Professor Roger Davies

Abstract:

We compare high angular resolution mid-infrared (mid-IR) and Atacama Large Millimeter/submillimeter Array (ALMA) far-infrared (far-IR) images of twelve nearby (median 21 Mpc) Seyfert galaxies selected from the Galaxy Activity, Torus, and Outflow Survey (GATOS). The mid-IR unresolved emission contributes more than 60% of the nuclear (diameters of 1.5″ ∼ 150 pc) emission in most galaxies. By contrast, the ALMA 870 μm continuum emission is mostlyresolved with a median diameter of 42 pc and typically along the equatorial direction of the torus (Paper I). The Eddington ratios and nuclear hydrogen column densities (N_H) of half the sample are favorable to launching polar and/or equatorial dusty winds, according to numerical simulations. Six of these show mid-IR extended emission approximately in the polar direction as traced by the narrow line region and perpendicular to the ALMA emission. In a few galaxies, the nuclear N_H might be too high to uplift large quantities of dusty material along the polar direction. Five galaxies have low N_H and/or Eddington ratios and thus polar dusty winds are not likely. We generated new radiative transfer CAT3D-WIND disk+wind models and model images at 8, 12, and 700 μm. We tailored these models to the properties of the GATOS Seyferts in this work. At low wind-to-disk cloud ratios, the far-IR model images have disk- and ring-like morphologies. The characteristic “X”-shape associated with dusty winds is seen better in the far-IR at intermediate-high inclinations for the extended-wind configurations. In most of the explored models, the mid-IR emission mainly comes from the inner part of the disk and cone. Extended biconical and one-sided polar mid-IR emission is seen in extended-wind configurations and high wind-to-disk cloud ratios. When convolved to the typical angular resolution of our observations, the CAT3D-WIND model images reproduce qualitative aspects of the observed mid- and far-IR morphologies. However, low to intermediate values of the wind-to-disk ratio are required to account for the observed large fractions of unresolved mid-IR emission in our sample. This work and Paper I provide observational support for the torus+wind scenario. The wind component is more relevant at high Eddington ratios and/or active galactic nucleus luminosities, and polar dust emission is predicted at nuclear column densities of up to ∼10²⁴ cm⁻². The torus or disk component, on the other hand, prevails at low luminosities and/or Eddington ratios.

Abstract:

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A ⋆ . We obtained light curves in the M , K , and H bands in the mid- and near-infrared and in the 2 − 8 keV and 2 − 70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νL ν ∝ ν 0.5 ± 0.2 ; the spectral slope observed in the X-ray band is νL ν ∝ ν −0.7 ± 0.5 . Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γ max , which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γ max ∼ 500, while cooled synchrotron model requires acceleration up to γ max ∼ 5 × 10 4 . The synchrotron self-Compton scenario requires electron densities of 10 10 cm −3 that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1 R S , the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γ max , implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.