Astronomers believe they have discovered a strange new type of cosmic object that is invisible to all wavelengths of light except radio.
The strange circular objects in question have been unofficially named “Odd Radio Circles” (ORC); three of them were discovered in recent data accumulated during a preliminary survey by the Australian Pathfinder Square Kilometer, an array of radio telescopes in Western Australia. A fourth Odd Radio Circle was discovered when researchers reviewed old data from 2013.
The new phenomenon is the focus of a new article. posted on the arXiv prepress website, which was sent to Nature Astronomy by a group of international astronomers. Not yet peer reviewed.
“Here we report the discovery of a kind of circular feature in radio images that does not appear to correspond to any of these known types of objects or artifacts, but appears to be a new class of astronomical objects,” the authors of the article noted. to write.
ORCs are mostly circular in shape, with the exception of one disk-shaped, and cannot be seen with infrared, optical, or X-ray telescopes. Three of them are brighter around the edges.
The circular nature of ORCs has generated some curiosity about their true nature. “Circular features are well known in radio astronomical imaging, and generally represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar layer, or a forehead disk such as a protoplanetary disk or a galaxy that forms stars,” the researchers they write.
Astronomers initially believed that the ORCs may have been a telescope failure, so the discovery of the fourth ORC, from Data collected in 2013 by the Giant MetreWave radio telescope in India was key to the finding. That observation ruled out the possibility that the phenomenon was simply an artifact from the specific set of Australian radio telescopes.
So what could these strange circular radio objects be? In the document, the researchers suggest a list of scenarios. First, they rule out that ORCs could be remnants of a supernova, mainly because of how rare ORCs are. Galactic planetary nebulae are also discarded, for the same reason. “[I]f the ORCs are [supernova remnants], which are very similar, so this implies a population of SNR [supernova remnants] in the galaxy about 50 times larger than the currently accepted figure, or otherwise, a new class of SNR that has not been previously reported, “the researchers explain.
Instead, they suspect that ORCs are a circular wave that appeared after some sort of extragalactic “transient” event, such as fast radio bursts, another mysterious but much better documented astronomical phenomenon.
“The brightness of the edges in some ORCs suggests that this circular image may represent a spherical object, which in turn suggests a spherical wave from some transient event,” the researchers write. “Several kinds of transient events, capable of producing a spherical shock wave, have recently been discovered, such as fast radio bursts, gamma-ray bursts, and neutron star mergers.”
The researchers add that due to the “large angular size,” the transient event in question “would have occurred in the distant past.”
Avi Loeb, chairman of the Harvard astronomy department, told Salon by email that he believes the ORCs are “likely the result of radio emission from a spherical shock that resulted from an energy source at its center.”
“They have a characteristic diameter of about one arc minute, which corresponds to a physical length of ten light years (a few parsecs) at our distance from most stars in the Milky Way or ten million light years (a few megaparsecs). at our distance from most galaxies in the visible universe, “said Loeb.” The first is a reasonable length scale for a supernova remnant, while the second is a reasonable scale for the range of jets produced by the most powerful quasars. “
However, since the distance to the origin of the event is unknown, it is unclear which interpretation is more likely.
Loeb added that the most likely explanation is that the ORCs are “the result of the departures from galaxies.”
“We know that galaxies have powerful winds, driven by supernova explosions and quasar activity at their nuclei,” said Loeb. “The collision of these outlets with the intergalactic medium is expected to produce radio layers on the scale of the distance between the galaxies, which are a few million light years, exactly as needed at a cosmological distance.”
Two decades ago, Loeb co-authored two articles that theoretically predicted these “radio halos.”
“Perhaps this is an indication that they exist,” he added.
