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It is known that the IR and X-ray luminosities of Active Galactic Nuclei(AGN) are correlated with L_{IR} ~ L_{X}. Moreover, the IR flux ratio betweenthe 5.8 and 3.6 \mu m bands is a good distinguishing characteristic of AGN orAGN-like behaviour. On the other hand, Galactic X-ray binaries (GXB) areunder-luminous in the IR with L_{IR} << L_{X}. Since Ultra-luminous X-raysources in nearby galaxies may be an intermediate class between AGN and GXB, itis interesting to study if their IR properties indicate which kind of objectsthey resemble. We use Spitzer IRAC images to identify mid-IR counterparts ofbright X-ray sources, detected by Chandra in the elliptical galaxy NGC 1399. Wefind that for sources with AGN-like IR flux ratios, the IR luminosity stronglycorrelates with that in X-rays, L_{IR} ~ L_{X}, while for the others, there isno correlation between the two. Some of the former objects may be backgroundAGN. If they are not strongly contaminated by background AGN, this resultextends the IR-X-ray luminosity correlation down to L_{X} ~ 10^{39} ergs/s. Wecalculate their g-z colours and find that the bright X-ray sources with IRcounterparts are typically blue in optical color. This is in contrast totypical X-ray sources, without IR counterparts which have predominantly redoptical counterparts. We highlight the need for IR or optical spectra of thesesources to distinguish background AGN and unveil the effect of the X-rayemission on the different environments of these systems.

Mid-Infrared and X-ray luminosity correlations of X-ray point sources in NGC 1399