The Spin of M87*

Abstract: Now that the mass of the central black hole in the galaxy M87 has been measured with great precision using different methods, the remaining parameter of the Kerr metric that needs to be estimated is the spin a*. We have modeled measurements of the average power of the relativistic jet and an upper limit to the mass accretion rate onto the black hole with general relativistic magnetohydrodynamic models of jet formation. This allows us to derive constraints on a* and the black hole magnetic flux phi. We find a lower limit on M87*'s spin and magnetic flux of a* > 0.4 and phi > 6 in the prograde case, and a* > 0.5 and phi > 10 in the retrograde case, otherwise the black hole is not able to provide enough energy to power the observed jet. These results indicate that M87* has a moderate spin at minimum and disfavor a variety of models typified by low values of phi known as ``SANE'', indicating that M87* prefers the magnetically arrested disk state. We discuss how different estimates of the jet power and accretion rate can impact a* and phi.

• The paper constrains the spin and magnetic flux of M87* using simulations, establishing lower limits for |a_*| and supporting a magnetically arrested disk state.
• The analysis calculates M87*'s jet production efficiency using the Blandford-Znajek process, finding it is at least 10% and possibly as high as 200%.
• The findings support magnetically arrested disk states, constraining theoretical models and suggesting refinements for future observational techniques.

Analysis of Black Hole Spin and Magnetic Flux in M87*

The paper entitled "The Spin of M87*" by Rodrigo Nemmen advances the understanding of the spin and magnetic flux of the supermassive black hole (SMBH) at the center of galaxy M87, commonly referred to as M87*. The research employs precise measurements of the black hole's mass and models the dynamics of its relativistic jet using general relativistic magnetohydrodynamic (GRMHD) simulations. This study is particularly significant due to its focus on constraining the dimensionless spin parameter $a_*$ and the magnetic flux $\Phi$ associated with the black hole, which, along with mass, are essential parameters of the Kerr metric.

Key Findings

1. Spin and Magnetic Flux Constraints:
   • The study establishes lower limits for M87*'s spin in both the prograde (spin direction same as the accretion disk) and retrograde modes (spin direction opposite to the disk). The prograde case indicates $|a_*| \geq 0.4$ and $\phi \gtrsim 6$, while the retrograde case requires $|a_*| \geq 0.5$ and $\phi \gtrsim 10$.
   • The findings identify a preference for a magnetically arrested disk (MAD), which includes higher magnetic flux values, over the standard and normal evolution (SANE) scenarios characterized by lower magnetic flux.
2. Jet Production Efficiency:
   • The analysis calculates a jet production efficiency $\eta$ using the Blandford-Znajek process, revealing it to be at least 10% and possibly as high as 200%, depending on the mass-loss behavior in the radiatively inefficient accretion flow (RIAF).
3. Model-Observation Consistency:
   • The study leverages measurements of X-ray cavities to determine the jet power and assesses accretion rates through submillimeter radio observations, achieving consistency with both empirical data and theoretical models.

Implications

The implications of this work are both theoretical and observational:

• Theory: This study provides constraints on the Kerr metric parameters, contributing to our understanding of relativistic jet formation mechanisms in accreting black holes. Such constraints aid in distinguishing between different accretion scenarios, namely SANE and MAD, and emphasize the potential need to incorporate more complex magnetic field models in black hole physics.
• Observational Astronomy: By reinforcing the preference for MAD states, the results support observations that detect strong magnetic field interactions with black holes. Furthermore, the study suggests potential avenues to refine observational techniques, potentially impacting future VLBI advancements, including space-based infrastructure.

Future Directions

The paper suggests areas for further exploration, notably:

• Enhanced Observational Techniques: Advances in very long baseline interferometry (VLBI) could provide increased resolution and sensitivity, enabling refined measurements of black hole properties. Polarimetric analyses of black hole environments could supply additional constraints on the spin and magnetic field orientation.
• Refinement of Theoretical Models: Incorporating non-ideal MHD effects and more detailed modeling of magnetic field geometries might yield improved descriptions of the accretion processes and jet dynamics.

Overall, Nemmen's study contributes to the nuanced modeling of supermassive black holes, offering a framework for future research in black hole spin dynamics and relativistic jet production in astrophysics.