Detection of Compact Nuclear X-Ray Emission in NGC 4736

摘要

We report the results from a deep ROSAT Position Sensitive Proportional Counter (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 timescales of minutes to hours in the ROSAT band. Simple power-law models, typical of active galactic nucleus (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 reveals 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.