Evidence for Rapidly Rotating Black Holes in FR I Radio Galaxies

TL;DR

This paper provides evidence that black holes in FR I radio galaxies are rapidly spinning, based on correlations between jet power, X-ray luminosity, and a model of jet production involving high black hole spin.

Contribution

It introduces a model linking jet power and X-ray emission to black hole spin, suggesting FR I black holes are rapidly spinning with spin parameter j>~0.9.

Findings

Black hole spin parameter j>~0.9 is required to match observations.

Jet power correlates with X-ray luminosity normalized by Eddington luminosity.

Black holes in FR I galaxies are likely rapidly spinning.

Abstract

We investigate the correlation between 151 MHz radio luminosity, L_151MHz, and jet power, P_jet, for a sample of low-power radio galaxies, of which the jet power is estimated from X-ray cavities. The jet power for a sample of FR I radio galaxies is estimated with the derived empirical correlation. We find that P_jet/L_Edd is positively correlated with Lx(2-10 keV)/L_Edd for FR Is, where L_Edd is the Eddington luminosity and Lx(2-10 keV) is 2-10 keV X-ray luminosity. We calculate the jet power of a hybrid model, as a variant of Blandford-Znajek model proposed by Meier, based on the global solution of the advection-dominated accretion flow (ADAF) surrounding a Kerr black hole (BH). Our model calculations suggest that the maximal jet power is a function of mass accretion rate and the black hole spin parameter j. The hard X-ray emission is believed to be mainly from the ADAFs in FR Is, and the mass accretion rate is therefore constrained with the X-ray emission in our ADAF model calculations. We find that the dimensionless angular momentum of BH j>~0.9 is required in order to reproduce the observed relation of P_jet/L_Edd-Lx(2-10 keV)/L_Edd for FR Is. Our conclusion will be strengthened if part of the X-ray emission is contributed by the jets. Our results suggest that BHs in FR I radio galaxies are rapidly spinning, which are almost not affected by the uncertainty of the black hole mass estimates.