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Astron. Astrophys. 349, 88-96 (1999)

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1. Introduction

Low-ionization nuclear emission line regions, LINERs, are characterized by their optical emission line spectrum which shows a lower degree of ionization than Seyfert galaxies (e.g., Heckman et al. 1980). Their major power source and line excitation mechanism has been a subject of lively debate ever since their discovery (for reviews see, e.g., Filippenko 1989, 1993, Ho 1998). LINERs manifest the most common type of activity in the local universe. If powered by accretion, they probably represent the low-luminosity end of the quasar phenomenon and their presence has relevance to, e.g., the evolution of quasars, the faint end of the AGN luminosity function, and the presence of supermassive black holes (SMBHs) in nearby galaxies. A detailed study of the LINER phenomenon is thus very important.

Many different mechanisms that might account for their optical emission line spectra have been examined, including collisional ionization and excitation (Burbidge & Burbidge 1962), shock heating (e.g., Fosbury et al. 1978, Heckman 1980, Dopita et al. 1996, Contini 1997), photoionization by hot stars (e.g., Shields 1992, Ho et al. 1993), photoionization by a non-stellar continuum source (e.g., Ferland & Netzer 1983, Halpern & Steiner 1983, Binette 1984,1985,1986, Ho et al. 1993), and photoionization by an absorption-diluted AGN continuum (Halpern & Steiner 1983, Schulz & Fritsch 1994). Despite this detailed shock and photoionization modelling the nature of the main ionizing source of LINERs remained elusive, although there is now growing evidence that they are accretion powered (e.g., Falcke et al. 1997, Falcke 1998, Ho 1998). Eracleous et al. (1995), in an effort to explain the UV bright centers detected in some but not all LINERs, suggested a duty cycle model where central activity in LINERs is governed by occasional tidal disruptions of stars by central black holes. In an alternative approach, Barth et al. (1998) suggested that dust extinction could cause the UV darkness of some LINERs.

X-rays are a powerful tool to investigate the presence of an AGN via X-ray variability, luminosity, and extent, and to explore the physical properties of LINERs in general. Nevertheless, not many LINERs have been examined in X-rays, particularly not larger samples in an homogeneous way. The largest previous one we are aware of was presented by Ptak et al. (1999, see also Serlemitsos et al. 1997) and consisted of several low-luminosity AGN (LLAGN) including 5 LINERs observed with ASCA . They find that the ASCA spectra are best described by a two-component model consisting of soft thermal emission and a powerlaw with photon index [FORMULA] [FORMULA], with varying relative contributions of the two spectral components from object to object. Several studies of individual objects (e.g., Mushotzky 1982, Koratkar et al. 1995, Ehle et al. 1995, Cui et al. 1997, Terashima et al. 1998, Pietsch et al. 1998, Roberts et al. 1999) revealed results consistent with the above spectral results. A fairly complex X-ray spectrum was recently reported for the LINER NGC 1052 (Weaver et al. 1999). Concerning the spatial extent of the X-ray emission, Koratkar et al. (1995; K95 hereafter) found their two LINERs to be consistent with a point source within the limits of the ROSAT HRI resolution. X-ray luminosities range up to [FORMULA] 1041 erg s-1 (e.g., Ptak et al. 1999, Stockdale et al. 1998, K95; see Halpern & Steiner 1983 for a collection of Einstein luminosities) and are presently biased towards the X-ray brightest objects.

Given the importance to better understand the LINER phenomenon and activity in nearby galaxies in general, with its potential bearing on the evolution of SMBHs in galaxies, the contribution to the faint end of the AGN luminosity function, and the soft X-ray background; and given the still limited number of objects previously studied in the X-ray spectral region, we examined a sample of 13 LINERs with the ROSAT (Trümper 1983) instruments (Pfeffermann et al. 1987). We report here the results of an investigation of the spectral, spatial, and temporal X-ray properties of these galaxies.

Our sample consists of spiral galaxies and lenticulars. The primary selection was according to the list of Huchra & Burg (1992, with most LINERs identified in Heckman 1980, Stauffer 1982, and Keel 1983) and Huchra (1998, priv. com.). We then excluded LINERs that were, on the basis of emission lines, re-classified as Seyferts or listed to contain a Seyfert-component as well according to the NED database. This resulted in 13 remaining LINERs. Results for a larger sample of LLAGN, including the objects with composite spectra and Seyfert 2 galaxies will be reported elsewhere. For the selected sources we analyzed ROSAT all-sky survey data. In addition, 8 of the galaxies were targets of, or serendipituously located in the field of view of, PSPC and/or HRI observations. All except one are detected, and for 5 of them a PSPC spectral analysis was possible. The brightest source turned out to be NGC 4450 which is studied in most detail below.

The paper is organized as follows: The data reduction is described in Sect. 2. In the next two Sections we present the general assumptions on which the data analysis is based (Sect. 3) and results for the individual objects (Sect. 4), including a discussion of (the reality of) further X-ray sources close to the target sources. The discussion (Sect. 5) is followed by the concluding summary in Sect. 6.

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Online publication: August 25, 1999