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Register a new userVisibility of Black Hole Shadows in Low-luminosity AGN
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Accreting black holes tend to display a characteristic dark central region called the black-hole shadow, which depends only on spacetime/observer geometry and which conveys information about the black holes mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black-hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black-hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black-hole shadow, and obscuration of the black-hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black-hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black-hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black-hole shadow size to an accuracy of 5%. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations.
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We present the results of the analysis of a sample of 17 low-luminosity (L_x < 1e42 erg/s), radio loud AGNs in massive galaxies. The sample is extracted from the SDSS database and it spans uniformly a wide range in optical [OIII] emission line and radio luminosity, but within a narrow redshift range (0.05 < z < 0.11) and a narrow super massive black hole mass range (~ 1e8 M_sun). For these sources we measured core X-ray emission with the Chandra X-ray telescope and radio emission with the VLA. Our main goal is to establish which emission component, if any, can be regarded as the most reliable accretion/jet-power estimator at these regimes. In order to do so, we studied the correlation between emission line properties, radio luminosity, radio spectral slopes and X-ray luminosity, as well as more complex multi-variate relations involving black hole mass, such as the fundamental plane of black hole activity. We find that 15 out of 17 sources of our sample can be classified as Low-Excitation Galaxies (LEG) and their observed properties suggest X-ray and radio emission to originate from the jet basis. We also find that X-ray emission does not appear to be affected by nuclear obscuration and can be used as a reliable jet-power estimator. More generally, X-ray, radio and optical emission appear to be related, although no tight correlation is found. In accordance with a number of recent studies of this class of objects these findings may be explained by a lack of cold (molecular) gaseous structures in the innermost region of these massive galaxies.
A distinct visual signature occurs in black holes that are surrounded by optically thin and geometrically thick emission regions. This signature is a sharp-edged dip in brightness that is coincident with the black-hole shadow, which is the projection of the black holes unstable-photon region on the observers sky. We highlight two key mechanisms responsible for producing the sharp-edged dip: i) the reduction of intensity observed in rays that intersect the unstable-photon region, and thus the perfectly absorbing event horizon, versus rays that do not (blocking), and ii) the increase of intensity observed in rays that travel along extended, horizon-circling paths near the boundary of the unstable-photon region (path-lengthening). We demonstrate that the black-hole shadow is a distinct phenomenon from the photon ring, and that models exist in which the former may be observed, but not the latter. Additionally, we show that the black-hole shadow and its associated visual signature differ from the more model-dependent brightness depressions associated with thin-disk models, because for geometrically thick and optically thin emission regions, the blocking and path-lengthening effects are quite general. Consequentially, the black-hole shadow is a robust and fairly model-independent observable for accreting black holes that are in the deep sub-Eddington regime, such as low-luminosity active galactic nuclei (LLAGN).
We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semi-analytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that in excess of a million SMBH shadows meeting these criteria are potentially accessible to interferometric observations with sufficient angular resolution and sensitivity. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Our model predicts that with modest improvements to sensitivity, as many as $sim$5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.
We put active galactic nuclei (AGNs) with low-mass black holes on the fundamental plane of black hole accretion---the plane that relates X-ray emission, radio emission, and mass of an accreting black hole---to test whether or not the relation is universal for both stellar-mass and supermassive black holes. We use new Chandra X-ray and Very Large Array radio observations of a sample of black holes with masses less than $10^{6.3} M_{scriptscriptstyle odot}$, which have the best leverage for determining whether supermassive black holes and stellar-mass black holes belong on the same plane. Our results suggest that the two different classes of black holes both belong on the same relation. These results allow us to conclude that the fundamental plane is suitable for use in estimating supermassive black hole masses smaller than $sim 10^7 M_{scriptscriptstyle odot}$, in testing for intermediate-mass black holes, and in estimating masses at high accretion rates.
Causal concept for the general black hole shadow is investigated, instead of the photon sphere. We define several `wandering null geodesics as complete null geodesics accompanied by repetitive conjugate points, which would correspond to null geodesics on the photon sphere in Schwarzschild spacetime. We also define a `wandering set, that is, a set of totally wandering null geodesics as a counterpart of the photon sphere, and moreover, a truncated wandering null geodesic to symbolically discuss its formation. Then we examine the existence of a wandering null geodesic in general black hole spacetimes mainly in terms of Weyl focusing. We will see the essence of the black hole shadow is not the stationary cycling of the photon orbits which is the concept only available in a stationary spacetime, but their accumulation. A wandering null geodesic implies that this accumulation will be occur somewhere in an asymptotically flat spacetime.
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