We investigate the correlation between 151 MHz radio luminosity , L _ { 151 MHz } , 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 L _ { X } ^ { 2 - 10 { keV } } / \it L _ { Edd } for FR Is , where L _ { Edd } is the Eddington luminosity and L _ { X } ^ { 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 \gtrsim 0.9 is required in order to reproduce the observed relation of P _ { jet } / L _ { Edd } - L _ { X } ^ { 2 - 10 { keV } } / \it 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 .