The relation of optical/UV and X-ray emission in low-luminosity active galactic nuclei

Abstract

We study the relation of optical/UV and X-ray emission in the low luminosity active galactic nuclei (LLAGNs), using a sample of 49 sources including 28 local Seyfert galaxies and 21 low-ionization nuclear emission-line regions (LINERs) with the optical/UV spectral luminosity at the wavelength λ=2500à , 23.0\leq \log L_{ν(2500à )(erg/s/Hz)\leq 27.7, and the X-ray spectral luminosity at 2 keV, 20.5\leq \log L_{ν(2 keV)}\leq 25.3. The strong correlations are found between the X-ray luminosity and the optical/UV to X-ray index, α_{ox}, with the optical/UV luminosity, with the slopes very similar to the findings for the luminous AGNs in the previous works. The correlation between α_{ox} and L_{ν(2 keV) is very weak as that found for the luminous AGNs in the majority of previous similar works. We also study the relation between $α_{\rm ox} and the Eddington ratio L_{bol}/L_{Edd} for our sample and find a significant anti-correlation for the sources with L_{bol}/L_{Edd}\lesssim 10^{-3}, which is opposite to the correlation between the two variables for the luminous AGNs. Using the advection dominated accretion flow (ADAF) model, we roughly reproduce this anti-correlationship for the two variables for the LLAGNs. This result strongly supports the ADAF as a candidate accretion mode in LLAGNs.

21 pages, 1 table, 5 figures. Accepted for publication in ApJ