The radio continuum spectrum of PSR 1937 + 214

The millisecond pulsar, PSR 1937+214, is a strong continuum radio source at low frequencies which displays two interesting, but presumably unrelated, characteristics. Although the flux density of this source has been observed since 1973, no time variations have been observed below 75 MHz, but variations do occur at higher frequencies. This is what would be expected if the variations were not intrinsic but, rather, were caused by slow interstellar scintillations. On the other hand, the spectrum of the source appears to steepen above 75 MHz. This steepening of the spectrum must be intrinsic to the source.