Black Hole Fundamental Plane in Low-Excitation Radio Galaxies

Abstract: The radio-X-ray slope in the fundamental plane of radio-loud active galactic nuclei (AGNs) is found to be steeper compared with that of radio-quiet AGNs in previous works. In this work, we reinvestigate the fundamental plane in radio-loud AGNs by compiling a sample of 13 low-excitation radio galaxies (LERG) from the 3CR radio galaxies, for the reason that the accretion mode in LERG is believed to be a radiatively inefficient accretion flow. All the sources in our sample possess the data available at both the 5 GHz core radio luminosity detected by VLA/VLBI/VLBA and the core X-ray luminosity detected by Chandra/XMM-Newton. Surprisingly, we find the slope in the fundamental plane ($\log L_{\rm R}=0.52 \log L_{\rm X}+ 0.84 \log M_{\rm BH} + 10.84$) of LERG is well consistent with that reported by \citet{m2003}. However, the normalization is found to be shifted by about 0.7 dex, which can be due to the difference on magnetic field strength in different objects. A shallower slope of $L_{\rm R}-L_{\rm X}$ relation ($L_{\rm R}\sim L_{\rm X}^{0.63}$) is also given by our sample, which demonstrates that the X-ray emission in LERG may come from accretion disc instead of jets as suggested by previous works.