Synapses of active galactic nuclei: Comparing X-ray and optical classifications using artificial neural networks

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. © ESO, 2014.