Norris, and 30 co-authors, reported in 2020 that they have found a class of circular radio objects in the Evolutionary Map of the Universe Pilot Survey, using the Australian Square Kilometer Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs, about one arcmin diameter, that are unlike other objects previously reported in the literature. The authors explore several possible mechanisms that might cause these objects, but none seems to be a compelling explanation.
The authors concluded that the anomaly is a potentially new class of radio-astronomical object, consisting of a circular disc seen only at radio wavelengths, which in some cases is limb-brightened, and sometimes contains a galaxy at its center. The authors know they are real, because they have been detected using three different telescopes using different software. A careful search of the literature has found no previous report of such circular radio objects, other than the well-known objects, such as supernova remnants and starburst rings, which the authors have shown above can not explain their observations. The authors therefore propose that they are a new class of radio source.
Such a discovery is not unexpected when a new telescope such as ASKAP observes the sky in a relatively unexplored part of the observational parameter space (Norris, 2017a,b). In particular, no previous telescope was able to survey large areas of sky with the low-surface brightness sensitivity, combined with good spatial resolution, of ASKAP. In this paper, the authors have shown that none of the known types of radio object seems able to explain the ORCs as a class. For example, if the ORCs are Galactic SNRs, which they strongly resemble, then this implies a population of SNRs in the Galaxy several orders of magnitude larger than the currently accepted figure, or else a new class of SNR which has not previously been reported. If the ORCs are starburst rings, which they also strongly resemble, then the authors should have been able to see the stellar ring counterpart in optical observations.
The authors speculated that the edge-brightening in some ORCs suggests that this circular image may represent a spherical object, which in turn suggests a spherical wave from some transient event. Several such classes of transient events, capable of producing a spherical shock wave, have recently been discovered, such as fast radio bursts (Bannister et al., 2019), gamma-ray bursts (Ayal & Piran, 2001), and neutron star mergers (Hotokezaka &
Piran, 2015). However, because of the large angular size Unexpected Circular Radio Objects 19 of the ORCs, any such transients would have taken place in the distant past. It also seems unlikely that such an event could cause two ORCs together, as seen in ORCs 2 and 3.
It is also possible that the ORCs represent a new category of a known phenomenon, such as the jets of a radio galaxy or blazar when seen end-on, down the “barrel” of the jet. Alternatively, they may represent some remnant of a previous outflow from a radio galaxy. However, no existing observations of this phenomenon closely resemble the ORCs in features such as their striking circularity, the edge-brightening in most cases,
or the absence of a visual blazar or radio galaxy at the center.
The authors also acknowledge the possibility that the ORCs may represent more than one phenomenon, and that they have been discovered simultaneously because they match the spatial frequency characteristics of the ASKAP observations, which occupy a part of the observational parameter space which has hitherto been poorly studied. For example, if ORCs 2 and 3 are explained in terms of an unusual radio galaxy, then a spherical wave from a transient event may explain ORCs 1 and 4.
Finally, we note that Kirillov & Savelova (2020) have proposed that an ORC may represent the throat of a wormhole. Further work is continuing to investigate the nature of these objects.
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