Constraining spins of supermassive black holes from TeV variability. II. fully general relativistic calculations

TL;DR

This paper uses fully general relativistic calculations to analyze TeV photon variability from M87, providing a method to constrain supermassive black hole spins by modeling accretion flows and radiation fields.

Contribution

It extends previous models by including all relativistic effects and applies the approach to estimate the spin of M87's black hole, reducing parameter degeneracy.

Findings

Optical depth depends strongly on accretion rate and black hole spin.

The minimum spin parameter for M87's black hole is estimated to be around 0.8.

The model can be used for future TeV observations to constrain SMBH spins.

Abstract

The fast variability of energetic TeV photons from the center of M87 has been detected, offering a new clue to estimate spins of supermassive black holes (SMBHs). We extend the study of Wang et al. (2008) by including all of general relativistic effects. We numerically solve the full set of relativistic hydrodynamical equations of the radiatively inefficient accretion flows (RIAFs) and then obtain the radiation fields around the black hole. The optical depth of the radiation fields to TeV photons due to pair productions are calculated in the Kerr metric. We find that the optical depth strongly depends on: (1) accretion rates as $\tautev\propto \dot{M}^{2.5-5.0}$; (2) black hole spins; and (3) location of the TeV source. Jointly considering the optical depth and the spectral energy distribution radiated from the RIAFs, the strong degeneration of the spin with the other free parameters in the RIAF model can be largely relaxed. We apply the present model to M87, wherein the RIAFs are expected to be at work, and find that the minimum specific angular momentum of the hole is $a\sim0.8$. The present methodology is applicable to M87-like sources with future detection of TeV emissions to constrain the spins of SMBHs.