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[2.13] The Central Engines of Liners Dissected by Chandra

M. Eracleous (Penn State U.), J. C. Shields (Ohio U.), E. C. Moran (U. C. Berkeley), J. P. Halpern (Columbia U.)

In order to understand the nature of the power sources in LINERs, we have used the ACIS-S camera on board Chandra\/ to obtain deep observations of two nearby objects, NGC~4579 and NGC~4736. The targets were selected because they both have UV sources in their nuclei, which are unresolved by the HST, and because ASCA\/ observations showed them to be bright X-ray sources. Moreover, the optical spectrum of NGC~4579 features broad Balmer lines, while that of NGC~4736 does not. The high spatial resolution of Chandra} allows separate the contributions from different X-ray sources in the field, thus avoiding the confusion that plagued earlier ASCA observatons. We find that NGC~4579 shows the traits of a low-luminosity AGN. The X-ray emission is dominated by a central, unresolved source, which has a \Gamma~1.4 power-law spectrum and a 2--10~keV luminosity of order 10⁴⁰~erg~s^-1. In addition, diffuse nebulosity and about a dozen luminous discrete sources (10³⁷ - 10³⁹~erg~s^-1) are detected outside the nuclear region up to a few kpc from the center. In contrast, the properties of NGC~4736 provide no convincing evidence for the presence of an AGN. The central 10^\prime\prime (i.e., 200~pc) of this galaxy harbors a bright diffuse source with a cluster of 5 discrete sources embedded in it. A multi-temperature thermal model is required to fit the spectrum of the diffuse emission, which has a luminosity of about 10³⁹~erg~s^-1. The spectra and luminosities of the discrete sources suggest that these are X-ray binaries (some may be young, compact supernova remnants). Surrounding the central, composite source is a cluster of two dozen discrete sources, with luminosities of 10³⁷ - 10³⁹~erg~s^-1, as well as a low-surface brightness halo. The combined X-ray luminosity of all of the sources is several times 10³⁹~erg~s^-1 Our results strongly favor a scenario in which LINERs are a heterogeneous class of objects that are powered in some cases by accretion onto a massive compact object and in others by stellar processes. This work is supported by NASA through grant number GO0-1152A from the SAO/CSC.