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Register a new userDisk corona interaction: mechanism for the disk truncation and spectrum change in low luminosity AGN
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The truncation of an optically thick, geometrically thin accretion disk is investigated in the context of low luminosity AGN (LLAGN). We generalize the disk evaporation model used in the interpretative framework of black hole X-ray binaries by including the effect of a magnetic field in accretion disks surrounding supermassive black holes. The critical transition mass accretion rate for which the disk is truncated is found to be insensitive to magnetic effects, but its inclusion leads to a smaller truncation radius in comparison to a model without its consideration. That is, a thin viscous disk is truncated for LLAGN at an Eddington ratio less than 0.03 for a standard viscosity parameter ($alpha = 0.3$). An increase of the viscosity parameter results in a higher critical transition mass accretion rate and a correspondingly smaller truncation distance, the latter accentuated by greater magnetic energy densities in the disk. Based on these results, the truncation radii inferred from spectral fits of LLAGN published in the literature are consistent with the disk evaporation model. The infrared emission arising from the truncated geometrically thin accretion disks may be responsible for the red bump seen in such LLAGN.
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Accretion flows toward black holes can be of a quite different nature, described as an optically thick cool gas flow in a disk for high accretion rates or as a hot coronal optically thin gas flow for low accretion rates, possibly affected by outflowing gas. The detection of broad iron emission lines in active galactic nuclei (AGN) indicates the coexistence of corona and disk. The appearance and relative strength of such flows essentially depends on their interaction. Liu et al. suggested that condensation of gas from the corona to the disk allows to understand accretion flows of comparable strength of emission. Matter inflow due to gravitational capture of gas is important for the condensation process. We discuss observational features predicted by the model. Data from simultaneous observations of AGN with {it {Swifts}} X-ray and UV-optical telescopes are compared with the theoretical predictions. The frequent detection of broad iron K$alpha$ emission lines and the dependence of the emitted spectra on the Eddington ratio, described by the values of the photon index $Gamma$ and the two-point spectral index $alpha_{rm{ox}}$ are in approximate agreement with the predictions of the condensation model; the latter, however, with a large scatter. The model further yields a coronal emission concentrated in a narrow inner region as is also deduced from the analysis of emissivity profiles. The accretion flows in bright AGN could be described by the accretion of stellar wind or interstellar medium and its condensation into a thin disk.
We compile a blue AGN sample from SDSS and investigate the ratio of hard X-ray to bolometric luminosity in dependence on Eddington ratio and black hole mass. Our sample comprises 240 radio-quiet Seyfert 1 galaxies and QSOs. We find that the fraction of hard X-ray luminosity (log$(L_{rm 2-10 kev}/L_{rm bol})$) decreases with the increase of Eddington ratio. We also find that the fraction of hard X-ray luminosity is independent on the black hole mass for the radio-quiet AGNs. The relation of log$(L_{rm 2-10 kev}/L_{rm bol})$ decreasing with increasing Eddington ratio indicates that X-ray bolometric correction is not a constant, from a larger sample supporting the results of Vasudevan & Fabian (2007). We interpret our results by the disk corona evaporation/condensation model (Meyer et al. cite{me200}; Liu et al. 2002a; Liu et al. 2007). In the frame of this model, the Compton cooling becomes efficient in cooling of the corona at high accretion rate (in units of Eddington rate), leading to condensation of corona gas to the disk. Consequently, the relative strength of corona to the disk becomes weaker at higher Eddington ratio. Therefore, the fraction of hard X-ray emission to disk emission and hence to the bolometric emission is smaller at higher Eddington ratio. The independence of the fraction of hard X-ray luminosity on the mass of the black hole can also be explained by the disk corona model since the corona structure and luminosity (in units of Eddington luminosity) are independent on the mass of black holes.
The correlation observed between monochromatic X-ray and UV luminosities in radiatively-efficient active galactic nuclei (AGN) lacks a clear theoretical explanation despite being used for many applications. Such a correlation, with its small intrinsic scatter and its slope that is smaller than unity in log space, represents the compelling evidence that a mechanism regulating the energetic interaction between the accretion disk and the X-ray corona must be in place. This ensures that going from fainter to brighter sources the coronal emission increases less than the disk emission. We discuss here a self-consistently coupled disk-corona model that can identify this regulating mechanism in terms of modified viscosity prescriptions in the accretion disk. The model predicts a lower fraction of accretion power dissipated in the corona for higher accretion states. We then present a quantitative observational test of the model using a reference sample of broad-line AGN and modeling the disk-corona emission for each source in the $L_X-L_{UV}$ plane. We used the slope, normalization, and scatter of the observed relation to constrain the parameters of the theoretical model. For non-spinning black holes and static coronae, we find that the accretion prescriptions that match the observed slope of the $L_X-L_{UV}$ relation produce X-rays that are too weak with respect to the normalization of the observed relation. Instead, considering moderately-outflowing Comptonizing coronae and/or a more realistic high-spinning black hole population significantly relax the tension between the strength of the observed and modeled X-ray emission, while also predicting very low intrinsic scatter in the $L_X-L_{UV}$ relation. In particular, this latter scenario traces a known selection effect of flux-limited samples that preferentially select high-spinning, hence brighter, sources.
We investigate the accretion process in high-luminosity AGNs (HLAGNs) in the scenario of the disk evaporation model. Based on this model, the thin disk can extend down to the innermost stable circular orbit (ISCO) at accretion rates higher than $0.02dot{M}_{rm Edd}$; while the corona is weak since part of the coronal gas is cooled by strong inverse Compton scattering of the disk photons. This implies that the corona cannot produce as strong X-ray radiation as observed in HLAGNs with large Eddington ratio. In addition to the viscous heating, other heating to the corona is necessary to interpret HLAGN. In this paper, we assume that a part of accretion energy released in the disk is transported into the corona, heating up the electrons and thereby radiated away. We for the first time, compute the corona structure with additional heating, taking fully into account the mass supply to the corona and find that the corona could indeed survive at higher accretion rates and its radiation power increases. The spectra composed of bremsstrahlung and Compton radiation are also calculated. Our calculations show that the Compton dominated spectrum becomes harder with the increase of energy fraction ($f$) liberating in the corona, and the photon index for hard X-ray($2-10 rm keV$) is $2.2 < Gamma < 2.7 $. We discuss possible heating mechanisms for the corona. Combining the energy fraction transported to the corona with the accretion rate by magnetic heating, we find that the hard X-ray spectrum becomes steeper at larger accretion rate and the bolometric correction factor ($L_{rm bol}/L_{rm 2-10keV}$) increases with increasing accretion rate for $f<8/35$, which is roughly consistent with the observational results.
The absence of intrinsic broad line emission has been reported in a number of active galactic nuclei (AGN), including some with high Eddington ratios. Such true type 2 AGN are inherent to the disk-wind scenario for the broad line region: Broad line emission requires a minimal column density, implying a minimal outflow rate and thus a minimal accretion rate. Here we perform a detailed analysis of the consequences of mass conservation in the process of accretion through a central disk. The resulting constraints on luminosity are consistent with all the cases where claimed detections of true type 2 AGN pass stringent criteria, and predict that intrinsic broad line emission can disappear at luminosities as high as about 4x$10^{46}$ erg s$^{-1}$ and any Eddington ratio, though more detections can be expected at Eddington ratios below about 1%. Our results are applicable to every disk outflow model, whatever its details and whether clumpy or smooth, irrespective of the wind structure and its underlying dynamics. While other factors, such as changes in spectral energy distribution or covering factor, can affect the intensities of broad emission lines, within this scenario they can only produce true type 2 AGN of higher luminosity then those prescribed by mass conservation.
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