Spins of the supermassive black hole in M87: new constraints from TeV observations

TL;DR

This study uses TeV gamma-ray variability in M87 to constrain the black hole's spin, showing that the transparency of the emission region to TeV photons indicates a rapidly spinning black hole.

Contribution

It introduces a method to estimate black hole spin by analyzing TeV photon absorption influenced by the accretion flow's radiation field.

Findings

Transparent radii differ significantly between maximally rotating and non-rotating black holes.

Observations suggest M87's black hole is spinning rapidly.

TeV photons can serve as diagnostics for black hole spin estimation.

Abstract

The rapid TeV $\gamma-$ray variability detected in the well-known nearby radio galaxy M87 implies an extremely compact emission region (5-10 Schwarzschild radii) near the horizon of the supermassive black hole in the galactic center. TeV photons are affected by dilution due to interaction with the radiation field of the advection-dominated accretion flow (ADAF) around the black hole, and can thus be used to probe the innermost regions around the black hole. We calculate the optical depth of the ADAF radiation field to the TeV photons and find it strongly depends on the spin of the black hole. We find that transparent radii of 10 TeV photons are of $5R_{\rm S}$ and $13R_{\rm S}$ for the maximally rotating and non-rotating black holes, respectively. With the observations, the calculated transparent radii strongly suggest the black hole is spinning fast in the galaxy. TeV photons could be used as a powerful diagnostic for estimating black hole spins in galaxies in the future.