Odd radio circle

{{Short description|Unexplained circular astronomical object detected only by radio waves}}

{{redirect|ORCs|the singular form|orc (disambiguation)}}

{{use dmy dates |date=July 2020}}

File:ORC J2103-6200 2022.jpg telescope superimposed on an optical image from the Dark Energy Survey (2022){{Cite journal |last1=Norris |first1=Ray P |last2=Collier |first2=J D |last3=Crocker |first3=Roland M |last4=Heywood |first4=Ian |last5=Macgregor |first5=Peter |last6=Rudnick |first6=L |last7=Shabala |first7=Stas |last8=Andernach |first8=Heinz |last9=da Cunha |first9=Elisabete |last10=English |first10=Jayanne |last11=Filipović |first11=Miroslav |date=2022-03-24 |title=MeerKAT uncovers the physics of an Odd Radio Circle |journal=Monthly Notices of the Royal Astronomical Society |volume=513 |language=en |pages=1300–1316 |arxiv=2203.10669 |doi=10.1093/mnras/stac701 |doi-access=free |issn=0035-8711}}{{Cite web |last=Norris |first=Ray |title='Odd radio circles' that baffled astronomers are likely explosions from distant galaxies |url=http://theconversation.com/odd-radio-circles-that-baffled-astronomers-are-likely-explosions-from-distant-galaxies-178290 |access-date=2022-03-30 |website=The Conversation |date=21 March 2022 |language=en}}]]

In astronomy, an odd radio circle (ORC) is a very large (over 50 times the diameter of our Milky Way ~ 3 million light years) unexplained astronomical object that, at radio wavelengths, is highly circular and brighter along its edges.{{cite news |last=Norris |first=Ray |title='WTF?': newly discovered ghostly circles in the sky can't be explained by current theories, and astronomers are excited |url=https://theconversation.com/wtf-newly-discovered-ghostly-circles-in-the-sky-cant-be-explained-by-current-theories-and-astronomers-are-excited-142812 |date=1 December 2020 |work=The Conversation Australia |access-date=14 December 2020 }} As of 27 April 2021, there have been five such objects (and possibly six more) observed.{{cite journal|author=Norris, Ray P.| display-authors=et al. |title=Unexpected Circular Radio Objects at High Galactic Latitude | journal=Publications of the Astronomical Society of Australia | year=2021 | volume=38 | doi=10.1017/pasa.2020.52 |arxiv=2006.14805v1 | bibcode=2021PASA...38....3N | s2cid=220128279 }}{{cite news |last=Johnson-Groh |first=Mara |title=4 mysterious objects spotted in deep space are unlike anything ever seen |url=https://www.livescience.com/circular-radio-objects-space.html |date=8 July 2020 |work=Live Science |access-date=9 July 2020 }}{{cite news |last=Murugesu |first=Jason Arunn |title=Circles in space made of radio waves are like nothing we've ever seen |url=https://www.newscientist.com/article/2247978-circles-in-space-made-of-radio-waves-are-like-nothing-weve-ever-seen/ |date=3 July 2020 |work=New Scientist |access-date=9 July 2020 }}{{cite news |last=Starr |first=Michelle |title=Astronomers Detect Unexpected Class of Mysterious Circular Objects in Space |url=https://www.sciencealert.com/mysterious-unidentified-circles-have-been-found-in-space |date=9 July 2020 |work=ScienceAlert|access-date=9 July 2020 }}{{cite news |last=Osborne |first=Hannah |title='Odd' Circles of Radio Waves Coming from Unknown Cosmic Source Discovered |url=https://www.newsweek.com/circle-radio-waves-space-source-unknown-1516549 |date=9 July 2020 |work=Newsweek|access-date=10 July 2020 }}{{cite news |last=Ferreira |first=Becky |title=Scientists Discover Unexplained Glowing Circles of Energy in Space - Scientists believe they have likely discovered a new and unexplained class of space object: the Odd Radio Circle, or ORC. |url=https://www.vice.com/en/article/scientists-discover-unexplained-glowing-circles-of-energy-in-space/ |date=10 July 2020 |work=Vice |access-date=11 July 2020 }}{{cite web |last1=Nowakowski |first1=Tomasz |title=Astronomers discover a new extragalactic circular radio source |url=https://phys.org/news/2021-05-astronomers-extragalactic-circular-radio-source.html |website=Phys.org |date=May 4, 2021 |access-date=4 May 2021}} The observed ORCs are bright at radio wavelengths, but are not visible at visible, infrared or X-ray wavelengths. This is due to the physical process producing this radiation, which is thought to be synchrotron radiation. Three of the ORCs contain optical galaxies in their centers, suggesting that the galaxies might have formed these objects.