Context. Many classes of active galactic nuclei (AGN) have been defined
entirely through optical wavelengths, while the X-ray spectra have been
very useful to investigate their inner regions. However, optical and
X-ray results show many discrepancies that have not been fully
understood yet. Aims: The main purpose of the present paper is to
study the synapses (i.e., connections) between X-ray and optical AGN
classifications. Methods: For the first time, the newly
implemented efluxer task allowed us to analyse broad band X-ray spectra
of a sample of emission-line nuclei without any prior spectral fitting.
Our sample comprises 162 spectra observed with XMM-Newton/pn of 90 local
emission line nuclei in the Palomar sample. It includes, from the
optical point of view, starbursts (SB), transition objects (T2),
low-ionisation nuclear emission line regions (L1.8 and L2), and Seyfert
nuclei (S1, S1.8, and S2). We used artificial neural networks (ANNs) to
study the connection between X-ray spectra and optical classes.
Results: Among the training classes, the ANNs are 90% efficient at
classifying the S1, S1.8, and SB classes. The S1 and S1.8 classes show a
negligible SB-like component contribution with a wide range of
contributions from S1- and S1.8-like components. We suggest that this
broad range of values is related to the high degree of obscuration in
the X-ray regime. When including all the objects in our sample, the S1,
S1.8, S2, L1.8, L2/T2/SB-AGN (SB with indications of AGN activity in the
literature), and SB classes have similar average X-ray spectra, but
these average spectra can be distinguished from class to class. The S2
(L1.8) class is linked to the S1.8 (S1) class with a larger SB-like
component than the S1.8 (S1) class. The L2, T2, and SB-AGN classes
constitute a class in the X-rays similar to the S2 class, albeit with
larger portions of SB-like component. We argue that this SB-like
component might come from the contribution of the host galaxy emission
to the X-rays, which is high when the AGN is weak. Up to 80% of the
emission line nuclei and, on average, all the optical classes included
in our sample show a significant fraction of S1-like or S1.8-like
components. Thus, an AGN-like component seems to be present in the vast
majority of the emission line nuclei in our sample. Conclusions:
The ANN trained in this paper is not only useful for studying the
synergies between the optical and X-ray classifications, but might also
be used to infer optical properties from X-ray spectra in surveys like
eRosita.
Table 1 and Appendices are available in electronic form at
http://www.aanda.org