A ROSAT HRI survey of bright nearby galaxies

We use the extensive public archive of ROSAT High Resolution Imager (HRI) observations to carry out a statistical investigation of the X-ray properties of nearby galaxies. Specifically we focus on the sample of 486 bright (BT ≤ 12.5) northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the context of their exploration of the optical spectroscopic properties of nearby galactic nuclei. Over 20% of HFS galaxies are encompassed in ROSAT HRI fields of reasonable (≥ 10ks) exposure. The X-ray sources detected within the optical extent of each galaxy are categorised as either nuclear or non-nuclear depending on whether the source is positioned within or outside of a 25 radius circle centred on the optical nucleus. A nuclear X-ray source is detected in over 70% of the galaxies harbouring either a Seyfert or LINER nucleus compared to a detection rate of only ∼ 40% in less active systems. The correlation of the Hα luminosity with nuclear X-ray luminosity previously observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the regime of ultra-low luminosity (LX∼ 10 −10 erg s) active galactic nuclei (AGN). The inferred accretion rates for this sample of low-luminosity AGN are significantly sub-Eddington. In total 142 non-nuclear sources were detected. In combination with published data for M31 this leads to a luminosity distribution (normalised to an optical blue luminosity of 10 L⊙) for the discrete X-ray source population in spiral galaxies of the form dN/dL38 = (1.0 ± 0.2)L −1.8 38 , where L38 is the X-ray luminosity in units of 10 erg s. The implied LX/LB ratio is ∼ 1.1 × 10 39 erg s(10 L⊙) . The nature of the substantial number of “super-luminous” non-nuclear objects detected in the survey is discussed.