ALBERT

Description: In this paper, we present the results obtained with a monitoring programme (23 days long) performed with Swift -XRT on the local Seyfert galaxy Mrk 915. The light-curve analysis shows a significant count rate variation (about a factor of 2–3) on a time-scale of a few days, while the X-ray colours show a change in the spectral curvature below 2 keV and the presence of two main spectral states. From the spectral analysis we find that the observed variations can be explained by the change of the intrinsic nuclear power (about a factor of 1.5) coupled with a change of the properties of an ionized absorber. The quality of the data prevents us from firmly establishing if the spectral variation is due to a change in the ionization state and/or in the covering factor of the absorbing medium. The latter scenario would imply a clumpy structure of the ionized medium. By combining the information provided by the light curve and the spectral analyses, we can derive some constraints on the location of the absorber under the hypotheses of either homogeneous or clumpy medium. In both cases, we find that the absorber should be located inside the outer edge of an extended torus and, in particular, under the clumpy hypothesis, it should be located near, or just outside, to the broad emission line region.

Description: We investigate the mid-infrared (mid-IR) properties of the largest (42 objects) sample of radio-loud narrow-line Seyfert 1 (RL NLS1) collected to date, using data from the Wide-field Infrared Survey Explorer . We analyse the mid-IR colours of these objects and compare them to what is expected from different combinations of AGN and galaxy templates. We find that, in general, the host galaxy emission gives an important contribution to the observed mid-IR flux in particular at the longest wavelengths ( W 3, at 12 μm, and W 4, at 22 μm). In about half of the sources (22 objects), we observe a very red mid-IR colour ( W 4 – W 3 〉 2.5) that can be explained only using a starburst galaxy template (M82). Using the 22 μm luminosities, corrected for the AGN contribution, we have then estimated the star formation rate (SFR) for 20 of these ‘red’ RL NLS1, finding values ranging from 10 to 500 M yr –1 . For the RL NLS1 showing bluer colours, instead, we cannot exclude the presence of a star-forming (SF) host galaxy although, on average, we expect a lower SFR. Studying the radio (1.4 GHz) to mid-IR (22 μm) flux ratios of the RL NLS1 in the sample, we found that in ~10 objects the SF activity could represent the most important component also at radio frequencies, in addition (or in alternative) to the relativistic jet. We conclude that both the mid-IR and the radio emission of RL NLS1 are a mixture of different components, including the relativistic jet, the dusty torus and an intense SF activity.

Description: In this paper, we present the results obtained with a monitoring programme (23 days long) performed with Swift -XRT on the local Seyfert galaxy Mrk 915. The light-curve analysis shows a significant count rate variation (about a factor of 2–3) on a time-scale of a few days, while the X-ray colours show a change in the spectral curvature below 2 keV and the presence of two main spectral states. From the spectral analysis we find that the observed variations can be explained by the change of the intrinsic nuclear power (about a factor of 1.5) coupled with a change of the properties of an ionized absorber. The quality of the data prevents us from firmly establishing if the spectral variation is due to a change in the ionization state and/or in the covering factor of the absorbing medium. The latter scenario would imply a clumpy structure of the ionized medium. By combining the information provided by the light curve and the spectral analyses, we can derive some constraints on the location of the absorber under the hypotheses of either homogeneous or clumpy medium. In both cases, we find that the absorber should be located inside the outer edge of an extended torus and, in particular, under the clumpy hypothesis, it should be located near, or just outside, to the broad emission line region.

Description: We present SDSS J143244.91+301435.3, a new case of a radio-loud narrow-line Seyfert 1 (RL NLS1) with a relatively high radio power ( P 1.4 GHz = 2.1 10 25  W Hz –1 ) and large radio-loudness parameter ( R 1.4 = 600 ± 100). The radio source is compact with a linear size below ~1.4 kpc but, in contrast to most of the RL NLS1 discovered so far with such a high R 1.4 , its radio spectrum is very steep (α = 0.93, S    –α ) and does not support a ‘blazar-like’ nature. Both the small mass of the central supermassive black hole and the high accretion rate relative to the Eddington limit estimated for this object (3.2 10 7  M and 0.27, respectively, with a formal error of ~0.4 dex for both quantities) are typical of the NLS1 class. Through modelling the spectral energy distribution of the source, we have found that the galaxy hosting SDSS J143244.91+301435.3 is undergoing quite intense star formation (SFR = 50 M  yr –1 ), which, however, is expected to contribute only marginally (~1 per cent) to the observed radio emission. The radio properties of SDSS J143244.91+301435.3 are remarkably similar to those of compact steep-spectrum (CSS) radio sources, a class of active galactic nuclei (AGN) mostly composed of young radio galaxies. This may suggest a direct link between these two classes of AGN, with CSS sources possibly representing the misaligned version (the so-called ‘parent population’) of RL NLS1 showing blazar characteristics.

Description: Although absorbed quasars are extremely important for our understanding of the energetics of the Universe, the main physical parameters of their central engines are still poorly known. In this work, we present and study a complete sample of 14 quasars (QSOs) that are absorbed in the X-rays (column density N H 〉 4 x 10 21 cm –2 and X-ray luminosity L 2-10 keV 〉 10 44 ergs –1 ; XQSO2) belonging to the XMM – Newton Bright Serendipitous Survey (XBS). From the analysis of their ultraviolet-to-mid-infrared spectral energy distribution, we can separate the nuclear emission from the host galaxy contribution, obtaining a measurement of the fundamental nuclear parameters, like the mass of the central supermassive black hole and the value of Eddington ratio, Edd . Comparing the properties of XQSO2s with those previously obtained for the X-ray unabsorbed QSOs in the XBS, we do not find any evidence that the two samples are drawn from different populations. In particular, the two samples span the same range in Eddington ratios, up to Edd ~ 0.5; this implies that our XQSO2s populate the ‘forbidden region’ in the so-called ‘effective Eddington limit paradigm’. A combination of low grain abundance, presence of stars inwards of the absorber, and/or anisotropy of the disc emission can explain this result.

Description: We have determined the relation between the AGN luminosities at rest-frame 6 μm associated with the dusty torus emission and at 2–10 keV energies using a complete, X-ray-flux-limited sample of 232 AGN drawn from the Bright Ultra-hard XMM-Newton Survey. The objects have intrinsic X-ray luminosities between 10 42 and 10 46 erg s –1 and redshifts from 0.05 to 2.8. The rest-frame 6 μm luminosities were computed using data from the Wide-field Infrared Survey Explorer and are based on a spectral energy distribution decomposition into AGN and galaxy emission. The best-fitting relationship for the full sample is consistent with being linear, L 6 μm $L_{\rm 2{\rm -}10\,keV}^{0.99\pm 0.03}$ , with intrinsic scatter,  log  L 6 μm  ~ 0.35 dex. The $L_{\rm 6\,\mu m}/L_{\rm 2{\rm -}10\,keV}$ luminosity ratio is largely independent of the line-of-sight X-ray absorption. Assuming a constant X-ray bolometric correction, the fraction of AGN bolometric luminosity reprocessed in the mid-IR decreases weakly, if at all, with the AGN luminosity, a finding at odds with simple receding torus models. Type 2 AGN have redder mid-IR continua at rest-frame wavelengths 〈12 μm and are overall ~1.3–2 times fainter at 6 μm than type 1 AGN at a given X-ray luminosity. Regardless of whether type 1 and type 2 AGN have the same or different nuclear dusty toroidal structures, our results imply that the AGN emission at rest-frame 6 μm is not isotropic due to self-absorption in the dusty torus, as predicted by AGN torus models. Thus, AGN surveys at rest-frame ~6 μm are subject to modest dust obscuration biases.

Description: The cosmic history of the growth of supermassive black holes in galactic centres parallels that of star formation in the Universe. However, an important fraction of this growth occurs inconspicuously in obscured objects, where ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio ( f x / f o ) is expected to be an efficient tool to find out these obscured accreting sources. We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the most extreme cases of this population (those with f x / f o  〉 50, EXO50 sources hereafter), using a well-defined sample of seven X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (~70 per cent of the sample) in the bright flux regime explored by our survey ( f (2–10 keV)  ≥ 1.5 10 –13 erg cm –2 s –1 ) are associated with obscured AGN ( N H  〉 10 22  cm –2 ), spanning a redshift range between 0.75 and 1 and characterized by 2–10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 10 44 erg s –1 ). We did not find compelling evidence of Compton thick active galacic nuclei (AGN). Overall, the EXO50 type 2 QSOs do not seem to be different from standard X-ray-selected type 2 QSOs in terms of nuclear absorption; a very high AGN/host galaxy ratio seems to play a major role in explaining their extreme properties. Interestingly, three out of five EXO50 type 2 QSO objects can be classified as extreme dust-obscured galaxies (EDOGs, f 24 μm /f R ≥ 2000), suggesting that a very high AGN/host ratios (along with the large amount of dust absorption) could be the natural explanation also for a part of the EDOG population. The remaining two EXO50 sources are classified as BL Lac objects, having rather extreme properties, and which are good candidates for TeV emission.

Description: This paper presents a new sample of BL Lac objects selected from a deep (30 mJy) radio survey of flat spectrum radio sources (the CLASS blazar survey). The sample is one of the largest well-defined samples in the low-power regime with a total of 130 sources of which 55 satisfy the ‘classical’ optical BL Lac selection criteria, and the rest have indistinguishable radio properties. The primary goal of this study is to establish the radio luminosity function (RLF) on firm statistical ground at low radio luminosities where previous samples have not been able to investigate. The gain of taking a peek at lower powers is the possibility to search for the flattening of the luminosity function which is a feature predicted by the beaming model but which has remained elusive to observational confirmation. In this study, we extend for the first time the BL Lac RLF down to very low radio powers ~10 22  W Hz –1 , i.e. two orders of magnitude below the RLF currently available in the literature. In the process, we confirm the importance of adopting a broader, and more physically meaningful set of classification criteria to avoid the systematic missing of low-luminosity BL Lacs. Thanks to the good statistics we confirm the existence of weak but significant positive cosmological evolution for the BL Lac population, and we detect, for the first time the flattening of the RLF at L  ~ 10 25  W Hz –1 in agreement with the predictions of the beaming model.

Description: We investigate the possible link between mergers and the enhanced activity of supermassive black holes (SMBHs) at the centre of galaxies, by comparing the merger fraction of a local sample (0.003 ≤ z  〈 0.03) of active galaxies – 59 active galactic nuclei host galaxies selected from the All-Sky Swift Burst Alert Telescope (BAT) Survey – with an appropriate control sample (247 sources extracted from the HyperLeda catalogue) that has the same redshift distribution as the BAT sample. We detect the interacting systems in the two samples on the basis of non-parametric structural indexes of concentration ( C ), asymmetry ( A ), clumpiness ( S ), Gini coefficient ( G ) and second-order momentum of light ( M 20 ). In particular, we propose a new morphological criterion, based on a combination of all these indexes, that improves the identification of interacting systems. We also present a new software – p y cass o ( python CAS software) – for the automatic computation of the structural indexes. After correcting for the completeness and reliability of the method, we find that the fraction of interacting galaxies among the active population (20 ${^{+ 7}_{- 5}}$  per cent) exceeds the merger fraction of the control sample (4 ${^{+ 1.7}_{- 1.2}}$  per cent). Choosing a mass-matched control sample leads to equivalent results, although with slightly lower statistical significance. Our findings support the scenario in which mergers trigger the nuclear activity of SMBHs.