Unusual radio source MG1131+0456 - A possible Einstein ring

Abstract

The discovery of extremely unusual structure in the radio source MG1131+0456 is reported. In a radio map with subsarcsecond resolution the object appears as an elliptical ring of emission accompanied by a pair of more compact sources, nearly diametrically opposed and offset by about 0.3 arcsec to the southwest of the ring. There is a faint, slightly extended, optical counterpart. A continuum but no emission lines is detected in its spectrum; therefore, the redshift of the counterpart is unknown. The radio morphology suggests that this source may be an example of an 'Einstein ring', a highly symmetric case of gravitational lensing in which the source is imaged into a ring. Other explanations are considered and excluded by the data. Another possibility is that MG1131+04565 represents a new type of astronomical object.