Radio haloes around BL Lacertae objects AO0235+164 and 4C03.59

Abstract

THE rapid variations in intensity and linear polarisation of several BL Lacertae sources at centimetre and optical wavelengths have indicated the presence of multiple non-thermal components, often appreciably smaller than 1 pc (refs 1, 2). But there is no clear evidence for radio structures extended over kiloparsecs around any of the known variable BL Lac sources which have been scanned with narrow beams of a few arc s^3–7. The absence of extended radio structure in brighter and more variable BL Lac sources has also been inferred from their flat or inverted radio spectra and has led to the suggestion that these are extremely ‘young’ QSOs which have yet to develop a steady, non-thermal background^8–10. We report here the lunar occupation and interplanetary scintillation (IPS) observations of two BL Lac sources, AO0235+164 and 4C03.59 at 327 MHz. One of the two occulations of the former source, which is known to be an exceptionally violent variable at centimetre, infrared and optical wavelengths, took place in November 1975 near the epoch of its major outburst^11–13. These observations indicate that at 327 MHz, part of the emission of both these sources originates in components larger than 1 arc s.