Chandra Traces the 'Treble Clef' Radio Shape to a Galaxy Cluster Merger
X-rays from Chandra and eROSITA show the Treble Clef radio structure sits in two merging galaxy clusters and likely began with a black hole outburst.
Astronomers have worked out the likely origin of the “Treble Clef,” a radio-emitting structure nicknamed for its resemblance to the musical symbol. A team led by Andrea Botteon of Italy’s National Institute for Astrophysics (INAF) used NASA’s Chandra X-ray Observatory and the eROSITA X-ray telescope. NASA’s Chandra team announced the result on October 14, 2026. The paper is published in Astronomy & Astrophysics, and the Chandra page lists it as accepted.
The structure sits in the cluster VLSS J0318.9+5755, about 1.55 billion light-years away in the constellation Camelopardalis. It was hard to explain because it lies almost directly behind the plane of the Milky Way, where our galaxy’s stars, gas and dust block much of the view. X-rays can get through that clutter. Chandra and eROSITA found diffuse, multimillion-degree gas at the site, which shows that a galaxy cluster hosts the radio source. Galaxy clusters are among the largest structures in the universe, holding hundreds of galaxies and vast reservoirs of superheated gas. The X-ray data further showed that this is actually two clusters in the middle of merging.
The team proposes a two-stage story:
- An earlier outburst from a supermassive black hole at the center of the cluster pushed clouds of charged particles and magnetic fields outward at close to the speed of light. The cloud first shone brightly in radio waves, then faded as its particles lost energy.
- The collision of the two clusters later compressed or reenergized the cloud, making it bright in radio again. Shock waves from the collision, similar to the sonic booms of supersonic jets, may have helped.
To find evidence of the merger, the authors subtracted a smooth, symmetrical oval from the Chandra X-ray image to leave only the gas that departs from that shape. The leftover emission is brightest to the left and right and faintest in the center. That points to a collision along the left-right direction in the image. The researchers think the clusters collided head-on a few hundred million years ago and are now moving apart.
The treble clef look is pareidolia, the human habit of seeing familiar shapes in random patterns, with no scientific link to music. The release notes that some viewers instead see a dragon seen from the side.
The composite image combines X-rays from Chandra and eROSITA with radio data from the LOFAR network in Europe and infrared data from the United Kingdom Infrared Telescope. The Chandra data come from four observations in April 2026 totaling 13 hours 34 minutes. For more on how structures of this scale are studied, see our galaxies guide and the Chandra hub.
Primary source
This summary is based on NASA Chandra and the peer-reviewed paper. Read the original for full details, data and caveats.
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