Three LINERs Under the Chandra X-Ray Microscope

Abstract

We use X-ray observations of three galaxies hosting LINERs (NGC 404, NGC 4736, NGC 4579) with Chandra to study their power sources. We find very diverse properties within this small group: NGC 404 has an X-ray-faint nucleus with a soft, thermal spectrum, NGC 4736 harbors a plethora of discrete X-ray sources in and around its nucleus, and NGC 4579 has a dominant nuclear point source embedded in a very extended, diffuse nebulosity. From their multi- wavelength properties we conclude the following: the nucleus of NGC 404 is the site of a weak, compact starburst, whose X-ray emission is due to gas heated by stellar winds and supernovae, NGC 4736 is in a recent or aging starburst phase, where the X-ray emission is dominated by a dense cluster of X-ray binaries, and NGC 4579 is powered by accretion onto a supermassive black hole. We detect 39 discrete sources in NGC 4736 and 21 in NGC 4579, most with L_X > 10^37 erg/s. One source in the disk of NGC 4579 could be an ultraluminous X-ray binary with L_X (2-10 keV) = 9x10^39 erg/s, but it could also be a background quasar. The most luminous discrete sources have simple power-law spectra, which along with their luminosities suggest that these are X-ray binaries accreting near or above the Eddington rate for a neutron star. By comparing the luminosity functions of discrete X-ray sources in these and other galaxies we find a potential connection between the age of the stellar population and the slope of the cumulative X-ray source luminosity function: galaxies with primarily old stellar populations have steeper luminosity functions than starburst galaxies. We suggest that this difference results from the contribution of high-mass X-ray binaries from the young stellar population to the upper end of the luminosity function.