Active Galaxies NewsletterAn electronic publication dedicated to the observations and theory of active galaxies
Below is a list of abstracts posted in the Newsletter in the past 6 months, along with relevant contact
Please note that the latest abstracts can be found in the recent issues of the newsletter.
Active Galactic Nuclei (AGN) emit energies of the order of 1044 erg s-1 in very compact regions, being one of the most energetic phenomena in the Universe. Nowadays, the most accepted theory is that this phenomena is consequence of accretion of matter onto a super massive black hole.
This thesis is centered on the variability study of the low luminosity AGN (LLAGN), including LINERs (low ionization nuclear emission line regions) and the well known Seyferts. The behaviour of the AGN families are also compared. Whereas Seyfert nuclei were discovered in 1943 and are characterized because their nuclei produce high ionized spectral emission lines, it was not until 1980 that Heckman discovered LINERs and classified them as a subclass of AGN, whose optical spectra present a lower ionization state than Seyferts.
The study of AGN in X-rays is ideal because the nucleus is accessible in this frequency range and the obscuration effect is much smaller compared to the ultraviolet (UV), optical or near-infrared. That is why the data presented in this thesis are mainly in X-rays, although complemented with simultaneous information at UV frequencies.
Variability is a property characterizing powerful AGN, which show variations in the whole electromagnetic spectrum, and allows to infer their physical properties. While this is well established for Seyfert galaxies, it is not obvious for LINERs, for which it has been studied only in a small sample of type 1s and a sample at UV frequencies. Moreover, at the beginning of this thesis the physical mechanism that originates variations in this kind of AGN was unknown, being this one of the main objectives of this thesis.
To analyze the X-ray variability, we have developed a methodology which allows the study at short- and long-term variations. The long-term variations are analyzed by fitting all the data with the same model. The short-term variations are studied from the light curves using standard procedures. This allows us to estimate not only flux variations but also spectral variations, that serves to know the variability pattern and at the same time permits to infer the physical properties of these objects. For this study archival data from Chandra and XMM-Newton satellites at different dates have been used. Moreover, XMM-Newton allows to simultaneously obtain variability information at X-rays and UV frequencies. The methodology is explained in detail in Chapter 2 (see also Hernández-García et al. 2013).
First, the methodology was applied to a sample of 18 LINER nuclei, including type 1 (they are actually type 1.9) and 2 (see Chapter 2). The nuclei were classified as AGN (non-AGN) when a point-like source was observed (or not) in the 4.5-8.0 keV energy band, following the work by González-Martín et al. (2009b). From the entire sample, three LINERs were classified as non-AGN, all of them being Compton-thick candidates (i.e., they are obscured by very high column densities, larger than 1024 cm-2), none show variations in X-rays and two of them vary at UV. None of the nuclei show short-term variations (between hours and days), whereas more than a half of the AGN candidates show long-term variations (between months and years). These variations are mainly related to intrinsic changes of the sources, while only one galaxy shows column density variations - what would be directly related with changes in the density of the dusty torus or the broad line region (BLR). The study in the UV shows that this type of galaxies are variable at UV frequencies. Therefore, it is found that LINERs are variable objects both at X-rays and UV frequencies. Furthermore, according to their black hole masses, accretion rates, and variability timescales, we find that LINERs follow the same variability plane (MBH - Lbol - TB) as more powerful AGN in X-rays. We have also studied the accretion mechanism from the relation between the spectral index and the Eddington rate, where an anticorrelation is given, indicating that accretion could be inefficient (compared to the efficient accretion obtained for more powerful sources). The results derived from this study have been published in Hernández-García et al. (2014).
The same methodology was applied to a sample of 26 Seyfert 2s (Chapter 3). None of the nuclei show short-term variations. In this case, and additional analysis was performed to select Compton-thick candidates because, since these galaxies are observed through the dusty torus, we expect a fraction of them being highly obscured. We find that 12 sources are Compton-thick candidates. Among the Compton-thick candidates, only one shows long-term variations; the most reasonable explanation is that part of the continuum is still transmitted and thus we can observe variations. Among the remaining nuclei we find that most of them are variable in timescales between months and years, the variability pattern being very similar to that found for LINERs: intrinsic changes of the energy source. Variations related to the column density were found only in four sources (30%). Finally, in contrast to that found for LINERs, none of the galaxies show variations at UV frequencies. The results of this study have been published in Hernández-García et al. (2015).
Therefore, the X-ray variability study in LLAGN shows that both AGN families are variable in timescales ranging from months to years, with amplitudes between 20% and 80% and a common variability pattern among them. At UV, in contrast, the variability study shows that LINERs are variable, but Seyfert 2s do not show changes at these frequencies, with the nucleus detected only in three cases. The fact that the nucleus varies in this frequency range might be because the dusty torus disappeared in LINERs, leaving the core uncovered, giving place to the observed variations. The work undertaken in this thesis shows that LINERs and Seyfert 2s spectrally behave similarly at X-ray frequencies. However, the analyzed Eddington ratios are consistent with different accretion mechanisms, which is efficient for Seyferts and inefficient for LINERs. This study is presented in Chapter 4.
This thesis is complemented by two chapters related to nuclear activity and the X-ray emission in extragalactic objects (Chapters 5 and 6). The first is a study about AGN in groups and clusters of galaxies using XMM-Newton and Chandra data, which includes the determination of the number of AGN on each cluster. We find from one to five AGN per galaxy cluster. The second is a study on the variability of ultraluminous X-ray sources (ULXs) using XMM-Newton data and applying Fourier techniques. We estimate non-linear variability of the ULX, which occurs in the same way as in black holes of different masses, and time lags that variations generate between two different energy bands.