Astronomers have discovered a new, bizarre type of cosmic object, ORC J0102–2450, while using the Australian Square Kilometer Array Pathfinder telescope. The colossal object is now the third odd radio circle (ORC) that is invisible to all wavelengths of light except radio with an elliptical galaxy at its geometrical center.
The objects appear in radio images as circular edge-brightened discs, observe the researchers. about one arcmin in diameter, that are unlike other objects previously reported in the literature. Astronomers have explored several possible mechanisms that might cause these objects, but none seems to be a compelling explanation.
The strange circular objects in question have been dubbed “Odd Radio Circles” (ORCs); three of them were recently discovered in data captured during a preliminary survey by the Australian Square Kilometer Array Pathfinder, a radio telescope array in Western Australia. A fourth was discovered when researchers sifted through old data from 2013.
No Known Object or Artefact
The new phenomenon was described in a new paper, Unexpected Circular Radio Objects at High Galactic Latitude, published on the preprint website arXiv, which was submitted to Nature Astronomy by a group of international astronomers. “Here we report the discovery of a class of circular features in radio images that do not seem to correspond to any of these known types of object or artefact, but rather appear to be a new class of astronomical object,” wrote the authors of the paper.
“Circular features are well-known in radio astronomical images, the researchers write, “and usually represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar shell, or a face-on disc such as a protoplanetary disc or a star-forming galaxy.”
ASKAP radio continuum contours overlaid onto an optical RGB color image above created from the Dark Energy Survey (DES)
Astronomers initially believed the ORCs may have been a telescope malfunction— which is why the discovery of the fourth ORC, from data that was gathered in 2013 by the Giant MetreWave Radio Telescope in India, was key to the finding. That observation confirmed that the phenomenon was not the result of a glitch of the specific Australian radio telescope array.
If Not That, Then What?
In the paper, the researchers suggest a list of hypotheses: First, they rule out that ORCs could be remnants of a supernova, mainly because of how rare ORC detections have been. Galactic planetary nebulae are ruled out, too, for the same reason. “[I]f the ORCs are [supernova remnants], which they strongly resemble, then this implies a population of SNRs [supernova remnants] in the Galaxy some 50 times larger than the currently accepted figure, or else a new class of SNR which has not previously been reported,” the researchers explain.
The researchers suspect the ORCs are a circular wave that appeared after some sort of extra-galactic “transient” event—like fast-radio bursts, another mysterious but far better documented astronomical phenomena.
“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,” the researchers write. “Several such classes 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 because of the “large angular size” the transient event in question “would have taken place in the distant past.”
The newly detected ORC has a radio ring diameter of around 70 arcseconds, or 978,000 light years. The source’s total radio flux was measured to be some 3.9 mJy, while its total radio luminosity was found to be approximately 140 billion TW/Hz. The object is most likely associated with the central elliptical galaxy DES J010224.33–245039.5.
The researchers conclude suggesting that it could be a relic lobe of a giant radio galaxy seen end-on or a giant blast wave, possibly from a binary supermassive black hole merger, resulting in a radio ring of such large size. A third scenario considered by the authors of the paper is that it could be a radio galaxy and intergalactic medium (IGM) interactions.
Source: Discovery of a new extragalactic circular radio source with ASKAP: ORC J0102-2450, arXiv:2104.13055
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Unexpected Circular Radio Objects at High Galactic Latitude and Salon