Bibcode
Hernández-García, L.; González-Martín, O.; Masegosa, J.; Márquez, I.
Bibliographical reference
Highlights of Spanish Astrophysics VII, Proceedings of the X Scientific Meeting of the Spanish Astronomical Society (SEA), held in Valencia, July 9 - 13, 2012, Eds.: J.C. Guirado, L.M. Lara, V. Quilis, and J. Gorgas., pp.443-443
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5
2013
Citations
0
Refereed citations
0
Description
Active galactic nuclei (AGN) are powered by energetic phenomena which
cannot be attributed to stars. Among AGN, several objects can be
described by the unified model (Antonucci 1993; Urry & Padovani
1995). However, several subclasses of objects that cannot be
accommodated into this scheme, as is the case of LINERs (low ionisation
narrow emission line regions). Variability across the whole
electromagnetic spectrum is one of the properties that characterized
AGNs. Therefore, searching for variability in LINERs could unequivocally
demonstrate the presence of a non-thermal source. Also, X-rays is one of
the best ways to search for AGN signature. In this work (which is part
of a larger study) we add more evidence about the X-ray variability in
LINERs and investigate its origin. We study two LINER nuclei; NGC 1052
(type 2) and NGC 4278 (type 1). The data consist on different
observations in different epochs (timescale of years), taken from
XMM-Newton and Chandra archives, respectively. To search for variability
we try to fit all the spectra with the same model using XSPEC; if we can
fit all the spectra with the same parameters, it is supposed that the
object is non-variable, whereas if we cannot fit them properly, it will
be variable. In the last case we need to let one or more parameters to
vary in the model, so it may provide clues to understand the nature of
this variability. For NGC 1052 we fit a model containing a thermal
component plus two power laws. This results in a variability due to
changes in the column density and the slope of the power law, both at
hard energies. This scenario is consistent with the variability
understood as variations in the clouds intersecting the line of sight of
the observer (see Rissalitti et al. (2007, 2010)), and is also
compatible with the framework of the clumpy torus model (Elitzur 2006).
For NGC 4278 the model contains a thermal component plus a single power
law. The spectral fitting results in variations of the slope and
normalization of the power law. This spectral fitting is consistent with
the results provided by Younes et al. (2010), who had already reported a
significant variability on this source. In this case the variability is
attributed to intrinsic variations of the nuclear source. The presence
of different mechanisms for LINERs have to be confirmed with a larger
sample of LINERs which is under study.