M87 Black Hole

M87 Black Hole

1. Introduction

   • Black Holes and General Relativity
     • Black holes are a fundamental prediction of general relativity, defined by the event horizon from which not even light can escape. They are at the heart of fundamental issues in the unification of general relativity with quantum physics.
   • Shadow of the Black Hole
     • Black holes, when surrounded by a transparent emission region, are expected to reveal a dark shadow caused by the gravitational deflection of light and the capture of photons at the event horizon.
   • Objective of the Event Horizon Telescope
     • The Event Horizon Telescope (EHT) was set up to image and study this shadow phenomenon. It is a global Very Long Baseline Interferometry (VLBI) array observing at a wavelength of 1.3 mm to reconstruct images on the scale of the event horizon of the candidate supermassive black hole at the center of the M87 galaxy. The EHT seeks to resolve the shadows of M87* and Sgr A*, the two supermassive black holes with the largest apparent angular sizes.

2. The Radio Core in M87

   • M87 Galaxy and its jet
     • M87 is a giant elliptical galaxy in which a linear feature, later called a "jet", has been detected. This jet is a bright and well-studied radio source at multiple wavelengths, believed to be powered by a supermassive black hole.
   • M87* Compact Radio Source
     • The upstream end of the jet is marked by a compact radio source (M87*). Observations indicate that, at a wavelength of 1.3 mm, M87* coincides with the supermassive black hole.
   • VLBI observations
     • VLBI observations at 1.3 mm revealed an emission region diameter of approximately 40 μas, comparable to the structure expected at the horizon scale, but were unable to image the shadow due to limited coverage.
   • Mass Estimates
     • Previous estimates of the mass of M87*, based on measurements of the stellar population and the kinematic structure of the gas disk, predict theoretical shadow diameters for a Schwarzschild black hole. The distance adopted for M87 is 16.8 ± 0.8 Mpc.

3. The Event Horizon Telescope

   • VLBI technique
     • The EHT is a VLBI experiment that directly measures "visibilities", which are Fourier components of the brightness distribution in the sky. As the Earth rotates, each pair of telescopes samples many spatial frequencies, effectively creating a virtual telescope that covers almost the entire diameter of the Earth.
   • Technical Developments
     • The extension of the VLBI to wavelengths of 1.3 mm required long-term technical developments, including challenges such as increased noise, atmospheric opacity, phase fluctuations and reduced efficiency of the telescopes. The EHT addressed these challenges by developing broadband systems and upgrading the stations' infrastructure.
   • EHT Array Configuration
     • For the 2017 observations, the EHT used a global array of eight stations in six geographical locations. The baselines ranged from 160 m to 10,700 km towards M87*, resulting in a theoretical diffraction-limited resolution of approximately 25 μas.

4. Observations, Correlation, and Calibration

   • Observation dates and conditions
     • M87* was observed on April 5, 6, 10 and 11, 2017. The weather was good to excellent, with low atmospheric opacities in most locations. The observations consisted of series of scans on M87* interspersed with observations of the quasar 3C 279 for calibration.
   • Data processing
     • The observation, correlation and calibration are described in detail in Paper III. The calibrated complex visibilities correspond to the Fourier components of the brightness distribution in the sky.
   • Visibility amplitudes
     • The visibility amplitudes of M87* for April 11 resemble those of a thin ring model (approximately 46 μas in diameter) with a first null at 3.4 Gλ. The visibilities at shorter baselines suggest that about half of the compact flow is resolved by the interferometer. Non-zero closure phases and differences in the depth of the first minimum indicate some degree of asymmetry in the source.

5. Images and Features

   • Challenges in Image Reconstruction
     • Reconstructing images from EHT data is challenging due to the limited sampling of the (u,v) plane, which makes the problem underdetermined, and uncertainties in phase and amplitude calibration.
   • Imaging Algorithms
     • Two types of algorithm have been explored: the traditional CLEAN approach (deconvolution) and the Regularized Maximum Likelihood approach (RML, direct modelling). Both incorporate assumptions to produce physically plausible or conservative images that are consistent with the data.
   • Two-Step Imaging Approach
     • To control for bias, four teams worked independently using initial data. Each team produced an image with a similar prominent feature: a ring with a diameter of approximately 38-44 μas and increased brightness to the south.
   • Final Images
     • The average image obtained from different pipelines shows a dominant ring with an asymmetrical azimuthal profile, oriented at approximately 170° east of north. The image features were consistently reproduced by different methods and on all four observation days.
   • Consistency with Models
     • The imaging results are consistent with the fit of geometric and general relativistic magnetohydrodynamic (GRMHD) models in the visibility domain.

6. Theoretical Modeling

   • GRMHD simulations
     • Turbulent, hot, magnetized disks around a Kerr black hole produce jets, a shadow, and an asymmetric emission ring in GRMHD models.
   • Simulation and Image Libraries
     • A vast set of synthetic images was generated by coupling GRMHD simulations with general-relativistic ray-tracing and radiative-transfer codes.
   • Simulation parameters
     • Models explore black-hole spin (a*) and magnetic flux (ϕ) under SANE (moderate flux) and MAD (high flux) regimes.
   • Emission Source and Temperature Ratio
     • 1.3 mm photons stem from synchrotron emission, with the electron/ion temperature ratio (Rhigh) setting disk- versus jet-dominated regions.
   • Physical interpretation
     • Strong gravitational lensing and relativistic beaming of a magnetized accretion flow near the Kerr horizon produce the asymmetric ring and central shadow.

7. Discussion

   • Robustness of Results
     • Independent techniques and four days of data ensure consistent image with predictions.
   • Consistency with General Relativity and Mass Estimation
     • Almost circular ring (deviation ≲10%) imposes initial limit on quadrupole moment.
   • Source Stability
     • Core at 1.3 mm remained stable despite flow variability.
   • Exclusion of Alternative Interpretations
     • Jets, clusters and Einstein rings do not explain the observations.

8. Conclusion and Outlook

   • Summary of Findings
     • A 42 ± 3 μas ring with >10:1 contrast reveals M87*’s shadow.
   • Mass and Spin Direction
     • Clockwise spin and relativistic beaming explain the brightness asymmetry.
   • Future Prospects
     • Further observations and polarimetric analysis will refine shadow shape, depth, and accretion flow.
   • Significance
     • Supermassive black-hole shadows have become observable, transforming theory into repeatable observations.