  We report on the discovery of a dramatic change in the energy spectrum of the
X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that
the cutoff power-law spectrum typical for accreting pulsars, including GX 304-1
at higher luminosities of $L_{\rm X}\sim 10^{36} - 10^{37}$ erg s$^{-1}$,
transformed at lower luminosity of $L_{\rm X}\sim 10^{34}$ erg s$^{-1}$ to a
two-component spectrum peaking around 5 and 40 keV. We suggest that the
observed transition corresponds to a change of the dominant mechanism
responsible for the deceleration of the accretion flow. We argue that the
accretion flow energy at low accretion rates is released in the atmosphere of
the neutron star, and the low-energy component in the source spectrum
corresponds to the thermal emission of the optically thick, heated atmospheric
layers. The most plausible explanations for the high-energy component are
either the cyclotron emission reprocessed by the magnetic Compton scattering or
the thermal radiation of deep atmospheric layers partly Comptonized in the
overheated upper layers. Alternative scenarios are also discussed.
