  Direct Chandra observations of a surface temperature of isolated neutron star
in Cassiopeia A (Cas A NS) and its cooling scenario which has been recently
simultaneously suggested by several scientific teams put stringent constraints
on poorly known properties of the superfluid neutron star core. It was found
also that the thermal energy losses from Cas A NS are approximately twice more
intensive than it can be explained by the neutrino emission. We use these
unique data and well-defined cooling scenario to estimate the strength of KSVZ
axion interactions with neutrons. We speculate that enlarged energy losses
occur owing to emission of axions from superfluid core of the neutron star. If
the axion and neutrino losses are comparable we find $c_{n}^{2}m_{a}^{2}\sim
5.7\times 10^{-6}\,\text{eV}^2$, where $m_{a}$ is the axion mass, and $c_{n}$
is the effective Peccei-Quinn charge of the neutron. (Given the QCD
uncertainties of the hadronic axion models, the dimensionless constant $c_{n}$
could range from $-0.05$ to $ 0.14$.)
