2.5. Clues from the X-rays
Compact soft X-ray emission on the scale of the ROSAT HRI camera (~ 5") has now been detected in a handful of LINERs (e.g., Worral and Birkinshaw 1994; Koratkar et al. 1995; Fabbiano and Juda 1997), although no statistical conclusions can yet be drawn based on the scant data available. Most of the core sources have luminosities clustering near L(0.5-2 keV) 1040-1041 ergs s-1 because of selection effects. The pointlike morphology of the ROSAT images certainly agrees with our expectation for an AGN source, but we must remember that the 5" point-spread function of the HRI subtends an uncomfortably large region (several hundred parsecs) at the typical distances of these objects. Images taken at much higher angular resolution and ideally at harder energies, such as would be possible with the ACIS camera on AXAF (see the concluding remarks at the end of the paper), are needed to put more stringent constraints on the nature of the X-ray emission.
In the meantime, progress can be made by examining the ASCA hard X-ray spectra of LINERs whose X-ray structure is found to be compact on HRI images. These data, again, are scarce, and current constraints by necessity bias the sample in favor of the brightest targets. Nonetheless, when the observations are considered collectively (Serlemitsos et al. 1996; Terashima et al. 1997; Ptak 1998; Ptak et al. 1998; Iyomoto et al. 1998; Nicholson et al. 1998; see contributions by H. Awaki and Y. Terashima), the following trends appear. (1) The 2-10 keV continuum can generally be modeled as a single power-law function modified by cold absorption; the best-fitting photon index, 1.7-1.8, agrees well with values normally measured in luminous AGNs. In some cases the fits require an additional soft thermal component with a temperature of kT 1 keV. (2) There is no evidence for significant amounts of cold material along the line of sight. Any measurable absorbing column in excess of the Galactic contribution usually does not exceed NH 1021 cm-22. (3) Broad Fe K emission at 6.4 keV, a feature common to many luminous Seyfert 1 nuclei, is usually either absent or unusually weak. The composite LINER spectrum of Terashima et al. (1997) shows no detectable Fe K line to an equivalent-width limit of 140 eV. In the few cases where an iron line has been detected, the rest energy is ~ 6.7 keV, consistent with ionized instead of neutral iron. And (4), these sources do not undergo rapid X-ray variability at the level expected from extrapolation of the variability behavior established for more luminous sources.