ALLDATA 1052 Portable
Achill Baldwin
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Methods. We used X-ray images from Chandra and radio images from Very Large Array to explore the morphology of the central area. We also studied the spectra of the nucleus and the surrounding region using observations from Chandra and XMM-Newton.
Conclusions. The extended emission is consistent with originating from extended jets and from jet-triggered shocks in the surrounding medium. The hard power-law emission from the nucleus and the lack of relativistic reflection supports the scenario of inefficient accretion in an advection-dominated accretion flow.
Low-ionisation nuclear emission-line regions (LINER) are characterised by intense optical lines from low-ionisation species (Heckman 1980). Although their optical properties define them as a class, there is not yet a consensus on their ionising mechanism. Indeed, the LINER class is highly heterogeneous, including sources with and without clear evidence of an active galactic nucleus (AGN), as well as both actively star-forming and passive galaxies. Alternatives to photo-ionisation explain the line excitation as shock-heated regions of the inter stellar medium (ISM; Dopita et al. 1997; Sugai & Malkan 2000), strongly star-forming regions (Armus et al. 1990; Balmaverde & Capetti 2015), or diffuse ionised plasmas (Collins et al. 2001). However, several LINER surveys have shown that the majority of sources are AGNs powered by low-luminosity accretion onto super-massive black holes (SMBHs; Ho 2008; Terashima et al. 2002; Gonzlez-Martn et al. 2009b; Hernndez-Garca et al. 2013, 2014). The evidence collected so far provides support for the scenario that these sources have advection-dominated accretion flows (ADAFs) embedded in standard accretion discs, as described by Narayan et al. (1998). This picture has been adopted to explain most LINER as low-luminosity active galactic nuclei (LLAGNs; Ho 2008). Since low and high ionisation states correspond to low and high accretion rates, LINERs can be seen as the low-accretion-rate extension of Seyfert galaxies (Alexander & Hickox 2012). LINERs thus offer an opportunity to study SMBHs in the low-accretion-rate regime, which is thought to dominate in the local universe (e.g. Hickox et al. 2009; Alonso-Herrero et al. 2008).
The X-ray emission from LINERs offers a powerful way of unveiling the presence of an AGN. Indeed, both spectral features and variability support the scenario of SMBH accretion in LINERs (Gonzlez-Martn et al. 2009a; Hernndez-Garca et al. 2014, 2016). One unambiguous piece of evidence of AGN-like accretion is the narrow iron Kα line at 6.4 keV. This line is ubiquitous in AGNs and originates from the outer parts of the accretion disc or regions further away, such as the Torus or broad line region. Active galactic nuclei commonly also exhibit a relativistically broadened iron line emitted from the innermost accretion disc, along with other reflection features such as the Compton hump above 10 keV (e.g. Garca et al. 2014). Such relativistic reflection is not expected to be observed in the ADAF scenario for LINERs, as the inner accretion flow is then optically thin and the iron is fully ionised. A relativistic line has been reported in only one source (NGC 1052, Brenneman et al. 2009), which is discussed further below. The narrow iron line is instead rather common (e.g. 42% in the sample of Terashima et al. 2002), offering evidence for the presence of AGNs.
From the current knowledge and previous work on NGC 1052, it is clear that open questions still remain. One of these concerns the nature and origin of the complex soft X-ray emission from the circum-nuclear region, including the possible role of star formation and the connection with the radio jets. The details of the circum-nuclear spectrum also affect the modelling of the nucleus, including the properties of the complex absorption, spectral variability, and the possible presence of relativistic reflection. The latter point is especially interesting because it is not expected in an ADAF and because there have been discordant results regarding the presence of a broad iron line in the literature.
This paper presents an analysis of one Chandra and four XMM-Newton observations of NGC 1052 which we use to explore the above mentioned aspects, which concern the nucleus and the circum-nuclear region, in more detail. The excellent spatial resolution of Chandra allows us to study the morphology of the central region and the X-ray spectra associated with the jets, the galaxy, and the nucleus. We also present a novel comparison between the long Chandra observation of 2005 and the Very Large Array (VLA) observations. The information obtained from Chandra is also used when analysing the XMM-Newton observations, which offer good-quality spectra but do not resolve the galaxy. This approach allows us to characterise the spectra of the nucleus in detail and to determine whether the variability is significant on timescales of years. We describe the observations in Sect. 2, present the analysis of images and spectra in Sect. 3, and discuss the results in Sect. 4. Our conclusions are summarised in Sect. 5.
The details of the four XMM-Newton observations of NGC 1052 analysed in this work are summarised in Table 1. There is also a more recent XMM-Newton observation available (from 2017 January 17), but we did not analyse this since the source lies very close to a chip gap in the p-type/n-type semiconductor camera (pn) and overlaps with a bright column in one of the two Metal Oxide Semi-conductor cameras (MOS), specifically the MOS2. The four observations analysed here are separated by time periods of between 7 months and 5 years, which is suitable for investigating possible spectral variability of the nucleus. From these observations we used all data except those where the source falls on a chip gap, which turns out to be the case for the first MOS observation of 2001 August 15 and the MOS2 observation from 2009 January 14. All the observations were performed in full frame mode with the medium filter.
We start the analysis by investigating the morphology of the galaxy and jet using Chandra and VLA images in Sect. 3.1. We then present the X-ray spectral analysis in Sect. 3.2, where we first characterise the Chandra spectrum of the extended emission and then analyse the XMM-Newton and Chandra spectra of the nucleus. We focus our analysis on time-averaged spectra, which allow us to investigate possible long-term variations. The light curves of the individual XMM-Newton observations (except the first short one) have already been studied by Hernndez-Garca et al. (2014), who showed that no significant variability was present during the observations.
The VLA image at 1.4 GHz shows two compact high-surface-brightness regions, or two hot spots, on the eastern lobe. These have also been seen in a MERLIN observations from 1995 (Kadler et al. 2004a, labelled as H1 and A in their Fig. 3). On the western lobe, one main hot spot is seen, with some indication of an inner secondary one. The latter was seen more clearly in the MERLIN observation (Kadler et al. 2004a). In the X-ray images, a hot spot below 2 keV is seen along the outer edge of the eastern radio lobe. This X-ray emission is likely due to shocks in the region where the extended jet interacts with the environment, as discussed in Kadler et al. (2004a). On the same lobe, there is also some X-ray emission below 1 keV at the position of the inner radio hot-spot. However, it is not clear if this is associated with the jet since X-ray emission at a similar level is also seen in the nearby regions that do not overlap with the radio contours. The western radio lobe has a less clear counterpart in the X-ray band, but we note some low-surface-brightness emission coincident with the southern boundary of the lobe.
The spectra of the extended emission extracted from the two rectangular regions in Fig. 1 show very similar features. We therefore fitted them together with all parameters tied except for a cross normalisation constant. Due to the limited statistics of the spectra we only considered simple models in the form of a power law (expected from jet emission) and a thermal mekal component. The latter may arise from the ISM, shocks, or starburst regions. The results from the fits are reported in Table 2. We found that a single component could not describe the spectra, whereas a pow+mekal and mekal+mekal provided acceptable fits. We adopt the former as our preferred model due to slightly better C statistic (cstat) and the high temperature of the second mekal component in the latter model, which lacks a clear physical interpretation. We also analysed the effects of adding intrinsic absorption to the models, but found that it was not required. The spectra and best-fitting model are shown in Fig. 3. We refer to our preferred model for the jet plus galaxy region as JGAL hereafter.
As the iron line is highly significant, we introduced a complete model which considers Compton reflection and fluorescence in a self-consistent way, considering that Compton reflection is expected together with fluorescence. In line with this, Osorio-Clavijo et al. (2020) found evidence for Compton reflection in NuSTAR and Suzaku observations of the source. To account for this reflection, we substituted the Gaussian line in our model with a pexmon component (Nandra et al. 2007). We refer to this model as PexJGAL. Pexmon includes fluorescent lines of Fe Kα, Fe Kβ, and Ni Kα, the Fe Kα Compton shoulder, and Compton reflection. We allowed only the normalisation of the pexmon component to vary in the fits. The inclination of the source is known to be high (Baczko et al. 2019) and we fixed it at 85, noting that its exact value does not affect the conclusions. We further tied the photon index to the value of the primary power law and kept the cutoff energy and abundances fixed at their default values. This model does not produce a satisfactory χ2 (see Table 3) and leaves residuals especially in the soft band below 3 keV. For this reason we included an additional absorption component, as also done in the previous Suzaku observation (Brenneman et al. 2009). The overall model in xspec was: tbabs(zpcfabs*absori*pow+pexmon+mekal+pow), whichis hereafter referred to as PexiJGAL.
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