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

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6. Summary and conclusions

We have investigated the soft X-ray properties of a number of LINERs based on ROSAT survey and pointed PSPC and HRI observations.

Luminosities range between [FORMULA] (NGC 404) and 40.8 (NGC 4450). The ratios [FORMULA]/[FORMULA], when compared to early-type galaxies, are located in the intermediate region and similar emission mechanisms may contribute to the observed X-ray luminosities.

Whereas the bulk of the X-ray emission is consistent with arising from a point source there is some extended emission seen at weak emission levels in some sources.

Spectra are best described by either a powerlaw of photon index [FORMULA] [FORMULA] (NGC 3898, NGC 4450, NGC 5371), or a Raymond-Smith model with heavily depleted abundances (NGC 2768, NGC 3642). Since the inferred metal abundances are implausibly low we take this latter model as indication of a more complex soft X-ray spectral shape/ emission mechanism (e.g., a powerlaw or other hard component plus RS emission from gas with [FORMULA] solar abundances; or plasma out of equilibrium). Some sources are best described by a single AGN-like powerlaw with X-ray luminosity above that expected from discrete stellar sources. These spectra most likely indicate the presence of low-luminosity AGNs in the centers of the LINER galaxies.

The absence of (short-timescale) variability is consistent with the earlier suggestion that LINERs may accrete in the advection-dominated mode. Only one source (NGC 2768) seems to be slightly variable on a timescale of months.

For several LINER galaxies nearby second X-ray sources are discovered with countrates roughly one-tenth those of the target sources. We cannot identify an obvious detector effect. If at the distances of the galaxies, their luminosities are on the order of several [FORMULA] erg s-1.

Given the spectral variety of LINERs with contributions from several emission components, future studies of both, spectra of individual objects as well as larger samples will certainly give further insight into the LINER phenomenon which provides an important link between active and `normal' galaxies.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999