Mid-Infrared and X-ray luminosity correlations of X-ray point sources in
  NGC 1399
  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 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
under-luminous in the IR with L_{IR} << L_{X}. Since Ultra-luminous 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 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} ~ 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} ~ 10^{39} ergs/s. We
calculate their g-z colours and find that the bright X-ray sources with IR
counterparts are typically blue in optical color. 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.
