X-ray properties of liners and low-luminosity Seyfert galaxies observed with ASCA. I. Observations and results

This paper presents a comprehensive study of the X-ray properties of low-ionization nuclear emission-line regions ( LINERs) and low-luminosity Seyfert galaxies based on observations obtained with the ASCA satellite. We analyzed data of 53 observations of 21 LINERs and 17 low-luminosity Seyferts. X-ray emission has been detected in all but one object. The X-ray luminosities in the 2-10 keV band range from 4 x 10(39) to 5 x 10(41) ergs s(-1), which are 1-3 orders of magnitude smaller than in classical Seyfert galaxies. The X-ray spectra of most objects are well described by a canonical model which consists of ( 1) a soft component from a thermal plasma with kT < 1 keV and ( 2) a hard component represented by a power law with a photon index of Gamma approximate to 1.8 or thermal bremsstrahlung emission with kT approximate to 10 keV. Several objects do not require the soft thermal component, and their continua are well fitted by a single power-law model. Some objects show heavy absorption with column densities in excess of 10(23) cm(-2). We detect in several objects Fe K line emission with equivalent widths ranging from 50 eV to 2 keV. Variability on timescales less than a day is uncommon in our sample. By comparing multiple observations made with ASCA or with published 2-10 keV observations from other satellites, we show that at least eight objects are variable on timescales of a week to several years. We find that the morphologies of many objects, both in the soft and hard bands, are consistent with being pointlike relative to the telescope point-spread function; a few are clearly extended in either or both energy bands. The second paper of this series will discuss the physical interpretation of the X-ray emission and its implications for low-luminosity active galactic nuclei.