from observed data of $\sigma$=0.61 based on 63 galaxies. In this formula and in the following, all the luminosities are expressed in solar units.

If, instead of the blue luminosity, we use the galaxy area $D^2_{kpc}$, calculated from the apparent diameter measured at the 25 mag arcsec$^2$ isophote and converted in kpc$^2$, we discover that the relation is still present, but with a larger spread. It becomes: $$Log L_X = Log D^2_{kpc} + 3.83$$ ($\sigma$=0.80) for a sample of 64 galaxies.

A relation similar to that of @ciotti has been found by some authors [@padovani; @david; @ranalli], but using 60 $\mu$m fluxes or FIR luminosities. The values of $L_{FIR}$ are calculated using the formula: $$Log L_{FIR}= 2.59+Log(2.58\ S_{60}+S_{100})+2 Log\ d$$ where L$_{FIR}$ is in solar luminosities, fluxes are in mJy and the galaxy distance d is in Mpc.

From our data it is possible to find a relation between L$_X$ and L$_{FIR}$ that fits the values of late-type galaxies. We found L$_X \propto$ L$_{FIR}^{0.90}$, similar to the L$_X \propto$ L$_{FIR}^{0.88}$ found by @ranalli for fluxes between 0.5 and 2 keV and to L$_X \propto$ L$_{FIR}^{0.95}$ found by @david using fluxes between 0.5 and 4.5 keV. Forcing the relation to a 