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

It is known that the infrared (IR) and X-ray luminosities of active galactic nuclei (AGN) are correlated with L-IR similar to L-X. Moreover, the IR flux ratio between the 5.8 and 3.6 mu m bands is a good distinguishing characteristic of AGN or AGN-like behaviour. On the other hand, Galactic X-ray binaries (GXB) are underluminous in the IR with L-IR < L-X. Since ultraluminous X-ray sources in nearby galaxies may be an intermediate class between AGN and GXB, it is interesting to study if their IR properties indicate which kind of objects they resemble. We use Spitzer InfraRed Array Camera (IRAC) images to identify mid-IR counterparts of bright X-ray sources, detected by Chandra in the elliptical galaxy NGC 1399. We find that for sources with AGN-like IR flux ratios, the IR luminosity strongly correlates with that in X-rays, L-IR similar to L-X, while for the others, there is no correlation between the two. Some of the former objects may be background AGN. If they are not strongly contaminated by background AGN, this result extends the IR-X-ray luminosity correlation down to L-X similar to 10(39) erg s(-1). We calculate their g - z colours and find that the bright X-ray sources with IR counterparts are typically blue in optical colour. This is in contrast to typical X-ray sources, without IR counterparts which have predominantly red optical counterparts. We highlight the need for IR or optical spectra of these sources to distinguish background AGN and unveil the effect of the X-ray emission on the different environments of these systems.