An unusually strong Einstein ring in the radio source PKS1830–211

RADIO observations of the strong, flat-spectrum radio source PKS1830–211 revealed a double structure, with a separation of 1 arcsec, suggesting that it might be a gravitationally lensed object1. We have now obtained high-resolution radio images of PKS1830–211 from several interferometric radiotelescope networks, which show an unusual elliptical ring-like structure connecting the two brighter components. The presence of the ring, and the similarity of the two brighter spots, argue strongly that this is indeed a gravitationally lensed system, specifically an Einstein ring in which lens and lensed object are closely aligned. Although the source is close to the galactic plane, it seems that both the lens and background (lensed) object are extragalactic. This object is one hundred times brighter than either of the two previously discovered radio Einstein rings, and is among the six brightest flat-spectrum sources in the sky. Its brightness makes it a peculiar object: it must involve either a chance alignment of a lensing object with an unusually bright background source, or an alignment with a less bright object but amplified to an unusual degree.