Observations of X-Ray Sources in LINERs and Models for Their Interpretation

In our quest for the power source of LINERs we have observed some objects with the Chandra X-ray observatory at high spatial resolution and analyzed archival Chandra images of many others. We have found that a significant fraction of LINERs host clusters of luminous X-ray binaries (XRBs; L x ~ 1038 erg/s), with X-ray luminosity functions flatter than those of elliptical galaxies and similar to those of star-forming galaxies. This suggests that these are HMXBs from a recent episode of star-formation. To interpret the observations we have simulated the evolution of XRBs from a burst of star formation of duration 20 Myr and star-formation rate 10 M⊙/yr. This population reaches a maximum 2-10 keV luminosity of ~ 1? 1040 erg/s after ~ 20 Myr. The X-ray luminous phase is sustained for a period of several tens of Myr by succeeding populations of systems with lighter secondary stars, i. e., it persists long after the star-formation episode has ended. The peak X-ray luminosity is consistent with observationally-derived correlations between the star-formation rate and the observed hard X-ray luminosity and is comparable to those of some Seyfert galaxies. Thus, X-ray observations at low spatial resolution, will not be able to distinguish such a compact starburst from a weak AGN. We also investigate the evolution of such a system, after the end of star formation, in the X-ray vs Halpha luminosity diagram (sometimes used as an aid in the classification of LINERs). After star formation ceases, the system moves away from the starburst locus, as the hot stars die and the Halpha luminosity declines rapidly. At late times, however, ( ~ 60 Myr after the end of star formation and up to 2 Gyr) post-AGB stars provide ionizing photons to sustain the Halpha luminosity. As a result the system lingers in the vicinity of the low-luminosity AGN locus.