a power law index of 1.8 are not expected (see Terashima 1999).
Nevertheless, this approach has been shown to be too simplistic in some cases ( i.e. NGC 3077,
Ott et al 2003). Therefore a carefull analysis of the SED has been performed in order to get
better estimations of the luminosities. A two component model has been formally fit to each
spectrum, using an absorbed Raymond-Smith model (to include a possible thermal
contribution) added to an absorbed Power-Law model.
In Table 1 the resulting parameters of the two models are given. Notice that too low values
for the spectral index are found for four galaxies (namely, NGC1808, NGC 2681, NGC4261
and NGC6240); nevertheless these cases have been shown to need the inclusion of additional
components. For 11 galaxies, no suitable fit is obtained including the power-law component.
For the remaining 19 galaxies, the spectral indices obtained range from 1.5 to 2.5, well within
the range shown by active galaxies. Our results indicate that Ho et al’s approximation to
estimate X-Ray luminosities cannot be applied for most of the LINERs. In fact,
overestimations as large as 4 orders of magnitude are obtained.
Figure 3.
Chandra spectrum of
NGC 4594.
(Up) Spectral fitting
(Down) Residuals
Conclusions
AGN activity has been detected in 50-60% of the LINERs, as
obtained from the morphology (compact sources) and the spectral
analysis.