Detection of Compact Nuclear X-Ray Emission in NGC 4736

Abstract

We report the results from a deep ROSAT PSPC observation of LINER galaxy NGC 4736. Two bright sources are detected, separated by only about 1', with the brighter one coinciding with the center of the galaxy. Neither source shows apparent X-ray variability on time-scales of minutes to hours in the ROSAT band. Simple power-law models, typical of AGN X-ray spectra, produce poor fits to the observed X-ray spectrum of the nuclear source. The addition of a Raymond-Smith component improves the fits significantly. This is consistent with the presence of hot gas in the nuclear region with kT=~0.3 keV, in addition to a compact nuclear source. However, a careful examination of the residuals reveal apparent features at low energies (< 0.25 keV). We find that the addition of a narrow emission line at about 0.22 keV is a significant improvement to the parameterization of the spectrum. We examine the results in the light of the accuracy of the PSPC spectral calibration. The derived photon index is about 2.3, which is similar to those for Seyfert 1 galaxies measured in the ROSAT energy range. On the other hand, the 0.1-2 keV luminosity for the compact source is only about 3.4x10^{39} erg/s, much fainter than typical Seyfert galaxies. We discuss the implications of these results on the connection between LINERs and AGNs. The off-center source is transient in nature. It has a hard X-ray spectrum, with a photon index of about 1.5, so is likely to be an X-ray binary. There is still some ambiguity regarding its association with the galaxy. If it is indeed located in the galaxy, the 0.1-2 keV luminosity would be greater than 5.1x10^{38} erg/s, making it a stellar-mass black hole candidate.