The size of the jet launching region in M87

Abstract: The supermassive black hole candidate at the center of M87 drives an ultra-relativistic jet visible on kiloparsec scales, and its large mass and relative proximity allow for event horizon scale imaging with very long baseline interferometry at millimeter wavelengths (mm-VLBI). Recently, relativistic magneto-hydrodynamic (MHD) simulations of black hole accretion flows have proven capable of launching magnetically-dominated jets. We construct time-dependent disc/jet models of the innermost portion of the M87 nucleus by performing relativistic radiative transfer calculations from one such simulation. We identify two types of models, jet-dominated or disc/jet, that can explain the spectral properties of M87, and use them to make predictions for current and future mm-VLBI observations. The Gaussian source size for the favored sky orientation and inclination from observations of the large-scale jet is 33-44 microarcseconds (~4-6 Schwarzschild radii) on current mm-VLBI telescopes, very similar to existing observations of Sgr A*. The black hole shadow, direct evidence of an event horizon, should be visible in future measurements using baselines between Hawaii and Mexico. Both models exhibit variability at millimeter wavelengths with factor of ~2 amplitudes on year timescales. For the low inclination of M87, the counter-jet dominates the event horizon scale millimeter wavelength emission from the jet-forming region.

Summary of "The Size of the Jet Launching Region in M87"

The paper, "The Size of the Jet Launching Region in M87" by Dexter, McKinney, and Agol, provides an in-depth exploration of the emission structures surrounding the supermassive black hole in the center of the galaxy M87. The authors employ general relativistic magneto-hydrodynamic (GRMHD) simulations to develop radiative disc/jet models, examining the spectral features and projected observational signatures of the ultra-relativistic jet launched from the vicinity of this black hole. This work aligns with recent advancements in very long baseline interferometry (VLBI), using millimeter wavelengths (mm-VLBI) to image the event horizon scales.

Key Findings and Methodology

1. Simulation and Modeling:

   • GRMHD simulations with a focus on magnetically dominated jet launching mechanisms were conducted to create time-dependent disc/jet models. The authors identify two distinct model types: jet-dominated and disc/jet, both capable of accounting for M87's observed spectral properties.
   • The paper investigates the black hole's emission via synchrotron radiation, incorporating relativistic ray tracing to capture time-variable disc and jet images. Central to this is the adoption of a power-law distribution of jet electrons, modulated by the ratio of magnetic to rest mass energy density.

2. Galactic Context and Observational Relevance:

   • M87's supermassive black hole presents a unique opportunity for mm-VLBI due to its massive scale and proximity. The authors focus on calculating the emission's angular size in micro-arcseconds (44–33 μas) as constrained by VLBI observations.

3. Emission Characteristics:

   • One key finding is that both jet and disc components can manifest as crescent-shaped images due to gravitational lensing and relativistic beaming effects. However, the location and dominance of the emission vary with parameters such as inclination and disc to jet energy ratios.
   • For low inclination angles pertinent to M87, the model suggests that the counter-jet might dominate the event horizon scale millimeter-wavelength emissions.

4. Predictions and Observations:

   • The paper provides concrete predictions for future mm-VLBI observations. It anticipates the detectability of the black hole's shadow with extended baselines in future arrays involving Mexico and Hawaii.

5. Implications for Jet Dynamics:

   • By utilizing variations in jet parameters, such as the energy fraction in non-thermal electrons and the electron Lorentz factor distribution, the study highlights how these contribute to the observed broadband spectra and variability. These findings emphasize the need for better understanding the physical conditions in jet acceleration regions, which are still uncertain.

Implications and Future Prospects

The research underscores the potential of emerging mm-VLBI technology to confirm theoretical predictions from GRMHD simulations. It bolsters the concept that GRMHD simulations can produce self-consistent, stable jet structures analogous to those seen in M87, offering a reliable framework to model supermassive black hole environments. The prospect of measuring the black hole's shadow allows for potential breakthroughs in confirming event horizon theories, paving the way for further exploration into relativistic jet dynamics and their launching mechanisms.

For researchers in high-energy astrophysics and observational astronomy, this work provides a predictive framework for interpreting high-resolution VLBI data, facilitating future discoveries on galactic nuclei dynamics. Moreover, the paper’s approach to modeling the interplay between accretion processes and jet dynamics offers valuable insights for formulating broader theories of active galactic nuclei emissions.