Resumen de An x-ray approach to low luminosity active galactic nuclei

Omaira Gonzalez Martin

• Active Galactic Nuclei (AGNs) are one of the most energetic phenomena ever observed in the Universe, They produce luminosities as high as 1E+44 erg/s in very compact volumes. The classification of AGNs is essentially observational and is based on their luminosities and optical spectral characteristics. As a consequence of that, the taxonomy of AGNs tend to be rather confusing and its complexity leads to an heterogeneous family of sources.

  Despite the lack of direct evidence it is today accepted that accretion of matter by supermassive black holes (SMBH) is the primary energy source in AGNs. The material is accreted onto the SMBH by an accretion disk and a region with high velocity clouds (Broad Line Region, BLR) is surrounding this accretion disk. Finally a region composed by low velocity clouds (Narrow Line Region, NLR), extend up to several kpc scales, shows evidence of interaction with a radio jet.

  Antonucci et al. (1993) proposed a unified model to explain all the families of AGNs. The key elements in this simple scheme is the obscuring torus between the BLR and the NLR. If the torus is seen face-on, our view of the central regions is unobscured and we detect the broad lines; if our view is closer to edge-on, the central regions are not seen directly and no broad lines are detected. However, although this general scenario is widely accepted, there are still several discrepancies that need to be explained.

  This thesis is focused in one of these subclasses of active nuclei known as Low Ionization Narrow Emission-line Regions (LINERs). They were discovered by Heckman (1980), which classified them as a subgroup of AGNs by an optical spectra dominated by emission lines of moderate intensities arising from gas in lower ionization states than classical AGN and line widths approaching those of the NLR in Seyfert galaxies.

  The X-ray domain is plenty of AGN signatures (X-ray luminosity, FeK emission line or hydrogen column density) resulting in one of the most important tools to determine the nature of doubtful AGN, as it is the case of LINERs.

  The analysis of a large sample of LINERs carried out in this thesis demonstrates that a high percentage of them (60% for the full sample, 68% for the sub sample with spectral fit) shows compact unresolved nuclear sources at high energies (4.5-8 keV) band, being excellent candidates to host a nuclear AGN. The modeling of their X-ray spectra shows that most of them agree with the Baseline model of AGNs. 70% of the sample need two power-laws while a thermal model at energies below 2 keV is needed in the vast majority of the cases (95%).

  The obscuring material scenario is consistent with a two absorber media claimed for AGNs, an extended medium associated with the host galaxy and a compact nuclear absorber likely associated with the BLR. Multiwavelength diagnostics also suggests that 50% of the LINER nuclei are good candidates to host a hidden AGN, even at X-ray frequencies (Compton-thick), higher than that reported for Seyfert nuclei (closest AGNs in terms of luminosities).

  Taken all the known multiwavelength tracers of AGN activity together, our main conclusion is that the vast majority of LINERs (around 90% of our sample) show evidence of AGN activity in their nuclei. However, the results of UV, radio and X-ray studies are controversial for some objects, i.e. showing AGN characteristics at UV that are not seen at X-ray frequencies. We argue that such variations might be compatible with the existence of a clumpy torus, which will be the responsible for such discrepancies due to the probability of finding a high-velocity thick cloud in the line-of-sight when non simultaneous observations are analyzed.

  The NLR and ENLR of a low luminosity Seyfert 1 galaxy NGC 4151 is studied at optical, radio and X-ray frequencies. It suggests that most of the emission seen at soft X-ray energies, both in the NLR and in the ENLR, is due to photoionized material from the central source, and not to in-situ stellar photoionization. The other important conclusion on this work is the explanation of the orientation mismatch of the optical NLR/ENLR and the radio jet. It is suggested that a different (or precessing) jet is the origin of the the X-ray and optical cone-like morphology.

  Finally, Ultra-Luminous X-ray sources (ULX, defined as X-ray sources with L(2-10 keV)> 1E+39 erg/s) seen in front of the High Velocity System of NGC\,1275, are mostly associated with a region of very active star formation, although the brightest does not seem to be connected. This relation of ULX with star formation regions are also reported by several archetypal galaxies as the Antennae or the Cartwheel system.