The black hole fundamental plane in low-excitation radio galaxies

The radio-X-ray slope in the fundamental plane of radio-loud active galactic nuclei (AGNs) has been found to be steeper than that of radio-quiet AGNs in previous studies. In this paper, we reinvestigate the fundamental plane in radio-loud AGNs by compiling a sample of 13 low-excitation radio galaxies (LERGs) from the 3CR radio galaxies, for the reason that the accretion mode in LERGs is believed to be a radiatively inefficient accretion flow. All the sources in our sample have data available both at the 5-GHz core radio luminosity as detected by VLA/VLBI/VLBA and at the core X-ray luminosity as detected by Chandra/XMM-Newton. Surprisingly, we find that the slope in the fundamental plane (logL_R = 0.52log L_X + 0.84log M_BH + 10.84) of LERGs is consistent with that reported by Merloni et al. However, the normalization is found to be shifted by about 0.7 dex, which could be due to the difference in the magnetic field strength in different objects. A shallower slope of the L_R-L_X relationship (L_R∼ L_X^{0.63}) is also found for our sample, which demonstrates that the X-ray emission in LERGs may come from the accretion disc, rather than jets suggested by previous works.