Detection of Nuclear X-Ray Sources in Nearby Galaxies with [ITAL]Chandra[/ITAL]

Abstract

We report preliminary results from an arcsecond-resolution X-ray survey of nearby galaxies using the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. The total sample consists of 41 low-luminosity active galactic nuclei (AGNs), including Seyfert galaxies, LINERs, and LINER/H II transition objects. In the initial subsample of 24 objects observed thus far, we detect in ~62% of the objects a compact, pointlike source astrometrically coincident with either the optical or radio position of the nucleus. The high detection rate strongly suggests that the majority of the objects do contain weakly active, AGN-like cores, presumably powered by central massive black holes. The 2-10 keV luminosities of the nuclear sources range from less than 10 38 to 10 41 ergs s -1 , with a median value of 2 × 10 38 ergs s -1 . Our detection limit corresponds to L X (2-10 keV) ≈ 8 × 10 37 ergs s -1 for the typical sample distance of 12 Mpc; this limit is 2 orders of magnitude fainter than the weakest sources of this kind previously studied using ASCA or BeppoSAX . The new data extend toward lower luminosities the known linear correlation between hard X-ray and Hα luminosity for broad-line AGNs. Many narrow-line objects do contain X-ray cores, consistent with either weak AGNs or X-ray binary systems, but they have X-ray luminosities a factor of 10 below the L X - L Hα relation of the broad-line sources. Their distributions of photon energies show no indication of exceptionally high absorption. The optical line emission in these nuclei is likely powered, at least in part, by stellar processes.