Selective enhancement of the QCD axion couplings
  We present a mechanism wherein the QCD axion coupling to nucleons, photons,
or electrons, can be enhanced selectively without increasing the axion mass. We
focus in particular on the axion-nucleon couplings, that are generally
considered to be largely model-independent, and we show how nucleophilic axion
models can be constructed. We discuss the implications of a nucleophilic axion
for astrophysics, cosmology and laboratory searches. We present a model with
enhanced axion couplings to nucleons and photons that can provide an excellent
fit to the anomalous emission of hard X-rays recently observed from a group of
nearby neutron stars, and we argue that such a scenario can be thoroughly
tested in forthcoming axion-search experiments.
