Evolution of the radio emission of the Crab nebula from long-term observations at 927 and 151.5 MHz

Abstract

Radio flux measurements of the Crab nebula have been performed over many years relative to Orion A at 927 MHz and relative to Cygnus A and Virgo A at 151.5 MHz. The inferred average secular rates of decrease in the radio flux of the Crab nebula are d _{927 MHz} = −0.18 ± 0.10% yr⁻¹ over 1977–2000 and d _{151.5 MHz} = −0.3 ± 0.1% yr⁻¹ over 1980–2003. The weighted mean flux-decrease rate averaged over several years of relative measurements at 86, 151.5, 927, and 8000 MHz is d _mw = −0.17 ± 0.02% yr⁻¹. The secular flux decrease is frequency independent, with an upper limit of |dα/dt| < 3 × 10⁻⁴ yr⁻¹ for the absolute value of the rate of change of the spectral index, and remains constant in time when averaged over long time intervals. The results of our measurements at 151.5 and 927 MHz combined with published absolute measurements at 81.5 and 8250 MHz are used to determine the radio spectrum of the Crab nebula for epoch 2010.0.