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Register a new userProperties of AGN coronae in the NuSTAR era II: hybrid plasma
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The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous Active Galactic Nuclei (AGN). The continuum has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electron-positron pair production, as explored earlier (Paper I). Here we examine hybrid plasmas in which a mixture of thermal and nonthermal particles is present. Pair production from the nonthermal component reduces the temperature leading to a wider temperature range more consistent with observations.
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The focussing optics of NuSTAR have enabled high signal-to-noise spectra to be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic Black Hole Binaries (BHB). Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects. Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size. Our results indicate that coronae are hot and radiatively compact, lying close to the boundary of the region in the compactness - temperature diagram which is forbidden due to runaway pair production. The coincidence suggests that pair production and annihilation are essential ingredients in the coronae of AGN and BHB and that they control the shape of the observed spectra.
We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the E-CDFS, EGS and COSMOS fields. The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between $f_{rm 3-24 keV}approx10^{-14}$ and $6times10^{-13}$ erg/cm$^2$/s ($f_{rm 8-24 keV}approx3times10^{-14}-3times10^{-13}$ erg/cm$^2$/s) and redshifts of $z=0.04-3.21$. We produce composite spectra from the Chandra+NuSTAR data ($Eapprox2-40$ keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) AGN, and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is $Gamma=1.65_{-0.03}^{+0.03}$, flatter than $Gammaapprox1.8$ typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values ($Gamma=1.79_{-0.01}^{+0.01}$), a significant flattening is seen in NL AGN and heavily-absorbed sources ($Gamma=1.60_{-0.05}^{+0.08}$ and $Gamma=1.38_{-0.12}^{+0.12}$, respectively), likely due to the effect of absorption and to the contribution from Compton reflection to the high-energy flux (E>10 keV). We find that the typical reflection fraction in our spectra is $Rapprox0.5$ (for $Gamma=1.8$), with a tentative indication of an increase of the reflection strength with column density. While there is no significant evidence for a dependence of the photon index with X-ray luminosity in our sample, we find that $R$ decreases with luminosity, with relatively high levels of reflection ($Rapprox1.2$) for $L_{rm 10-40 keV}<10^{44}$ erg/s and $Rapprox0.3$ for $L_{rm 10-40 keV}>10^{44}$ erg/s AGN, assuming $Gamma=1.8$.
Aims. Study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser Active Galactic Nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with available physical quantities from VLBI mapping. Results. Most of the sources analyzed are heavily obscured, showing a column density in excess of $sim 10^{23}$ cm$^{-2}$. In particular, $79%$ are Compton-thick ($N_{rm H} > 1.5 times 10^{24}$ cm$^{-2}$). Using column densities measured by NuSTAR, with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, the torus dimensions can be predicted. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds connecting the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.
We present the analysis of a sample of 35 candidate Compton thick (CT-) active galactic nuclei (AGNs) selected in the nearby Universe (average redshift <z>~0.03) with the Swift-BAT 100-month survey. All sources have available NuSTAR data, thus allowing us to constrain with unprecedented quality important spectral parameters such as the obscuring torus line-of-sight column density (N_{H, z}), the average torus column density (N_{H, tor}) and the torus covering factor (f_c). We compare the best-fit results obtained with the widely used MyTorus (Murphy et al. 2009) model with those of the recently published borus02 model (Balokovic et al. 2018) used in the same geometrical configuration of MyTorus (i.e., with f_c=0.5). We find a remarkable agreement between the two, although with increasing dispersion in N_{H, z} moving towards higher column densities. We then use borus02 to measure f_c. High-f_c sources have, on average, smaller offset between N_{H, z} and N_{H, tor} than low-f_c ones. Therefore, low f_c values can be linked to a patchy torus scenario, where the AGN is seen through an over-dense region in the torus, while high-f_c objects are more likely to be obscured by a more uniform gas distribution. Finally, we find potential evidence of an inverse trend between f_c and the AGN 2-10 keV luminosity, i.e., sources with higher f_c values have on average lower luminosities.
A soft X-ray excess above the 2-10 keV power law extrapolation is generally observed in AGN X-ray spectra. Presently there are two competitive models to explain it: blurred ionized reflection and warm Comptonisation. In the latter case, observations suggest a corona temperature $sim$ 1 keV and a corona optical depth $sim$ 10. Moreover, radiative constraints from spectral fits with Comptonisation models suggest that most of the accretion power should be released in the warm corona. The disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. The true radiative properties of such a warm and optically thick plasma are not well-known, however. For instance, the importance of the Comptonisation process, the potential presence of strong absorption/emission features or the spectral shape of the output spectrum have been studied only very recently. We present in this paper simulations of warm and optically thick coronae using the TITAN radiative transfer code coupled with the NOAR Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by a hard X-ray emission and from below by an optically thick accretion disk is taken into account as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering almost entirely a non-dissipative accretion disk. These results confirm the warm Comptonisation as a valuable model for the soft X-ray excess.
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