Determine the electron distribution parameter κ for M87*

Determine the physically appropriate value, or spatially and temporally varying behavior, of the non-thermal electron-distribution parameter \(\kappa\) in the horizon-scale accretion flow of M87*.

Background

The paper uses a non-thermal κ\kappa distribution to model relativistic electrons, with the parameter κ\kappa controlling the high-energy power-law slope. For the main calculations, the authors fix κ=4.25\kappa=4.25, motivated loosely by the observed large-scale jet spectral index, while noting that the horizon-scale electron population need not share this value.

The parameter is therefore not determined by the adopted GRMHD simulations or by the emission model. The paper explores only a fixed representative value and explicitly identifies κ\kappa as an unknown quantity that must be assumed; determining it observationally or from a physical electron-heating model remains unresolved.

References

The value of the parameter $\kappa$ is typically unknown and must be assumed (a condition must be satisfied that $\kappa>3$ for $w>0$).

— Probing the details of relativistic electrons with multifrequency observations of M87 black hole  (2609.11609 - Keuper et al., 10 Sep 2026) in Section 2.2, Synchrotron emission models; Section 2.3.1, Model parameters and finding \(M_{\rm unit}\)