Conditional Image Diffusion with Interferometric Closure Invariants: Independent EHT Imaging of Centaurus A and 3C 279

A mathematically consistent rotating black hole model in loop quantum gravity (LQG) is yet lacking. The scarcity of rotating black hole solutions in LQG substantially hampers the development of testing LQG from observations, e.g., from the Event Horizon Telescope (EHT) observations. The EHT observation revealed event horizon-scale images of the supermassive black holes Sgr A* and M87*. The EHT results are consistent with the shadow of a Kerr black hole of general relativity. We present LQG-motivated rotating black hole (LMRBH) spacetimes, which are regular everywhere and asymptotically encompass the Kerr black hole as a particular case. The LMRBH metric describes a multi-horizon black hole in the sense that it can admit up to three horizons, such that an extremal LMRBH, unlike the Kerr black hole, refers to a black hole with angular momentum a > M. The metric, depending on the parameters, describes (1) black holes with only one horizon (BH-I), (2) black holes with an event horizon and a Cauchy horizon (BH-II), (3) black holes with three horizons (BH-III), or (4) no-horizon spacetime, which we show is almost ruled out by EHT observations. We constrain the LQG parameter with the aid of the EHT shadow observational results of M87* and Sgr A*, respectively, for inclination angles of 17° and 50°. In particular, the VLTI bound for Sgr A*, δ ∈ (−0.17, 0.01), constrains the parameters (a, l) such that for 0 < l ≤ 0.347851M (l ≤ 2 × 106 km), the allowed range of a is (0, 1.0307M). Together with the EHT bounds of Sgr A* and M87* observables, our analysis concludes that a substantial part of BH-I and BH-II parameter space agrees with the EHT results of M87* and Sgr A*. While the EHT M87* results totally rule out BH-III, but not that by Sgr A*.

In 2005 Zakharov et al. predicted an opportunity to reconstruct a shadow in Sgr A* with ground based or space—ground interferometer acting in mm or sub-mm band (the Millimetron was mentioned for such needs). The prediction was confirmed in May 2022 since the Event Horizon Telescope (EHT) Collaboration presented results of a shadow reconstruction for our Galactic Center (the shadow around the supermassive black hole in M87 was reconstructed in 2019). These reconstructions were based on EHT observations done in 2017. In 2005 Zakharov et al. also derived analytical expressions for shadow size as a function of charge for Reissner–Nordström metric and later these results were generalized for a tidal charge case. We discuss opportunities to evaluate parameters of alternative theories of gravity with shadow size estimates done by the EHT Collaboration, in particular, a tidal charge could be estimated from these observations. We also discuss opportunities to use Millimetron facilities for shadow reconstructions in M87* and Sgr A*. In our recent studies we discuss shadow formations for cases where naked singularities, wormholes or more exotic models substitute conventional black holes in galactic centers.

Context. In a series of publications, we describe a comprehensive comparison of Event Horizon Telescope (EHT) data with theoretical models of the observed Sagittarius A* (Sgr A*) and Messier 87* (M87*) horizon-scale sources. Aims. In this article, we report on improvements made to our observational data reduction pipeline and present the generation of observables derived from the EHT models. We make use of ray-traced general relativistic magnetohydrodynamic simulations that are based on different black hole spacetime metrics and accretion physics parameters. These broad classes of models provide a good representation of the primary targets observed by the EHT. Methods. We describe how we combined multiple frequency bands and polarization channels of the observational data to improve our fringe-finding sensitivity and stabilization of atmospheric phase fluctuations. To generate realistic synthetic data from our models, we took the signal path as well as the calibration process, and thereby the aforementioned improvements, into account. We could thus produce synthetic visibilities akin to calibrated EHT data and identify salient features for the discrimination of model parameters. Results. We have produced a library consisting of an unparalleled 962 000 synthetic Sgr A* and M87* datasets. In terms of baseline coverage and noise properties, the library encompasses 2017 EHT measurements as well as future observations with an extended telescope array. Conclusions. We differentiate between robust visibility data products related to model features and data products that are strongly affected by data corruption effects. Parameter inference is mostly limited by intrinsic model variability, which highlights the importance of long-term monitoring observations with the EHT. In later papers in this series, we will show how a Bayesian neural network trained on our synthetic data is capable of dealing with the model variability and extracting physical parameters from EHT observations. With our calibration improvements, our newly reduced EHT datasets have a considerably better quality compared to previously analyzed data.

We show how Event Horizon Telescope (EHT) observations of the supermassive object at the center of M87 can constrain deviations from General Relativity (GR) in a relatively model-independent way. We focus on the class of theories whose deviations from GR modify black holes into alternative compact objects whose properties approach those of an ordinary black hole sufficiently far from the would-be event horizon. We examine this class for two reasons: (i) they tend to reproduce black-hole expectations for astrophysical accretion disks (and so do not undermine the evidence linking black holes to active galactic nuclei); (ii) they lend themselves to a robust effective-field-theory treatment that expands in powers of ℓ/r, where ℓ is the fundamental length scale that sets the distance over which deviations from GR are significant and r is a measure of distance from the would-be horizon. At leading order the observational impact of these types of theories arise as modifications to the transmission and reflection coefficients of modes as they approach the horizon. We show how EHT observations can constrain this reflection coefficient, assuming only that the deviations from GR are small enough to be treated perturbatively. Our preliminary analysis indicates that such reflection coefficients can already be constrained to be less than of order 10% (corresponding to ℓ ≲ 100 μ m), and so can rule out some benchmark cases used when seeking black-hole echoes. The precise bounds depend on the black hole spin, as well as on detailed properties of the reflection coefficient (such as its dependence on angular direction).

Black hole (BH) shadow observations and gravitational wave astronomy have become crucial approaches for exploring BH physics and testing gravitational theories in extreme environments. This paper investigates the charged black hole with scalar hair (CBH-SH) derived from the Einstein–Maxwell-conformal coupled scalar (EMCS) theory. We first constrain the parameter space $$(Q/M, s/M^2)$$ ( Q / M , s / M 2 ) of the BH using the Event Horizon Telescope (EHT) observations of M87* and Sgr A*. The results show that M87* provides stronger constraints on positive scalar hair, constraining the scalar hair s within $$0\le s/M^2\le 0.4632$$ 0 ≤ s / M 2 ≤ 0.4632 and the charge Q within the range $$0\le Q/M\le 0.6806.$$ 0 ≤ Q / M ≤ 0.6806 . In contrast, Sgr A* imposes tighter constraints on negative scalar hair. When Q approaches zero, s is constrained within the range $$0\ge s/M^2\ge -0.0277.$$ 0 ≥ s / M 2 ≥ - 0.0277 . Overall, EHT observations can provide constraints at most on the order of $${\mathcal {O}}\left( {10}^{-1}\right) .$$ O 10 - 1 . Subsequently, we construct extreme mass ratio inspiral (EMRI) systems and calculate their gravitational waves to assess the detection capability of the LISA detector for these BHs. The results indicate that for central BHs of $$M={10}^6M_\odot ,$$ M = 10 6 M ⊙ , LISA is expected to detect scalar hair $$s/M^2$$ s / M 2 at the $${\mathcal {O}}\left( {10}^{-4}\right) $$ O 10 - 4 level and charge Q/M at the $${\mathcal {O}}\left( {10}^{-2}\right) $$ O 10 - 2 level, with detection sensitivity far exceeding the current EHT capabilities. This demonstrates the immense potential of EMRI gravitational wave observations in testing EMCS theory.

We investigate the presence and spatial characteristics of the jet base emission in M87* at $230 GHz $, enabled by the significantly enhanced ̆v coverage in the 2021 Event Horizon Telescope (EHT) observations. The integration of the 12 Kitt Peak Telescope (USA) and NOEMA (France) stations into the array introduces two critical intermediate-length baselines to SMT (USA) and IRAM 30 (Spain), providing sensitivity to emission structures at spatial scales of ∼ 250 μ as and ∼ 2500 μ as (∼ 0.02 pc and ∼ 0.2 pc ). Without these new baselines, previous EHT observations of the source in 2017 and 2018 lacked the capability to constrain emission on large scales, where a ``missing flux" of order ∼ 1,Jy is expected to reside. To probe these scales, we analyzed closure phases---robust against station-based gain calibration errors---and model the jet base emission using a simple Gaussian component offset from the compact ring emission at spatial separations $&gt; 100 μ as $. Our analysis revealed a Gaussian feature centered at (Δ R.A. ≈ 320 μ as Δ Dec. ≈ 60 μ as ), projected separation of ≈ 5500 AU with an estimated flux density of only ∼ 60 mJy implying that most of the missing flux identified in previous EHT studies had to originate from different, larger scales. Brighter emission at the relevant spatial scales is firmly ruled out, and the data do not favor more complex models. This component aligns with the inferred position of the large-scale jet and is therefore physically consistent with the emission of the jet base. While our findings point to detectable jet base emission at $230 GHz $, the limited coverage provided by only two intermediate baselines limits our ability to robustly reconstruct its morphology. Consequently, we treated the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate baseline coverage will be essential to constrain the structure and nature of this component with higher precision.

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

The Event Horizon Telescope (EHT) recently released an image of the supermassive black hole Sgr A* showing an angular shadow diameter d sh = 48.7 ± 7 μas and Schwarzschild shadow deviation , using a black hole mass . The EHT image of Sgr A* is consistent with a Kerr black hole’s expected appearance, and the results directly prove the existence of a supermassive black hole at the center of the Milky Way. Here, we use the EHT observational results for Sgr A* to investigate the constraints on its charge with the aid of Kerr-like black holes, paying attention to three leading rotating models, namely Kerr–Newman, Horndeski, and hairy black holes. Modeling the supermassive black hole Sgr A* as these Kerr-like black holes, we observe that the EHT results for Sgr A* place stricter upper limits on the parameter space of Kerr–Newman and Horndeski black holes than those placed by the EHT results for M87*. A systematic bias analysis reveals that observational results from future EHT experiments will place more precise limits on the charge of the black hole Sgr A*. Thus, the Kerr-like black holes and Kerr black holes are indiscernible in a substantial region of the EHT-constrained parameter space; the claim is substantiated by our bias analysis.

Event Horizon Telescope (EHT) observations of M87* provide a means of constraining the parameters of both the black hole and its surrounding plasma. However, the intrinsic variability of the emitting material introduces major sources of uncertainty, which complicates parameter inference. The precise nature of this variability remains uncertain, and previous studies have largely relied on general relativistic magnetohydrodynamic simulations to estimate its effects. Here, we fit a semianalytic, dual-cone model of the emitting plasma to multiple years of EHT observations to empirically assess the impact of intrinsic variability and improved array coverage on key measurements, including the black hole mass-to-distance ratio, spin, and viewing inclination. Despite substantial differences in the images of the two epochs, we find that the inferred mass-to-distance ratio remains stable and mutually consistent. The black hole spin is unconstrained for both observations, despite the improved baseline coverage in 2018. We show that intrinsic variability can contribute significantly to the inference error and that the inferred position angle and inclination of the black hole spin axis are discrepant between the two years. Our findings highlight both the promise and challenges of multiepoch EHT observations: while they can refine parameter constraints, they also reveal the limitations of simple parametric models in capturing the full source complexity. Our analysis—the first to fit semianalytic emission models to 2018 EHT observations—underscores the importance of quantifying data contributions from intrinsic variability in future high-resolution imaging studies of black hole environments and the role of repeated observations in quantifying these uncertainties.

We demonstrate the unprecedented capabilities of the Event Horizon Telescope (EHT) to image the innermost dark matter profile in the vicinity of the supermassive black hole at the center of the M87 radio galaxy. We present the first model of the synchrotron emission induced by dark matter annihilations from a spiky profile in the close vicinity of a supermassive black hole, accounting for strong gravitational lensing effects. Our results show that the EHT should readily resolve dark matter spikes if present. Moreover, the photon ring surrounding the silhouette of the black hole is clearly visible in the spike emission, which introduces observable small-scale structure into the signal. We find that the dark matter-induced emission provides an adequate fit to the existing EHT data, implying that in addition to the jet, a dark matter spike may account for a sizable portion of the millimeter emission from the innermost (subparsec) region of M87. Regardless, our results show that the EHT can probe very weakly annihilating dark matter. Current EHT observations already constrain very small cross sections, typically down to a few 10^{-31} cm^{3} s^{-1} for a 10 GeV candidate, close to characteristic values for p-wave-suppressed annihilation. Future EHT observations will further improve constraints on the DM scenario.

The Event Horizon Telescope (EHT) provides the unprecedented ability to directly resolve the structure and dynamics of black hole emission regions on scales smaller than their horizons. This has the potential to critically probe the mechanisms by which black holes accrete and launch outflows, and the structure of supermassive black hole spacetimes. However, accessing this information is a formidable analysis challenge for two reasons. First, the EHT natively produces a variety of data types that encode information about the image structure in nontrivial ways; these are subject to a variety of systematic effects associated with very long baseline interferometry and are supplemented by a wide variety of auxiliary data on the primary EHT targets from decades of other observations. Second, models of the emission regions and their interaction with the black hole are complex, highly uncertain, and computationally expensive to construct. As a result, the scientific utilization of EHT observations requires a flexible, extensible, and powerful analysis framework. We present such a framework, Themis, which defines a set of interfaces between models, data, and sampling algorithms that facilitates future development. We describe the design and currently existing components of Themis, how Themis has been validated thus far, and present additional analyses made possible by Themis that illustrate its capabilities. Importantly, we demonstrate that Themis is able to reproduce prior EHT analyses, extend these, and do so in a computationally efficient manner that can efficiently exploit modern high-performance computing facilities. Themis has already been used extensively in the scientific analysis and interpretation of the first EHT observations of M87.

Dynamical Chern-Simons (dCS) gravity has been attracting plenty of attentions due to the fact that it is a parity-violating modified theory of gravity that corresponds to a well-posed effective field theory in weak coupling approximation. In particular, a rotating black hole in dCS gravity is in contrast to the general relativistic counterparts. In this paper, we revisit the shadow of analytical rotating black hole spacetime in dCS modified gravity, based on which we study the shadow observables, discuss the constraint on the model parameters from the Event Horizon Telescope (EHT) observations, and analyze the real part of quasi-normal modes (QNMs) in the eikonal limit. In addition, we explore the deflection angle in weak gravitational field limit with the use of Gauss-Bonnet theorem. We find that the shadow related physics and the weak gravitational lensing effect are significantly influenced by the CS coupling, which could provide theoretical predictions for a future test of the dCS theory with EHT observations.

We investigate the astrophysical properties of the rotating Kazakov-Solodukhin (KS) black hole, a quantum-corrected extension of the Schwarzschild solution that removes the central singularity via an effective mass function. Extending the KS solution to the rotating case, we analyze its geodesic structure and explore the influence of the quantum correction parameter on observable quantities. In particular, we study the effective mass profile, the ergoregion, and photon motion, and compute the black hole shadow. Comparing the resulting shadows with the classical Kerr solution, we find that quantum corrections tend to enlarge the shadow radius and alter its shape. Using these theoretical predictions, we further constrain the parameter space of the rotating KS black hole by confronting the shadow properties with Event Horizon Telescope (EHT) observations of M87* and Sgr A*. Our results demonstrate that high-resolution black hole imaging provides a promising avenue for probing semiclassical modifications of general relativity.

The fast developments of radio astronomy open a new window to explore the properties of Dark Matter (DM). The recent direct imaging of the supermassive black hole (SMBH) at the center of M87 radio galaxy by the Event Horizon Telescope (EHT) collaboration is expected to be very useful to search for possible new physics. In this work, we illustrate that such results can be used to detect the possible synchrotron radiation signature produced by DM annihilation from the innermost region of the SMBH. Assuming the existence of a spike DM density profile, we obtain the flux density due to DM annihilation induced electrons and positrons, and derive new limits on the DM annihilation cross section via the comparison with the EHT integral flux density at 230 GHz. Our results show that the parameter space can be probed by the EHT observations is largely complementary to other experiments. For DM with typical mass regions of being weakly interacting massive particles, the annihilation cross section several orders of magnitude below the thermal production level can be excluded by the EHT observations under the density spike assumption. Future EHT observations may further improve the sensitivity on the DM searches, and may also provide a unique opportunity to test the interplay between DM and the SMBH.

We aim to leverage the transformational science enabled by the Event Horizon Telescope (EHT) to study the physics of, and near, the black holes in a sample of galaxies covering a large parameter space in supermassive black hole mass, accretion rate, and jet power. To this end, we work on a sample of nearby galaxies whose directly measured black hole masses and distances imply that 40 micro‐arcsec EHT observations will resolve the central engine at &lt;100 Schwarzschild radius resolution. To complement the near‐future EHT observations, we attempt to constrain the accretion properties of nearby active galactic nuclei by modeling the broadband spectral energy distribution using a general relativistic Monte Carlo radiative transfer code. These results will provide apriori information that is vital for an accurate ray‐tracing of EHT millimeter maps of accretion flows in the future.