Detection of Compact Nuclear X‐Ray Emission in NGC 4736

We report the results from a deep ROSAT PSPC observation of LINER galaxy NGC4736. Two bright sources are detected, separated by only about 1', with thebrighter one coinciding with the center of the galaxy. Neither source showsapparent X-ray variability on time-scales of minutes to hours in the ROSATband. Simple power-law models, typical of AGN X-ray spectra, produce poor fitsto the observed X-ray spectrum of the nuclear source. The addition of aRaymond-Smith component improves the fits significantly. This is consistentwith the presence of hot gas in the nuclear region with kT=~0.3 keV, inaddition to a compact nuclear source. However, a careful examination of theresiduals reveal apparent features at low energies (< 0.25 keV). We find thatthe addition of a narrow emission line at about 0.22 keV is a significantimprovement to the parameterization of the spectrum. We examine the results inthe light of the accuracy of the PSPC spectral calibration. The derived photonindex is about 2.3, which is similar to those for Seyfert 1 galaxies measuredin the ROSAT energy range. On the other hand, the 0.1-2 keV luminosity for thecompact source is only about 3.4x10^{39} erg/s, much fainter than typicalSeyfert galaxies. We discuss the implications of these results on theconnection 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 isstill some ambiguity regarding its association with the galaxy. If it is indeedlocated in the galaxy, the 0.1-2 keV luminosity would be greater than5.1x10^{38} erg/s, making it a stellar-mass black hole candidate.Comment: 15 pages, LaTeX (aaspp4.sty), macro rotate.tex, 10 postscript figures (including 3 color prints) and rotate.tex available at ftp://space.mit.edu/pub/cui/ngc4736/ . Accepted for publication in Ap