A New Class of Radio Source Spectra

Abstract

THE flux densities of 35 non-thermal radio sources were measured at 8,000 Mc/s (λ 3.75 cm) with the University of Michigan's 85-ft. reflector. Comparison of these measurements with the spectra of the sources at frequencies less than 3,200 Mc/s (ref. 1) showed that most spectra were either straight (constant spectral index) or curved downward (steepening spectrum) at the high-frequency end of the spectrum. However, the radio source 3C 84 which has been identified with the Seyfert galaxy NGC 1275 (refs. 2 and 3) was found to have a flux density at 8,000 Mc/s which was four times greater than that expected from a straight line extrapolation of the spectrum observed at frequencies less than 3,200 Mc/s. This is the first reported instance of a radio spectrum having such a pronounced upward turn in its spectrum with increasing frequency. (The novel spectrum of NGC 1275 was first reported by us at the International Scientific Radio Union General Assembly in Tokyo, September 9–20, 1963.) Thus NGC 1275 may represent a new class of radio source spectra. The radio source 3C 279 was also investigated because a pronounced upward turn of its spectrum was predicted by J. A. Roberts from measurements up to 2,650 Mc/s; we found its spectrum to be similar to NGC 1275. A third source 3C 273 has a spectral flattening at the high frequency end, but does not have a positive spectral index as do the foregoing two sources.