ALBERT

Description: Hard X-ray spectra of 28 bright Seyfert galaxies observed with INTEGRAL were analysed together with the X-ray spectra from XMM–Newton , Suzaku and RXTE . These broad-band data were fitted with a model assuming a thermal Comptonization as a primary continuum component. We tested several model options through a fitting of the Comptonized continuum accompanied by a complex absorption and a Compton reflection. Both the large data set used and the model space explored allowed us to accurately determine a mean temperature kT e of the electron plasma, the Compton parameter y and the Compton reflection strength R for the majority of objects in the sample. Our main finding is that a vast majority of the sample (20 objects) is characterized by kT e 〈 100 keV, and only for two objects we found kT e 〉 200 keV. The median kT e for entire sample is 48 $_{-14}^{+57}$  keV. The distribution of the y parameter is bimodal, with a broad component centred at 0.8 and a narrow peak at 1.1. A complex, dual absorber model improved the fit for all data sets, compared to a simple absorption model, reducing the fitted strength of Compton reflection by a factor of about 2. Modest reflection (median R 0.32) together with a high ratio of Comptonized to seed photon fluxes point towards a geometry with a compact hard X-ray emitting region well separated from the accretion disc. Our results imply that the template Seyferts spectra used in the population synthesis models of active galactic nuclei (AGN) should be revised.

Description: We present X-ray observations of the active galactic nucleus (AGN) in NGC 4785. The source is a local Seyfert 2 which has not been studied so far in much detail. It was recently detected with high significance in the 15–60 keV band in the 66-month Swift /BAT (Burst Array Telescope) all sky survey, but there have been no prior pointed X-ray observations of this object. With Suzaku , we clearly detect the source below 10 keV, and find it to have a flat continuum and prominent neutral iron fluorescence line with equivalent width 1 keV. Fitting the broad-band spectra with physical reflection models shows the source to be a Compton-thick AGN with N H of at least 2 10 24 cm –2 and absorption-corrected 2–10 keV X-ray power L 2–10  ~ few times 10 42  erg s –1 . Realistic uncertainties on L 2–10 computed from the joint confidence interval on the intrinsic power-law continuum photon index and normalization are at least a factor of 10. The local bona fide Compton-thick AGN population is highly heterogeneous in terms of WISE mid-infrared source colours, and the nucleus of NGC 4785 appears especially sub-dominant in the mid-infrared when comparing to other Compton-thick AGN. Such sources would not be easily found using mid-infrared selection alone. The extent of host galaxy extinction to the nucleus is not clear, though NGC 4785 shows a complex core with a double bar and inner disc, adding to the list of known Compton-thick AGN in barred host galaxies.

Description: We investigate the relationship between X-ray and optical line emission in 340 nearby ( z ~= 0.04) AGN selected above 10 keV using Swift BAT. We find a weak correlation between the extinction corrected [O iii ] and hard X-ray luminosity ( $L_{\left[{\rm O}\,\small {III}\right] }^{\text{int}} \propto L_{14\text{--}195}$ ) with a large scatter ( R Pear = 0.64, = 0.62 dex) and a similarly large scatter with the intrinsic 2–10 keV to [O iii ] luminosities ( R Pear = 0.63, = 0.63 dex). Correlations of the hard X-ray fluxes with the fluxes of high-ionization narrow lines ([O iii ], He ii , [Ne iii ] and [Ne v ]) are not significantly better than with the low-ionization lines (H α, [S ii ]). Factors like obscuration or physical slit size are not found to be a significant part of the large scatter. In contrast, the optical emission lines show much better correlations with each other ( = 0.3 dex) than with the X-ray flux. The inherent large scatter questions the common usage of narrow emission lines as AGN bolometric luminosity indicators and suggests that other issues such as geometrical differences in the scattering of the ionized gas or long-term AGN variability are important.

Description: In this paper, we report the results of an X-ray monitoring campaign on the heavily obscured Seyfert galaxy, Markarian 3, carried out between the fall of 2014 and the spring of 2015 with NuSTAR , Suzaku and XMM–Newton . The hard X-ray spectrum of Markarian 3 is variable on all the time-scales probed by our campaign, down to a few days. The observed continuum variability is due to an intrinsically variable primary continuum seen in transmission through a large, but still Compton-thin column density ( N H ~ 0.8–1.1 x 10 24  cm –2 ). If arranged in a spherical-toroidal geometry, the Compton scattering matter has an opening angle ~=66°, and is seen at a grazing angle through its upper rim (inclination angle ~=70°). We report a possible occultation event during the 2014 campaign. If the torus is constituted by a system of clouds sharing the same column density, this event allows us to constrain their number (17 ± 5) and individual column density, [~= (4.9 ± 1.5) x 10 22  cm –2 ]. The comparison of IR and X-ray spectroscopic results with state-of-the art ‘torus’ models suggests that at least two-thirds of the X-ray obscuring gas volume might be located within the dust sublimation radius. We report also the discovery of an ionized absorber, characterized by variable resonant absorption lines due to He- and H-like iron. This discovery lends support to the idea that moderate column density absorbers could be due to clouds evaporated at the outer surface of the torus, possibly accelerated by the radiation pressure due to the central AGN emission leaking through the patchy absorber.

Description: We investigate the 1.4-GHz radio properties of 92 nearby ( z 〈 0.05) ultra-hard X-ray selected active galactic nuclei (AGNs) from the Swift Burst Alert Telescope (BAT) sample. Through the ultra-hard X-ray selection, we minimize the biases against obscured or Compton-thick AGNs as well as confusion with emission derived from star formation that typically affect AGN samples selected from the ultraviolet, optical and infrared wavelengths. We find that all the objects in our sample of nearby, ultra-hard X-ray selected AGNs are radio quiet; 83 per cent of the objects are classed as high-excitation galaxies and 17 per cent as low-excitation galaxies. While these low- z BAT sources follow the radio–far-infrared correlation in a similar fashion to star-forming galaxies, our analysis finds that there is still significant AGN contribution in the observed radio emission from these radio-quiet AGNs. In fact, the majority of our BAT sample occupy the same X-ray–radio Fundamental Plane as has been observed in other samples, which include radio-loud AGNs – evidence that the observed radio emission (albeit weak) is connected to the AGN accretion mechanism, rather than star formation.

Description: We present a NuSTAR and XMM – Newton monitoring campaign in 2014/2015 of the Compton-thick Seyfert 2 galaxy, NGC 1068. During the 2014 August observation, we detect with NuSTAR a flux excess above 20 keV (32 ± 6 per cent) with respect to the 2012 December observation and to a later observation performed in 2015 February. We do not detect any spectral variation below 10 keV in the XMM – Newton data. The transient excess can be explained by a temporary decrease of the column density of the obscuring material along the line of sight (from N H ~= 10 25  cm –2 to N H  = 6.7 ± 1.0  x  10 24  cm –2 ), which allows us for the first time to unveil the direct nuclear radiation of the buried active galactic nucleus in NGC 1068 and to infer an intrinsic 2–10 keV luminosity L $_{\rm X}=7^{+7}_{-4} \times 10^{43}$  erg s –1 .

Description: The X-ray Baldwin effect is the inverse correlation between the equivalent width (EW) of the narrow component of the iron Kα line and the X-ray luminosity of active galactic nuclei (AGN). A similar trend has also been observed between Fe Kα EW and the Eddington ratio ( Edd ). Using Chandra /High Energy Grating results of Shu et al. and bolometric corrections we study the relation between EW and Edd , and find that log EW = ( – 0.13 ± 0.03)log Edd + 1.47. We explore the role of the known positive correlation between the photon index of the primary X-ray continuum and Edd on the X-ray Baldwin effect. We simulate the iron Kα line emitted by populations of unabsorbed AGN considering three different geometries of the reflecting material: toroidal, spherical–toroidal and slab. We find that the – Edd correlation cannot account for the whole X-ray Baldwin effect, unless a strong dependence of on Edd , such as the one recently found by Risaliti et al. and Jin et al., is assumed. No clear correlation is found between EW and . We conclude that a good understanding of the slope of the – Edd relation is critical to assess whether the trend plays a leading or rather a marginal role in the X-ray Baldwin effect.

Description: The narrow Fe Kα line is one of the main signatures of the reprocessing of X-ray radiation from the material surrounding supermassive black holes, and it has been found to be omnipresent in the X-ray spectra of active galactic nuclei (AGN). In this work, we study the characteristics of the narrow Fe Kα line in different types of AGN. Using the results of a large Suzaku study, we find that Seyfert 2s have on average lower Fe Kα luminosities than Seyfert 1s for the same 10–50 keV continuum luminosity. Simulating dummy Seyfert 1s and Seyfert 2s populations using physical torus models of X-ray reflected emission, we find that this difference can be explained by means of different average inclination angles with respect to the torus, as predicted by the unified model. Alternative explanations include differences in the intensities of Compton humps, in the photon index distributions or in the average iron abundances. We show that the ratio between the flux of the broad and narrow Fe Kα line in the 6.35–6.45 keV range depends on the torus geometry considered, and is on average 〈25 per cent and 〈15 per cent for type-I and type-II AGN, respectively. We find evidence of absorption of the narrow Fe Kα line flux in Compton-thick AGN, which suggests that part of the reflecting material is obscured. We estimate that on average in obscured AGN the reflected radiation from neutral material is seen through a column density which is 1/4 of that absorbing the primary X-ray emission. This should be taken into account in synthesis models of the CXB and when studying the luminosity function of heavily obscured AGN. We detect the first evidence of the X-ray Baldwin effect in Seyfert 2s, with the same slope as that found for Seyfert 1s, which suggests that the mechanism responsible for the decrease of the equivalent width with the continuum luminosity is the same in the two classes of objects.

Description: The primary source of emission of active galactic nuclei (AGNs), the accretion disc, is surrounded by an optically and geometrically thick dusty structure (‘the so-called dusty torus’). The infrared radiation emitted by the dust is nothing but a reprocessed fraction of the accretion disc emission, so the ratio of the torus to the AGN luminosity ( L torus / L AGN ) should corresponds to the fraction of the sky obscured by dust, i.e. the covering factor. We undertook a critical investigation of the L torus / L AGN as the dust covering factor proxy. Using state-of-the-art 3D Monte Carlo radiative transfer code, we calculated a grid of spectral energy distributions (SEDs) emitted by the clumpy two-phase dusty structure. With this grid of SEDs, we studied the relation between L torus / L AGN and the dust covering factor for different parameters of the torus. We found that in the case of type 1 AGNs the torus anisotropy makes L torus / L AGN underestimate low covering factors and overestimate high covering factors. In type 2 AGNs L torus / L AGN always underestimates covering factors. Our results provide a novel easy-to-use method to account for anisotropy and obtain correct covering factors. Using two samples from the literature, we demonstrated the importance of our result for inferring the obscured AGN fraction. We found that after the anisotropy is properly accounted for, the dust covering factors show very weak dependence on L AGN , with values in the range of 0.6–0.7. Our results also suggest a higher fraction of obscured AGNs at high luminosities than those found by X-ray surveys, in part owing to the presence of a Compton-thick AGN population predicted by population synthesis models.

Description: We investigate the observed relationship between black hole mass ( M BH ), bolometric luminosity ( L bol ) and Eddington ratio ( Edd ) with optical emission-line ratios ([N  ii ] 6583/Hα, [S  ii ] 6716, 6731/Hα, [O  i ] 6300/Hα, [O  iii ] 5007/Hβ, [Ne  iii ] 3869/Hβ and He  ii 4686/Hβ) of hard X-ray-selected active galactic nuclei (AGN) from the BAT AGN Spectroscopic Survey. We show that the [N  ii ] 6583/Hα ratio exhibits a significant correlation with Edd ( R Pear = –0.44, p -value = 3 x 10 –13 , = 0.28 dex), and the correlation is not solely driven by M BH or L bol . The observed correlation between [N  ii ] 6583/Hα ratio and M BH is stronger than the correlation with L bol , but both are weaker than the Edd correlation. This implies that the large-scale narrow lines of AGN host galaxies carry information about the accretion state of the AGN central engine. We propose that [N  ii ] 6583/Hα is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure Edd and thus M BH from the measured L bol , even for high-redshift obscured AGN. We briefly discuss possible physical mechanisms behind this correlation, such as the mass–metallicity relation, X-ray heating, and radiatively driven outflows.