Cooling of a Neutron Star by the "Urca" Process

The energy loss of a degenerate neutron gas by the "Urca" process is calculated in this paper; it is $7.36\times {10}^4{\left(\frac{T}{{10}^9}\right)}^8$ erg ${\mathrm{g}}^{-1\mathrm{}}$ for a density of 6×${10}^{14}$ g ${\mathrm{cm}}^{-3\mathrm{}}$. It follows from this energy loss that the Urca process alone should have cooled the core of the neutron star created in the type I supernova of 1054 a.d. to a temperature around 5×${10}^8$ °K. The emission power of the star should then be about one order of magnitude smaller than that of the x-ray source discovered recently in the Crab nebula; the source cannot be interpreted therefore simply as the thermal radiation of the star. This conclusion is consistent with the result of a recent experiment performed by the method of lunar occultation, indicating an angular size of the source comparable to that of the Crab nebula. A more refined experiment performed by the same method should, on the other hand, make it possible to decide whether a neutron star exists in the Crab nebula.