Event Horizon Telescope Research Articles

Strong gravitational lensing by static black holes in effective quantum gravity

We investigate strong gravitational lensing by two static black hole models (Model-1 and Model-2) within the Effective Quantum Gravity (EQG) framework, characterized by mass M and parameter ζ. For ζ=0, they reduce to the Schwarzschild solution, and depending on the parameters, they describe black holes with an event and Cauchy horizon (Model-1), a single horizon (Model-2), or no horizons. Using supermassive black holes (SMBHs), Sgr A* and M87*, as lenses and integrating theoretical predictions with recent Event Horizon Telescope (EHT) data, we identify significant differences in lensing signatures due to quantum corrections. For Model-1, the deviations of the lensing observables |δθ∞| of black holes in EQG from Schwarzschild black hole, for SMBHs Sgr A* and M87*, can reach as much as 1.75μas and 1.32μas, while |δs| is about 30.12 nas for Sgr A* and 22.63 nas for M87*. The flux ratio of the first image to all subsequent packed images indicates that EQG black hole images are brighter than their Schwarzschild counterparts, with a deviation in the brightness ratio |δrmag| reaching up to 2.02. The time delays between the second and first images, denoted |δT2,1|, exhibit substantial deviations from the GR counterpart, reaching up to 1.53 min for Sgr A* and 1159.9 min for M87*. The Event Horizon Telescope (EHT) constraint on θsh of Sgr A* and M87*, within the 1-σ region, limits the parameter ζ. Our analysis concludes that EQG black holes are consistent with the EHT observations within a finite parameter space.

Bridging Scales: Coupling the Galactic Nucleus to the Larger Cosmic Environment

Abstract Coupling black hole (BH) feeding and feedback involves interactions across vast spatial and temporal scales that are computationally challenging to model. Tracking gas inflows and outflows from kiloparsec scales to the event horizon for non-spinning BHs in the presence of strong magnetic fields, H. Cho et al. report strong suppression of accretion on horizon scales and low (2%) feedback efficiency. In this letter, we explore the impact of these findings for the supermassive BHs M87* and Sgr A*, using high-resolution, non-cosmological, magnetohydrodynamic simulations with the FIRE-2 model. Without feedback, we find rapid BH growth due to “cooling flows,” with 2% feedback efficiency, while accretion is suppressed, the rates still remain higher than constraints from Event Horizon Telescope (EHT) data for M87* and Sgr A*. To match the EHT observations of M87*, an efficiency greater than 15% is required, suggesting the need to include enhanced feedback from BH spin. Similarly, a feedback efficiency of >15% is needed for Sgr A* to match the observationally estimated star formation rate of ≲2M ⊙ yr−1. Even with 100% feedback efficiency, the simulation-predicted Sgr A* accretion rate remains higher than EHT-inferred levels on average, while only episodically matching it, suggesting that Sgr A* is currently in a temporary quiescent phase. Bridging accretion and feedback across scales, we conclude that higher feedback efficiencies, possibly due to nonzero BH spin, are necessary to suppress “cooling flows” and match both the observed accretion and star formation rates in M87* and Sgr A*.

How Theory-laden are Observations of Black Holes?

Abstract We evaluate the roles general relativistic assumptions play in simulations used in recent observations of black holes including LIGO-Virgo and the Event Horizon Telescope. In both experiments simulations play an ampliative role, enabling the extraction of more information from the data than would be possible otherwise. This comes at a cost of theory-ladenness. We discuss the issue of inferential circularity, which arises in some applications; classify some of the epistemic strategies used to reduce the extent of theory-ladenness; and discuss ways in which these strategies are model independent.

Signatures of Black Hole Spin and Plasma Acceleration in Jet Polarimetry

Abstract We study the polarization of black hole jets on scales of 10−103 GM/c 2 and show that large spatial swings in the polarization occur at three characteristic distances from the black hole: the radius where the counter-jet dims, the radius where the magnetic field becomes azimuthally dominated (the light cylinder), and the radius where the plasma reaches its terminal Lorentz factor. To demonstrate the existence of these swings, we derive a correspondence between axisymmetric magnetohydrodynamic outflows and their force-free limits, which allows us to analytically compute the plasma kinematics and magnetic field structure of collimated, general relativistic jets. We then use this method to ray trace polarized images of black hole jets with a wide range of physical parameters, focusing on roughly face-on jets like that of M87. We show that the location of the polarization swings is strongly tied to the location of the light cylinder and thus to the black hole’s spin, illustrating a new method of measuring spin from polarized images of the jet. This signature of black hole spin should be observable by future interferometric arrays like the (Next Generation) Event Horizon Telescope, which will be able to resolve the polarized emission of the jet down to the near-horizon region at high dynamic range.

Traces of quantum fuzziness on the black hole shadow and particle deflection in the multi-fractional theory of gravity

Abstract In this paper, we investigate the properties of black holes within the framework of multi-fractional theories of gravity, focusing on the effects of q-derivatives and weighted derivatives. These modifications, which introduce scale-dependent spacetime geometries, alter black hole solutions in intriguing ways. Within these frameworks, we analyze two key observable phenomena - black hole shadows and particle deflection angle in the weak field limit - using both analytical techniques and observational data from the Event Horizon Telescope (EHT) for M87* and Sgr A*. The study from $q$-derivative formalism reveals that the multi-scale length $\ell_*$ influences the size of the black hole shadow in two ways, and modifies the weak deflection angle. Constraints on $\ell_*$ are derived from the EHT observations, showing significant deviations from standard Schwarzschild black hole predictions, which range from $10^{9}$ to $10^{10}$ orders of magnitude. Additionally, the weak deflection angle is computed using the non-asymptotic generalization of the Gauss-Bonnet theorem, revealing the effects of finite-distance and multi-scale parameters. Using the Sun for Solar System test, the constraints for $\ell_*$ range from $10^{8}$ to $10^{9}$ orders of magnitude. Results from the weighted derivative formalism generates a dS/AdS-like behavior, where smaller deviations are found in the strong field regime than in the weak field regime. The results suggest that while these effects are subtle, they provide a potential observational signature of quantum gravity effects. The findings presented here contribute to the broader effort of testing alternative theories of gravity through black hole observations, offering a new perspective on the quantum structure of spacetime at cosmological and astrophysical scales.

Shadow Cast by the Kerr MOG Black Hole under the Influence of Plasma and Constraints from EHT Observation

Abstract The study of black hole (BH) shadows provide crucial insights into the nature of strong gravitational effects and the intricate structure of the spacetime surrounding BHs. In this paper, we explore the shadow of Kerr MOG BH within a plasma environment, investigating how much the presence of plasma influences the characteristics of the observed shadow compared to those in vacuum conditions. Our analysis reveals that the shadow characteristics of M87* and Sgr A* are more compatible with event horizon telescope (EHT) observational data in nonhomogeneous plasma spacetime compared to homogeneous distributions. For small metric deformation parameter $\alpha$, the shadow aligns within $2\sigma$ uncertainty for homogeneous plasma and within $1\sigma$ for nonhomogeneous plasma. Next, we determine the energy emission rate for the Kerr MOG BH and analyze the influence of parameters $\alpha$, $k_o$, $k_\theta$, and $k_r$ on particle emissions in the BH vicinity. We further analyze the deflection angle in the presence of homogeneous and nonhomogeneous plasma profiles. The findings indicate notable differences from the vacuum scenario, underscoring the importance of accounting for plasma effects in studying light propagation around compact objects.

A Survey of General Relativistic Magnetohydrodynamic Models for Black Hole Accretion Systems

Abstract General relativistic magnetohydrodynamics (GRMHD) simulations are an indispensable tool in studying accretion onto compact objects. The Event Horizon Telescope (EHT) frequently uses libraries of ideal GRMHD simulations to interpret polarimetric, event-horizon-scale observations of supermassive black holes at the centers of galaxies. In this work, we present a library of 10 nonradiative, ideal GRMHD simulations that were utilized by the EHT Collaboration in their analysis of Sagittarius A*. The parameter survey explores both low (SANE) and high (MAD) magnetization states across five black hole spins a * = −15/16, −1/2, 0, +1/2, +15/16 where each simulation was run out to 30,000 GM/c −3. We find the angular momentum and energy flux in SANE simulations closely matches the thin-disk value, with minor deviations in prograde models due to fluid forces. This leads to spin equilibrium around a * ∼ 0.94, consistent with previous studies. We study the flow of conserved quantities in our simulations and find mass, angular momentum, and energy transport in SANE accretion flows to be primarily inward and fluid dominated. MAD models produce powerful jets with outflow efficiency >1 for a * = + 0.94, leading to black hole spin-down in prograde cases. We observe outward directed energy and angular momentum fluxes on the horizon, as expected for the Blandford–Znajek mechanism. MAD accretion flows are sub-Keplerian and exhibit greater variability than their SANE counterpart. They are also hotter than SANE disks within r ≲ 10 GM/c −2. This study is accompanied by a public release of simulation data at http://thz.astro.illinois.edu/.

The existence and distribution of photon spheres near spherically symmetric black holes: a geometric analysis

Photon sphere has attracted significant attention since the capture of black hole shadow images by Event Horizon Telescope. Recently, a number of studies have highlighted that the number of photon spheres and their distributions near black holes are strongly constrained by black hole properties. Specifically, for black holes with event horizons and proper asymptotic behaviors, the number of stable and unstable photon spheres satisfies the relation nstable-nunstable=-1. In this study, we provide a new proof on this relation using a geometric analysis, which is carried out using intrinsic curvatures in the optical geometry of black hole spacetimes. Firstly, we demonstrate the existence of photon spheres near black holes assuming most general asymptotic behaviors (asymptotically flat black holes, asymptotically de-Sitter and anti-de-Sitter black holes). Subsequently, we prove that the stable and unstable photon spheres near black holes must be one-to-one alternatively separated from each other, such that each unstable photon sphere is sandwiched between two stable photon spheres (and each stable photon sphere is sandwiched between two unstable photon spheres). Our analysis in this study is applicable to any spherically symmetric black hole spacetimes.

Survey of radiative, two-temperature magnetically arrested simulations of the black hole M87* I: Turbulent electron heating

Abstract We present a set of eleven two-temperature, radiative, general relativistic magnetohydrodynamic (2TGRRMHD) simulations of the black hole M87* in the magnetically arrested (MAD) state, surveying different values of the black hole spin a*. Our 3D simulations self-consistently evolve the temperatures of separate electron and ion populations under the effects of adiabatic compression/expansion, viscous heating, Coulomb coupling, and synchrotron, bremsstrahlung, and inverse Compton radiation. We adopt a sub-grid heating prescription from gyrokinetic simulations of plasma turbulence. Our simulations have accretion rates $\dot{M}=(0.5-1.5)\times 10^{-6}\dot{M}_{\rm Edd}$ and radiative efficiencies εrad = 3 − 35 %. We compare our simulations to a fiducial set of otherwise identical single-fluid GRMHD simulations and find no significant changes in the outflow efficiency or black hole spindown parameter. Our simulations produce an effective adiabatic index for the two-temperature plasma of Γgas ≈ 1.55, larger than the Γgas = 13/9 value often adopted in single-fluid GRMHD simulations. We find moderate ion-to-electron temperature ratios in the 230 GHz emitting region of R = Ti/Te ≈ 5. While total intensity 230 GHz images from our simulations are consistent with Event Horizon Telescope (EHT) results, our images have significantly more beam-scale linear polarization (〈|m|〉 ≈ 30 %) than is observed in EHT images of M87* (〈|m|〉 < 10 %). We find a trend of the average linear polarization pitch angle ∠β2 with black hole spin consistent with what is seen in single-fluid GRMHD simulations, and we provide a simple fitting function for ∠β2(a*) motivated by the wind-up of magnetic field lines by black hole spin in the Blandford-Znajek mechanism.

Black hole accretion and radiation variability in general relativistic magnetohydrodynamic simulations with Rezzolla–Zhidenko spacetime

The Event Horizon Telescope (EHT) has unveiled the horizon-scale radiation properties of Sagittarius A* (Sgr A*), the supermassive black hole at the center of our galaxy, providing a novel platform for testing gravitational theories by comparing observations with theoretical models. A key next step is to investigate the nature of accretion flows and spacetime structures near black holes by analyzing the time variability observed in EHT data alongside general relativistic magnetohydrodynamic (GRMHD) simulations. We explored the dynamics of accretion flows in spherically symmetric black hole spacetimes with deviations from general relativity utilizing two dimensional GRMHD simulations based on the Rezzolla–Zhidenko parameterized spacetime. This study marks the first systematic investigation into how variability amplitudes in light curves, derived from non-Kerr GRMHD simulations, depend on deviations from the Schwarzschild spacetime. The deviation parameters are consistent with the constraints from weak gravitational fields and the size of Sgr A*’s black hole shadow. We find that the dynamics of accretion flows systematically depend on these parameters. In spacetimes with a deeper gravitational potential, fluid and Alfvén velocities consistently decrease relative to the Schwarzschild metric, indicating weaker dynamical behavior. We also examined the influence of spacetime deviations on radiation properties by computing luminosity fluctuations at 230 GHz using general relativistic radiative transfer simulations, in line with EHT observations. The amplitude of these fluctuations exhibits a systematic dependence on the deviation parameters, decreasing for deeper gravitational potentials compared to the Schwarzschild metric. These features are validated using one of the theoretically predicted metrics, the Hayward metric, a model that describes nonsingular black holes. This characteristic is expected to have similar effects in more comprehensive simulations that include more realistic accretion disk models and electron cooling in the future, potentially aiding in distinguishing black hole solutions that explain the variability of Sgr A*.

Shadows of Renormalisation Group Improved Black Holes Surrounded by Plasma

Abstract The study of black hole shadows after the discovery of the M87 black hole by the Event Horizon Telescope (EHT) has gained major interest recently. In this work, we have studied the shadows of a renormalization group improved (RGI) black hole system surrounded by two types of plasma media, viz homogeneous and inhomogeneous plasma. We have found that the associated free parameter ξ related to the momentum cutoff scale and the plasma parameter k plays a significant role in determining the photon sphere radius and hence the black hole shadows.

Jet formation studies in active galactic nuclei: A search for new targets

In recent years, the jet formation region in active galaxies has been imaged through millimeter very long baseline interferometry (mm-VLBI) for a few ideal targets, in particular, M,87. An important leap forward for understanding jet launching could be taken by identifying a larger number of suitable objects, characterized by different accretion modes and jet powers. In this article, we present 1,cm and 7,mm VLBI data of a sample of 16 poorly explored radio galaxies, comprising both high-excitation (HEGs) and low-excitation galaxies (LEGs), spanning a broad range in terms of radio power. There are several γ-ray emitters among this sample. The sources proximity (z<0.1) combined with a high black hole mass M_ BH ≳ 8.5) implies a high spatial resolution in units of Schwarzschild radii ($<10^3-10^4$,R_ S), necessary for probing the region where the jet is initially accelerated and collimated. We aim to identify the best candidates for follow-up observations with current and future VLBI facilities. The observations were performed with the High Sensitivity Array (HSA), including the Effelsberg telescope and the phased Very Large Array (VLA). The addition of elements with a large collecting area has allowed us to characterize the sub-parsec properties of these faint jets and to estimate their core brightness temperature and orientation. The number of sources imaged on scales lesssim 10^3,R_ S has more than doubled thanks to the present study. All targets were detected at both frequencies, with several of them presenting two-sided jet structures. Several LEG jets show hints of limb brightening. The core brightness temperatures are generally below the equipartition value, indicating that equipartition has not yet been reached and/or that the emission is de-boosted. Among the LEGs in the sample, we identified 3C,31, 3C,66B, and 3C,465 as the most promising, as they combine a relatively high flux density ($>50$,rm mJy) with a superb spatial resolution ($<500$,R_ S) at 7,mm. The powerful HEG 3C,452 is interesting as well, due to its highly symmetric, two-sided jet base. Most sources are expected to become prime targets for future experiments with the next generation Event Horizon Telescope (ngEHT) and next generation Very Large Array (ngVLA).

Shadows and accretion disk images of charged rotating black hole in modified gravity theory

In this paper, we study the shadows and images of the accretion disk of Kerr–Newman (KN) black hole (BH) in modified gravity (MOG) theory by using the backward ray-tracing method. And, the influence of spin parameter (a), charge (Q), and MOG parameter (α) on the observed features of BHs are carefully addressed. Interestingly, as α increases, the flat edge of the BH’s shadow gradually becomes more rounded, the size of shadow enlarges, and the deviation rate (δs) correspondingly decreases. By tracing the photon around BH, we observe that the trajectory of photon exhibits distortion behavior, i.e., the formation of two “tails” near the Einstein ring, which elongate as a increases. For the accretion disk, it shows that the inner shadow expands with α, while decreases with Q. The increase of α exhibits an increasing effect on redshift. At the same parameter level, α has a more obvious effect on inner shadow and image of BH by comparing with that of Q. Our study implies that both α and Q have relatively significant effects on the image of the KN-MOG BH with the thin disk accretion, but the influence of α is much greater. So, this indicates that α plays a dominant role in this spacetime.

A multifrequency study of sub-parsec jets with the Event Horizon Telescope

The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230,GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed, including the main science targets, Sgr,A* and M,87, and various calibrators. Sixteen sources were AGN. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-K"onigl (BK) jet model. In particular, we aimed to study the signatures of jet acceleration and magnetic energy conversion. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components (1749+096, 1055+018, BL,Lac, J0132--1654, J0006--0623, CTA,102, and 3C,454.3) and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86,GHz range. Combining these data into a multifrequency EHT+ data set, we studied the dependences of the VLBI core component flux density, size, and brightness temperature on the frequency measured in the AGN host frame (and hence on the distance from the central black hole), characterizing them with power law fits. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our observations spanning event horizon to parsec scales indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Only some of the discrepancies may be alleviated by tweaking the model parameters or the jet collimation profile. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations. For our sample, we estimate a general radial dependence of the Doppler factor δ ∝ r^ ≤0.5 . This interpretation is consistent with a magnetically accelerated sub-parsec jet. We also estimate a steep decrease of the magnetic field strength with radius B ∝ r^ hinting at jet acceleration or efficient magnetic energy dissipation.

The persistent shadow of the supermassive black hole of M87

The Event Horizon Telescope (EHT) observation of M87∗ in 2018 has revealed a ring with a diameter that is consistent with the 2017 observation. The brightest part of the ring is shifted to the southwest from the southeast. In this paper, we provide theoretical interpretations for the multi-epoch EHT observations for M87∗ by comparing a new general relativistic magnetohydrodynamics model image library with the EHT observations for M87∗ in both 2017 and 2018. The model images include aligned and tilted accretion with parameterized thermal and nonthermal synchrotron emission properties. The 2018 observation again shows that the spin vector of the M87∗ supermassive black hole is pointed away from Earth. A shift of the brightest part of the ring during the multi-epoch observations can naturally be explained by the turbulent nature of black hole accretion, which is supported by the fact that the more turbulent retrograde models can explain the multi-epoch observations better than the prograde models. The EHT data are inconsistent with the tilted models in our model image library. Assuming that the black hole spin axis and its large-scale jet direction are roughly aligned, we expect the brightest part of the ring to be most commonly observed 90 deg clockwise from the forward jet. This prediction can be statistically tested through future observations.

Thermodynamic and observational constraints on black holes with primary hair in Beyond Horndeski gravity: Stability and shadows

In this paper, we find that unlike in General Relativity, the shift-symmetric subclass of Beyond Horndeski theories permits black holes with primary hair that are thermodynamically stable and align with current Event Horizon Telescope observations of the M87* and Sgr A* black holes. This work begins by investigating thermodynamic properties, analyzing how primary hair influences thermodynamic quantities and local stability, which imposes strict constraints on the allowed range of primary hair values. The null geodesics near this black hole are then examined, demonstrating how scalar hair affects the shadow diameter. Specifically, when the parameter of the Beyond Horndeski function F 4 is negative, increasing scalar hair enlarges the shadow; in contrast, when this parameter is positive, greater scalar hair reduces the shadow size. Further constraints on the scalar hair are derived using observational data, highlighting its sensitivity to other black hole parameters. To explore additional observational features, face-on two-dimensional images of spherically infalling accretion disks are simulated, revealing how primary scalar hair shapes the black hole's shadow. Finally, all relevant constraints are combined to identify black holes that are both stable and consistent with observational data.

Evaluation of South African Candidate Sites for an Expanded Event Horizon Telescope

Evaluation of South African Candidate Sites for an Expanded Event Horizon Telescope

Circular light orbits of a general, static, and spherical symmetrical wormhole with Z2 symmetry

Recently, the ring images or the shadow images of the centers of the galaxy M87 and the Milky way have been reported by Event Horizon Telescope Collaboration. It is believed that the ring images imply that the central objects form unstable light circular orbits. Some of wormholes with Z2 symmetry against a throat are wrongly excluded from the candidates at the centers of M87 and the Milky way due to the overlooking the unstable light circular orbits on the throat. A general asymptotically-flat, static, and spherical symmetrical wormhole without a thin shell has at least one unstable circular light orbit at the throat or elsewhere. If the wormhole has Z2 symmetry against the throat, it has the unstable circular light orbits on the throat or it has stable circular light orbits on the throat and unstable ones near the throat. We need to analyze the throat carefully to make sure we do not unfairly rule out the Z2-symmetrical wormholes. In this study, we categorize the numbers of the circular light orbits of the Z2-symmetrical wormhole and their stability from the derivatives of an effective potential at the throat and we investigate the circular light orbits around a Simpson–Visser black-bounce spacetime, a Damour–Solodukhin wormhole spacetime, a Reissner–Nordström black-hole-like wormhole spacetime or a charged Damour–Solodukhin wormhole spacetime as examples. We give complete treatments including degenerated circular light orbits made from more than one stable and unstable circular light orbits on and off the throat.

MLody—Deep Learning–generated Polarized Synchrotron Coefficients

Abstract Polarized synchrotron emission is a fundamental process in high-energy astrophysics, particularly in the environments around black holes and pulsars. Accurate modeling of this emission requires precise computation of the emission, absorption, rotation, and conversion coefficients, which are critical for radiative transfer simulations. Traditionally, these coefficients are derived using fit functions based on precomputed ground truth values. However, these fit functions often lack accuracy, particularly in specific plasma conditions not well represented in the data sets used to generate them. In this work, we introduce MLody, a deep neural network designed to compute polarized synchrotron coefficients with high accuracy across a wide range of plasma parameters. We demonstrate MLody's capabilities by integrating it with a radiative transfer code to generate synthetic polarized synchrotron images for an accreting black hole simulation. Our results reveal significant differences, up to a factor of 2, in both linear and circular polarization compared to traditional methods. These differences could have important implications for parameter estimation in Event Horizon Telescope observations, suggesting that MLody could enhance the accuracy of future astrophysical analyses.

Supermassive Black Hole Spin Constraints from Polarimetry in an Equatorial Disk Model

Abstract The Event Horizon Telescope has released polarized images of the supermassive black holes Messier 87* (M87*) and Sagittarius A* accretion disks. As more images are produced, our understanding of the average polarized emission from near the event horizon improves. In this Letter, we use a semianalytic model for optically thin, equatorial emission near a Kerr black hole to study how spin constraints follow from measurements of the average polarization spiral pitch angle. We focus on the case of M87* and explore how the direct, weakly lensed image spiral is coupled to the strongly lensed indirect image spiral, and how a precise measurement of both provides a powerful spin tracer. We find a generic result that the spin twists the direct and indirect image polarization in opposite directions. Using a grid search over model parameters, we find a strong dependence of the resulting spin constraint on plasma properties near the horizon. Grid constraints suggest that, under reasonable assumptions for the accretion disk, a measurement of the direct and indirect image spiral pitch angles to ±5° yields a dimensionless spin amplitude measurement with uncertainty σ ∣ a * ∣ ∼ 0.25 for radially infalling models but otherwise provides only weak constraints; an error of 1∘ can reach σ ∣ a * ∣ ∼ 0.15 . We also find that a well-constrained rotation measure greatly improves spin measurements. Assuming that equatorial velocity and magnetic field are oppositely oriented, we find that the observed M87* polarization pattern favors models with strong radial velocity components, which are close to optimal for future spin measurements.