Credit: X. Zhang and M. Kluge (MPE), B. Koribalski (CSIRO)
A Cloverleaf for an ORC
Mysterious ORCs haunt deep space. These ORCs, or "odd radio circles" were discovered in 2021 using data from the Australian Square Kilometer Array Pathfinder (ASKAP for short). ORCs are large (more than 50 thousand times the diameter of the Milky Way), edge-brigthened circles of radio emission. The nature of these objects is unknown. Typically, gigantic cosmic explosions (like supernovae) can produce spherical structures (like supernova remnants) which can appear nearly circular in projection on the sky, but ORCs seem too large to be produced by conventional supernova explosions. Perhaps ORCS are created by extraordinary events, like a fast radio burst, gamma-ray burst, or something even more extreme. A search for X-ray emission from these objects, using the eROSITA X-ray telescope (which mapped the entire X-ray sky from 2019 to 2022) suggested that one ORC, called as the "Cloverleaf", might be an X-ray emitter. Follow-up observations with the XMM-Newton X-ray observatory confirmed that the Cloverleaf was indeed connected to a diffuse patch of X-ray emitting gas. The image above is a composite image of the Cloverleaf: pink shows the radio image (from ASKAP), blue the XMM-Newton X-ray image, and white and yellow an optical image from the Dark Energy Spectroscopic Instrument Legacy Survey. The X-ray image shows hot gas filling the interior of the Cloverleaf, the first time X-ray emission has been detected from an ORC. The X-ray emission from the Cloverleaf suggests that this ORC is the powerful shock wave produced by the gravitational merger of two groups consisting of about a dozen galaxies.
Published: March 17, 2026
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 24-Mar-2025 12:05:43 EDT