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According to theory, the torus of active galactic nuclei (AGN) is sustained from a wind coming off the accretion disc, and for low efficient AGN, it has been proposed that such structure disappears. However, the exact conditions for its disappearance remain unclear. This can be studied throughout the reflection component at X-rays, which is associated with distant and neutral material at the inner walls of the torus in obscured AGN. We select a sample of 81 AGNs observed with NuSTAR with a distance limit of D< 200 Mpc and Eddington rate |$\rm {\lambda _{Edd} \equiv L_{bol}/L_{Edd}\lt 10^{-3}}$|⁠. We fit the 3–70 keV spectra using a model accounting for a partial-covering absorber plus a reflection component from neutral material. We find that the existence of the reflection component spans in a wide range of black hole mass and bolometric luminosities, with only ∼13 per cent of our sample (11 sources) lacking of any reflection signatures. These sources fall in the region in which the torus may be lacking in the L-MBH diagram. For the sources with a detected reflection component, we find that the vast majority of them are highly obscured (⁠|$\rm {\log \ N_H \gt 23}$|⁠), with |$\rm {\sim 20{{\ \rm per\ cent}}}$| being Compton-thick. We also find an increase in the number of unobscured sources and a tentative increase on the ratio between |$\rm {FeK\alpha }$| emission line and Compton-hump luminosities toward |$\rm {\lambda _{Edd}=10^{-5}}$|⁠ , suggesting that the contribution of the |$\rm {FeK\alpha }$| line changes with Eddington ratio. [ABSTRACT FROM AUTHOR]

In this paper, we present a multi-epoch analysis of NGC 1052, a prototypical low-luminisity active galactic nucleus, using XMM –Newton, Suzaku and NuSTAR observations taken from 2001 to 2017. This is the first time that results from NuSTAR observations have been reported for NGC 1052. Regarding technical aspects, we found a wavelength-dependent calibration issue between simultaneous XMM –Newton and NuSTAR spectra, characterized by a change in the photon index of |$\rm { \Gamma _{NuSTAR}- \Gamma _{XMM-Newton}=0.17\pm 0.04}$|⁠. We use ancillary Chandra data to decontaminate the nuclear spectrum from circumnuclear contributors. We find that two baseline models can fit the broad (0.5–50 keV) X-ray spectrum of the source. One consists of a power-law-like continuum that is absorbed by a uniform absorber, and is reflected by neutral material, and a separate power-law component in the soft band. The second model consists of a clumpy absorber. The reflection component is still present, but not the soft-band power law. Instead, absorption by a warm absorber is necessary to fit the spectra. This is the first time that a reflection component has been established in this object, thanks to high-energy data from NuSTAR. This component is constant in flux and shape, supporting the idea that it is produced away from the central source (probably in the torus). We find flux, spectral slope and absorption variations on time-scales of months to years. We also find that a patchy absorber can explain the behaviour of this source better, as it is ∼200 times more likely than the uniform absorber and yields smaller intrinsic variations. [ABSTRACT FROM AUTHOR]

Context. Seyfert 1.8/1.9 are sources showing weak broad Hα components in their optical spectra. According to unification schemes, they are seen with an edge-on inclination, similar to type 2 Seyfert galaxies, but with slightly lower inclination angles. Aims. We aim to test whether Seyfert 1.8/1.9 have similar properties at UV and X-ray wavelengths. Methods. We used the 15 Seyfert 1.8/1.9 in the Véron Cetty and Véron catalog with public data available from the Chandra and/or XMM-Newton archives at different dates, with timescales between observations ranging from days to years. All the spectra of the same source were simultaneously fit with the same model and different parameters were left free to vary in order to select the variable parameter(s). Whenever possible, short-term variations from the analysis of the X-ray light curves and long-term UV variations from the optical monitor onboard XMM-Newton were studied. Our results are homogeneously compared with a previous work using the same methodology applied to a sample of Seyfert 2. Results. X-ray variability is found in all 15 nuclei over the aforementioned ranges of timescales. The main variability pattern is related to intrinsic changes in the sources, which are observed in ten nuclei. Changes in the column density are also frequent, as they are observed in six nuclei, and variations at soft energies, possibly related to scattered nuclear emission, are detected in six sources. X-ray intra-day variations are detected in six out of the eight studied sources. Variations at UV frequencies are detected in seven out of nine sources. Conclusions. A comparison between the samples of Seyfert 1.8/1.9 and 2 shows that, even if the main variability pattern is due to intrinsic changes of the sources in the two families, these nuclei exhibit different variability properties in the UV and X-ray domains. In particular, variations in the broad X-ray band on short timescales (days to weeks), and variations in the soft X-rays and UV on long timescales (months to years) are detected in Seyfert 1.8/1.9 but not in Seyfert 2. Overall, we suggest that optically classified Seyfert 1.8/1.9 should be kept separated from Seyfert 2 galaxies in UV/X-ray studies of the obscured AGN population because their intrinsic properties might be different. [ABSTRACT FROM AUTHOR]

Context. Obscured active galactic nuclei (AGNs) are thought to be very common in the Universe. Observations and surveys have shown that the number of sources increases for near galaxies and at the low-luminosity regime (the so-called LLAGNs). Furthermore, many AGNs show changes in their obscuration properties at X-rays that may suggest a configuration of clouds very close to the accretion disk. However, these variations could also be due to changes in the intrinsic continuum of the source. It is therefore important to study nearby AGN to better understand the locus and distribution of clouds in the neighbourhood of the nucleus. Aims. We aim to study the nuclear obscuration of LLAGN NGC835 and its extended emission using mid-infrared observations. Methods. We present sub-arcsecond-resolution mid-infrared 11.5 µm imaging of the LLAGN galaxy NGC835 obtained with the instrument CanariCam in the Gran Telescopio CANARIAS (GTC), archival Spitzer/IRS spectroscopy, and archival Chandra data observed in 2000, 2008, and 2013. Results. The GTC/CanariCam 11.5 µm image reveals faint extended emission out to ~6 arcsec. We obtained a nuclear flux of F(11:5 µm) ~ 18 mJy; whereas the extended emission accounts for 90% of the total flux within the 6 arcsec. This means that the low angular resolution (~4 arcsec) IRS spectrum is dominated by this extended emission and not by the AGN. This is clearly seen in the Spitzer/IRS spectrum, which resembles that of star-forming galaxies. Although the extended soft X-ray emission shows some resemblance with that of the mid-infrared, the knots seen at X-rays are mostly located in the inner side of this mid-infrared emission. The nuclear X-ray spectrum of the source has undergone a spectral change between 2000/2008 and 2013. We argue that this variation is most probably due to changes in the hydrogen column density from ~8 × 1023 cm–2 to ~3 × 1023 cm–2. NGC835 therefore is one of the few LLAGN, together with NGC1052, in which changes in the absorber can be claimed. [ABSTRACT FROM AUTHOR]

Context. Variability across the electromagnetic spectrum is a property of active galactic nuclei (AGN) that can help constrain the physical properties of these galaxies. Nonetheless, the way in which the changes happen and whether they occur in the same way in every AGN are still open questions. Aims. This is the third in a series of papers with the aim of studying the X-ray variability of different families of AGN. The main purpose of this work is to investigate the variability pattern(s) in a sample of optically selected Seyfert 2 galaxies. Methods. We use the 26 Seyfert 2s in the Véron-Cetty and Véron catalog with data available from Chandra and/or XMM-Newton public archives at different epochs, with timescales ranging from a few hours to years. All the spectra of the same source were simultaneously fitted, and we let different parameters vary in the model. Whenever possible, short-term variations from the analysis of the light curves and/or long-term UV flux variations were studied. We divided the sample into Compton-thick and Compton-thin candidates to account for the degree of obscuration. When transitions between Compton-thick and thin were obtained for different observations of the same source, we classified it as a changing-look candidate. Results. Short-term variability at X-rays was studied in ten cases, but variations are not found. From the 25 analyzed sources, 11 show long-term variations. Eight (out of 11) are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look candidates are also variable. The main driver for the X-ray changes is related to the nuclear power (nine cases), while variations at soft energies or related to absorbers at hard X-rays are less common, and in many cases these variations are accompanied by variations in the nuclear continuum. At UV frequencies, only NGC5194 (out of six sources) is variable, but the changes are not related to the nucleus. We report two changing-look candidates, MARK273 and NGC7319. Conclusions. A constant reflection component located far away from the nucleus plus a variable nuclear continuum are able to explain most of our results. Within this scenario, the Compton-thick candidates are dominated by reflection, which suppresses their continuum, making them seem fainter, and they do not show variations (except MARK3), while the Compton-thin and changing-look candidates do. [ABSTRACT FROM AUTHOR]

Aims. This programme is aimed at obtaining one of the largest X-ray selected samples of identified active galactic nuclei to date in order to characterise such a population at intermediate fluxes, where most of the Universe's accretion power originates.We present the XMM-NewtonWide Angle Survey (XWAS), a new catalogue of almost a thousand X-ray sources spectroscopically identified through optical observations. Methods. A sample of X-ray sources detected in 68 XMM-Newton pointed observations was selected for optical multi-fibre spectroscopy. Optical counterparts and corresponding photometry of the X-ray sources were obtained from the SuperCOSMOS Sky Survey. Candidates for spectroscopy were initially selected with magnitudes down to R ~ 21, with preference for X-ray sources having a flux F0.5-4.5keV ⩾ 10-14 erg s-1 cm-2. Optical spectroscopic observations were made using the Two Degree Field of the Anglo Australian Telescope, and the resulting spectra were classified based on optical emission lines. Results. We have identified through optical spectroscopy 940 X-ray sources over Ω ~ 11.8 deg2 of the sky. Source populations in our sample can be summarised as 65% broad line active galactic nuclei (BLAGN), 16% narrow emission line galaxies (NELGs), 6% absorption line galaxies (ALGs) and 13% stars. An active nucleus is also likely to be present in the large majority of the X-ray sources spectroscopically classified as NELGs or ALGs. Sources lie in high-galactic latitude (∣b∣ > 20 deg) XMM-Newton fields mainly in the southern hemisphere. Owing to the large parameter space in redshift (0 ⩽ z ⩽ 4.25) and flux (10-15 ⩽ F0.5-4.5keV ⩽ 10-12 erg s-1 cm-2) covered by the XWAS this work provides an excellent resource for the further study of subsamples and particular cases. The overall properties of the extragalactic objects are presented in this paper. These include the redshift and luminosity distributions, optical and X-ray colours and X-ray-to-optical flux ratios. [ABSTRACT FROM AUTHOR]

Context. One of the most distinctive features in active galactic nuclei (AGN) is the variability of their emission. Variability has been discovered at X-ray, UV, and radio frequencies on timescales from hours to years. Among the AGN family and according to theoretical studies, low-ionization nuclear emission line region (LINER) nuclei would be objects variable on long timescales. Aims. Our purpose is to investigate spectral X-ray variability in LINERs and to understand the nature of these types of objects, as well as their accretion mechanism. Methods. Chandra and XMM-Newton public archives were used to compile X-ray spectra of seven LINER nuclei at different epochs with timescales of years. To search for variability we fit all spectra from the same object with a set of models to identify the parameters responsible for the variability pattern. We also analyzed the light curves to search for short timescale (from hours to days) variability. Whenever possible, UV variability was also studied. Results. We found spectral variability in four objects (NGC1052, NGC3226, NGC4278, and NGC4552), with variations mostly related to hard energies (2-10 keV). These variations are generated by several possible changes that act either alone or in combination: changes in the soft excess or in the absorber. Added to this can be intrinsic variations of the source, which may also be responsible by themselves for the spectral variability. These variations occurred within years, the shortest timescale being found for NGC4278 (two months). Another two galaxies (NGC4261 and NGC 5846) apparently do not vary. No short timescale variations during individual observations were found. Our analysis confirms the previously reported anticorrelation between the X-ray spectral index, Γ, and the Eddington ratio, Lbol/LEdd, and also the correlation between the X-ray to UV flux ratio, αox, and the Eddington ratio, Lbol/LEdd. These results support an advection dominated accretion flow as the accretion mechanism in LINERs. [ABSTRACT FROM AUTHOR]

Context. Active galactic nuclei (AGN), powered by accretion onto supermassive black holes (SMBHs), are thought to be scaled up versions of Galactic black hole X-ray binaries (BH-XRBs). In the past few years evidence of such correspondence include similarities in the broadband shape of the X-ray variability power spectra, with characteristic bend times-scales scaling with mass. Aims. The aim of this project is to characterize the X-ray temporal properties of a sample of AGN to study the connection among different classes of AGN and their connection with BH-XRBs. Methods. We have performed a uniform analysis of the power spectrum densities (PSDs) of 104 nearby (z < 0.4) AGN using 209 XMM-Newton/pn observations. These PSDs span ≃ 3 decades in temporal frequencies, ranging from minutes to days. The PSDs have been estimated in three energy bands: 0.2-10 (total), 0.2-2 (soft), and 2-10 keV (hard). The sample comprises 61 Type-1 AGN, 21 Type-2 AGN, 15 NLSy1, and 7 BLLACS. We have fitted each PSD to two models: (1) a single power-law model and (2) a bending power-law model. Results. Among the entire sample, 72% show significant variability in at least one of the three bands tested. A high percentage of low-luminosity AGN do not show any significant variability (86% of LINERs). The PSD of the majority of the variable AGN was well described by a simple power-law with a mean index of α = 2.01 ?± 0.01. In 15 sources we found that the bending power law model was preferred with a mean slope of α = 3.08 ± 0.04 and a mean bend frequency of ? ?b ? ≃ 2 × 10-4 Hz. Only KUG?1031+398 (RE?J1034+396) shows evidence for quasi-periodic oscillations. The "fundamental plane" relating variability timescale, black hole mass, and luminosity is demonstrated using the new X-ray timing results presented here together with a compilation of the previously detected timescales from the literature. Conclusions. Both quantitative (i.e. scaling with BH mass) and qualitative (overall PSD shapes) found in this sample of AGN are in agreement with the expectations for the SMBHs and BH-XRBs being the same phenomenon scaled-up with the size of the BH. The steep PSD slopes above the high frequency bend bear a closer resemblance to those of the "soft/thermal dominated" BH-XRB states than other states. [ABSTRACT FROM AUTHOR]