The variable ionized absorber in the Seyfert 2 Mrk 348

Abstract

We present the results of the analysis of the X-ray spectrum of the Seyfert 2 Mrk 348, observed by Suzaku and XMM-Newton. The overall spectrum of Mrk 348 can be described by a primary power law continuum seen through three layers of absorption, of which one is neutral and two are ionised. Comparing Suzaku (2008) and XMM-Newton (2002) observations we find variability of the X-ray spectral curvature. We suggest that the variability can be explained through the change of column density of both the neutral and one of the ionised absorbers, together with a variation of the ionisation level of the same absorber. We thus confirm one of the main features presented in past works, where intrinsic column density variability up to ∼ 10 23 cm −2 was observed on time scales of months. We also find that the photon index of the underlying power law continuum (Γ ∼ 1.8) is in agreement with the previous observations of this Seyfert 2.

X-ray spectral variability of Seyfert 2 galaxies

“…We recall that variations are not due to changes in the power law index, Γ, but related to its normalization. It has been shown that hard X-ray variability is usual in Seyfert 2 galaxies (e.g., Turner et al 1997;Trippe et al 2011;Marchese et al 2014). In fact, this kind of variation has already been reported in the literature for objects included in the present work from intrinsic flux variations indicating changes in the nuclear continuum (Isobe et al 2005) or because they needed to set free the normalization of the power law for a proper fit to the data (LaMassa et al 2011).…”

“…This galaxy was observed twice with XMM-Newton in 2002 and 2013 and once with Chandra in 2010. Recently, Marchese et al (2014) have compared XMM-Newton and Suzaku data from 2002 and 2008. They fitted the data with a power law component transmitted through three absorbers (one neutral and two ionized), obtaining intrinsic luminosities of log(L(2−10 keV)) = 43.50 and 43.51, respectively.…”

“…In fact, these variations are usually observed in Seyferts 1−1.9 (e.g., NGC 1365, Risaliti et al 2007;NGC 4151, Puccetti et al 2007;MARK 766, Risaliti et al 2011), where it has been shown that the changes are most probably related to the broad line region (BLR), although it has been suggested that multiple absorbers may be present in an AGN, located at different scales (Braito et al 2013). However, it is not so clear whether variations due to absorbers are common for optically classified Seyfert 2s, for which this kind of variation has only been reported in a few cases (e.g., MARK 348, Marchese et al 2014;NGC 4507, Braito et al 2013 andMarinucci et al 2013;MARK 1210, Risaliti et al 2010.…”

“…However, we cannot discard variations due to components that we did not fit in the models. For instance, Marchese et al (2014) analyzed the XMM-Newton and Suzaku data of MARK 348 (also included in the present work), and report variations due to a neutral plus an ionized absorbers, together with a change in the ionization parameter of the ionized absorber. Their analysis is based on the residuals of the spectral fitting, where they include as many components as required, and the variability analysis is performed by testing different scenarios, including a variable continuum plus a constant reflection component Fig.…”

“…However, a number of Seyfert 2s actually do show variations. The study of the variability has been approached in different ways from the analysis of the light curves to study of short-term variations (Awaki et al 2006), through count-rate or flux variations (Isobe et al 2005;Trippe et al 2011), or comparisons of spectra of the same source at different epochs (LaMassa et al 2011;Marinucci et al 2013;Marchese et al 2014). The observed variations may be related with absorbing material that crosses our line of sight (Risaliti et al , 2010 and/or can be intrinsic to the sources (Evans et al 2005;Sobolewska & Papadakis 2009;Braito et al 2013).…”