The naked nuclei of LINERs

TL;DR

This study shows a strong correlation between X-ray and emission line luminosities in LINERs, indicating we observe their nuclei directly without obscuring tori, due to low gas and dust content.

Contribution

It provides evidence that LINER nuclei are directly visible in X-rays, lacking the typical obscuring torus, and links their low accretion rates to their nuclear environment.

Findings

Strong X-ray and emission line luminosity correlation over three orders of magnitude.

Absence of circumnuclear obscuring structures in LINERs.

Low gas and dust content explains their low accretion and luminosity.

Abstract

We analyze HST spectra and Chandra observations of a sample of 21 LINERs, at least 18 of which genuine AGN. We find a correlation between the X-rays and emission lines luminosities, extending over three orders of magnitude and with a dispersion of 0.36 dex; no differences emerge between LINERs with and without broad lines, or between radio-loud and radio-quiet sources. The presence of such a strong correlation is remarkable considering that for half of the sample the X-ray luminosity can not be corrected for local absorption. This connection is readily understood since the X-ray light is associated with the same source producing the ionizing photons at the origin of the line emission. This implies that we have a direct view of the LINERs nuclei in the X-rays: the circumnuclear, high column density structure (the torus) is absent in these sources. Such a conclusion is also supported by mid-infrared data. We suggest that this is due to the general paucity of gas and dust in their nuclear regions that causes also their low rate of accretion and low bolometric luminosity.