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Register a new userThe mid IR -- hard X-ray correlation in AGN and its implications for dusty torus models
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Context: We investigate mid-infrared and X-ray properties of the dusty torus invoked in the unification scenario for active galactic nuclei. Aims: We use the relation between mid IR and hard X-ray luminosities to constrain the geometry and physical state of the dusty torus. Methods: We present new VISIR observations of 17 nearby AGN and combine these with our earlier VISIR sample of 8 Seyfert galaxies. Combining these observations with X-ray data from the literature we study the correlation between their mid IR and hard X-ray luminosities. Results: A statistically highly significant correlation between the rest frame 12.3 mircon (L_MIR) and 2-10 keV (L_X) luminosities is found. Furthermore, with a probability of 97%, we find that Sy 1 and Sy 2 have the same distribution of L_MIR over L_X. Conclusions: The high resolution of our MIR imaging allows us to exclude any significant non-torus contribution to the AGN mid IR continuum,thereby implying that the similarity in the L_MIR / L_X ratio between Sy 1s and Sy 2s is intrinsic to AGN. We argue that this is best explained by clumpy torus models. The slope of the correlation is in good agreement with the expectations from the unified scenario and indicates little to no change of the torus geometry with luminosity. In addition, we demonstrate that the high angular resolution is crucial for AGN studies in the IR regime.
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We describe a complete volume limited sample of nearby active galaxies selected by their 14-195keV luminosity, and outline its rationale for studying the mechanisms regulating gas inflow and outflow. We describe also a complementary sample of inactive galaxies, selected to match the AGN host galaxy properties. The active sample appears to have no bias in terms of AGN type, the only difference being the neutral absorbing column which is two orders of magnitude greater for the Seyfert 2s. In the luminosity range spanned by the sample, log L_{14-195keV} [erg/s] = 42.4-43.7, the optically obscured and X-ray absorbed fractions are 50-65%. The similarity of these fractions to more distant spectroscopic AGN samples, although over a limited luminosity range, suggests that the torus does not strongly evolve with redshift. Our sample confirms that X-ray unabsorbed Seyfert 2s are rare, comprising not more than a few percent of the Seyfert 2 population. At higher luminosities, the optically obscured fraction decreases (as expected for the increasing dust sublimation radius), but the X-ray absorbed fraction changes little. We argue that the cold X-ray absorption in these Seyfert 1s can be accounted for by neutral gas in clouds that also contribute to the broad line region (BLR) emission; and suggest that a geometrically thick neutral gas torus co-exists with the BLR and bridges the gap to the dusty torus.
This is the second in a series of papers devoted to explore a set of six dusty models of active galactic nuclei (AGN) with available spectral energy distributions (SEDs). These models are the smooth torus by Fritz et al. (2006), the clumpy torus by Nenkova et al. (2008B), the clumpy torus by Hoenig & Kishimoto (2010), the two phase torus by Siebenmorgen et al. (2015), the two phase torus by Stalevski et al. (2016), and the wind model by Hoenig & Kishimoto (2017). The first paper explores discrimination among models and the parameter restriction using synthetic spectra (Gonzalez-Martin et al. 2019A). Here we perform spectral fitting of a sample of 110 AGN drawn from the Swift/BAT survey with Spitzer/IRS spectroscopic data. The aim is to explore which is the model that describes better the data and the resulting parameters. The clumpy wind-disk model by Hoenig & Kishimoto (2017) provides good fits for ~50% of the sample, and the clumpy torus model by Nenkova et al. (2008B) is good at describing ~30% of the objects. The wind-disk model by Hoenig & Kishimoto (2017) is better for reproducing the mid-infrared spectra of Type-1 Seyferts while Type-2 Seyferts are equally fitted by both models. Large residuals are found irrespective of the model used, indicating that the AGN dust continuum emission is more complex than predicted by the models or that the parameter space is not well sampled. We found that all the resulting parameters for our AGN sample are roughly constrained to 10-20% of the parameter space. The derived outer radius of the torus is smaller for the smooth torus by Fritz et al. (2006) and the two phase torus by Stalevski et al. (2016) than the one derived from the clumpy torus by (Nenkova et al. 2008B). Covering factors and line-of-sight viewing angles strongly depend on the model used. The total dust mass is the most robust derived quantity.
At distances from the active galaxy nucleus (AGN) where the ambient temperature falls below ~1500-1800 K, dust is able to survive. It is thus possible to have a large dusty structure present which surrounds the AGN. This is the first of two papers aiming at comparing six dusty torus models with available SEDs, namely Fritz et al. (2006), Nenkova et al. (2008B), Hoenig & Kishimoto (2010), Siebenmorgen et al. (2015), Stalevski et al. (2016), and Hoenig & Kishimoto (2017). In this first paper we use synthetic spectra to explore the discrimination between these models and under which circumstances they allow to restrict the torus parameters, while our second paper analyzes the best model to describe the mid-infrared spectroscopic data. We have produced synthetic spectra from current instruments: GTC/CanariCam and Spitzer /IRS and future JWST/MIRI and JWST/NIRSpec instruments. We find that for a reasonable brightness (F12um > 100mJy) we can actually distinguish among models except for the two pair of parent models. We show that these models can be distinguished based on the continuum slopes and the strength of the silicate features. Moreover, their parameters can be constrained within 15% of error, irrespective of the instrument used, for all the models but Hoenig & Kishimoto (2017). However, the parameter estimates are ruined when more than 50% of circumnuclear contributors are included. Therefore, future high spatial resolution spectra as those expected from JWST will provide enough coverage and spatial resolution to tackle this topic.
With mid-IR and near-IR long-baseline interferometers, we are now mapping the radial distribution of the dusty accreting material in AGNs at sub-pc scales. We currently focus on Type 1 AGNs, where the innermost region is unobscured and its intrinsic structure can be studied directly. As a first systematic study of Type 1s, we obtained mid-/near-IR data for small samples over ~3-4 orders of magnitudes in UV luminosity L of the central engine. Here we effectively trace the structure by observing dust grains that are radiatively heated by the central engine. Consistent with a naive expectation for such dust grains, the dust sublimation radius R_in is in fact empirically known to be scaling with L^1/2 from the near-IR reverberation measurements, and this is also supported by our near-IR interferometry. Utilizing this empirical relationship, we normalize the radial extent by R_in and eliminate the simple L^1/2 scaling for a direct comparison over the samples. We then find that, in the mid-IR, the overall size in units of R_in seems to become more compact in higher luminosity sources. More specifically, the mid-IR brightness distribution is rather well described by a power-law, and this power-law becomes steeper in higher luminosity objects. The near-IR flux does not seem to be a simple inward extrapolation of the mid-IR power-law component toward shorter wavelengths, but it rather comes from a little distinct brightness concentration at the inner rim region of the dust distribution. Its structure is not well constrained yet, but there is tentative evidence that this inner near-IR-emitting structure has a steeper radial distribution in jet-launching objects. All these should be scrutinized with further observations.
The detection of new clusters of galaxies or the study of known clusters of galaxies in X-rays can be complicated by the presence of X-ray point sources, the majority of which will be active galactic nuclei (AGN). This can be addressed by combining observations from a high angular resolution observatory (such as Chandra) with deeper data from a more sensitive observatory that may not be able to resolve the AGN (like XMM). However, this approach is undermined if the AGN varies in flux between the epochs of the observations. To address this we measure the characteristic X-ray variability of serendipitously detected AGN in 70 pairs of Chandra observations, separated by intervals of between one month and thirteen years. After quality cuts, the full sample consists of 1511 sources, although the main analysis uses a subset of 416 sources selected on the geometric mean of their flux in the pairs of observations, which eliminates selection biases. We find a fractional variability that increases with increasing interval between observations, from about 0.25 for observations separated by tens of days up to about 0.45 for observations separated by $sim 10$ years. As a rule of thumb, given the precise X-ray flux of a typical AGN at one epoch, its flux at a second epoch some years earlier or later can be predicted with a precision of about $60%$ due to its variability (ignoring any statistical noise). This is larger than the characteristic variability of the population by a factor of $sqrt{2}$ due to the uncertainty on the mean flux of the AGN due to a single prior measurement. The precision can thus be improved with multiple prior flux measurements (reducing the $sqrt{2}$ factor), or by reducing the interval between observations to reduce the characteristic variability.
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