Lunar Occultation of the Radio Source CTA 21

Abstract

THE radio sources CTA 21 and CTA 102 have been of considerable interest since they were first reported by Harris and Roberts1 from observations at 958 Mc/s. Later work2 showed that the radio spectra of these sources differed from those of most other sources and that each source had a maximum of emission near 900 Mc/s. It has been suggested3,4 that this form of spectrum is produced by self-absorption of synchrotron radiation within a source of small angular size, and from the shape of their spectra the very small angular diameters of ∼ 0.01 sec of arc have been predicted for both CTA 21 and CTA 102. CTA 102 has recently been identified5 as a quasi-stellar radio source, but for CTA 21 no optical object can be detected on the 48-in. Sky Survey plates within the limits of error (± 6 sec of arc) of the existing radio source position6,7. It appears that an identification of CTA 21 with an optical object can only be made if the radio source position is more precisely known and the region of sky can be examined with more powerful optical techniques.