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Astronomers confused by “Odd Radio Circles”, a newly discovered, very rare space phenomenon

Astronomers believe they have discovered a new, bizarre type of cosmic object that is invisible to all wavelengths of light except radio.

The strange circular objects in question were unofficially referred to as “Odd Radio Circles” (ORCs). Three of them were recently discovered in data collected during a preliminary survey by the Australian Square Kilometer Array Pathfinder, a radio telescope array in Western Australia. A fourth Odd Radio Circle was discovered when researchers searched old data from 2013.

The new phenomenon is the focus of a new paper published on the preprint website arXiv, which was presented by a group of international astronomers in natural astronomy. It still has to be assessed by experts.

“Here we report the discovery of a class of circular features in radio images that do not appear to correspond to any of these known types of objects or artifacts, but appear to be a new class of astronomical objects,”

; the authors of the paper write.

The ORCs usually have a circular shape, with the exception of a disk-shaped one, and cannot be seen with infrared, optical or X-ray telescopes. Three of them are lighter on the edges.

The circularity of the ORCs has led to a certain curiosity about their true nature. “Circular features are known in astronomical radio images and usually represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar shell or a front screen such as a protoplanetary disk or a star-forming galaxy.” The researchers write.

The astronomers initially believed that the ORCs could have been a telescope error – which is why the discovery of the fourth ORC occurred Data collected by the Giant MetreWave Radio Telescope in India in 2013 was the key to this result. This observation ruled out the possibility that the phenomenon was merely an artifact of the specific Australian radio telescope array.

What could these strange, circular radio objects be? In the work, the researchers propose a list of scenarios. First, they rule out that ORCs may be remnants of a supernova, largely due to the rarity of ORCs. For the same reason, galactic nebulae are also excluded. “”[I]f are the ORCs [supernova remnants]what they closely resemble implies a population of SNRs [supernova remnants] in the galaxy about 50 times larger than the currently accepted number or a new class of SNR that has not yet been reported, “the researchers explain.

Instead, they suspect that the ORCs are a circular wave that occurred after an extragalactic “transient” event – like rapid radio waves, another mysterious but much better documented astronomical phenomenon.

“Edge brightening in some ORCs suggests that this circular image is a spherical object, which in turn indicates a spherical wave due to a transient event,” the researchers write. “Recently, several such classes of transient events have been discovered that can produce a spherical shock wave, such as fast radio bursts, gamma-ray bursts, and neutron star fusions.”

The researchers add that, due to the “large angular size”, the temporary event in question “had occurred in the distant past”.

Avi Loeb, chair of the Harvard Astronomy Department, emailed Salon that the ORCs “are likely to be the result of radio emissions from a spherical shock caused by an energy source at their center”.

“”They have a characteristic diameter of about one minute of arc, which corresponds to a physical length of ten light years (some parsecs) in our distance from most stars in the Milky Way or ten million light years (some megaparsecs) in our distance from most galaxies in the visible universe “said Loeb.” The former is a reasonable length scale for a supernova remnant, while the latter is a reasonable scale for the range of the jets produced by the most powerful quasars. “

However, since the distance to the source of the event is unknown, it remains unclear which interpretation is more likely.

Loeb added that the most likely explanation is that the ORCs are “the result of galaxy outflows”.

“We know that galaxies have strong winds that are driven by supernova explosions and quasar activity in their nuclei,” said Loeb. “It is predicted that the collision of these drains with the intergalactic medium will generate radio shells of the order of the distance between galaxies, which is several million light years, just as is required at a cosmological distance.”

Two decades ago, Loeb co-authored two articles that theoretically predict these “radio halos”.

“Perhaps this is an indication that they exist,” he added.

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