| Abstract |
It is known that the Infrared and X-ray luminosities of Active Galactic Nuclei (AGN) are correlated with L_IR ~ L_X. Moreover, the Infrared flux ratio between the 5.8 and 3.6 micron bands is a good distinguishing characteristic of AGN or AGN-like behavior. On the other hand, Galactic X-ray binaries (GXB) are under-luminous in the Infrared with L_IR << L_X. Since Ultra-luminous X-ray sources in nearby galaxies maybe 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-infrared 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 the 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 optical g-z colors, to show 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. |
