VLSS J0318.9+5755, a looped radio source hidden behind the dusty disk of the Milky Way, has been identified as the remnants of an ancient outburst from a supermassive black hole. The source lies within a massive galaxy cluster about 1.5 billion light-years away, where a collision between clusters may have revived its fading radio emission.
A radio source hidden behind the Milky Way
VLSS J0318.9+5755 was discovered by Dr. Andrea Botteon of Italy’s INAF and colleagues while they were studying data from LOFAR’s 2-meter aerial survey. Its looped, thread-like structure led astronomers to nickname it the Treble Clef.
The object has appeared in surveys since the 1960s. Because it is located close to the plane of the Milky Way, it was once suspected to be a pulsar. A 2004 study tentatively linked it to a galaxy cluster, but the source had not been investigated in detail.
New observations from the Giant Meter Radio Telescope, low-frequency data from LOFAR, and X-ray observations from NASA’s Chandra X-ray Observatory and the SRG/eROSITA telescope have now confirmed that a massive galaxy cluster lies behind the radio source.
A revived “radio phoenix”
The cluster is approximately 1.5 billion light-years away. The Treble Clef extends for about 2.5 million light-years and consists of thin filaments. Its radio spectrum is very steep, meaning that its emission falls rapidly at higher frequencies.
Astronomers classify the object as a radio phoenix: old, faded plasma produced by an earlier episode of black-hole jet activity that was probably revived by gas motion caused by collisions between galaxy clusters.
The team estimates that the plasma is approximately 190 million to 200 million years old.
“The treble clef was likely formed in a two-step process,” the researchers said.
“First, a previous explosion from the supermassive black hole at the center of the galaxy cluster dislodged a cloud of charged particles and a magnetic field at speeds close to the speed of light.”
“This caused a kind of cosmic crescendo, producing bright radio waves at first, but later fading away as the particles lost energy.”
“Then, the collision between the two clusters compressed or reactivated the cloud, making it bright again in the radio waves.”
“Shock waves from the impact (similar to the sonic boom from a supersonic jet) may have played a role in this reenergization process.”
Study publication
The team’s paper is scheduled for publication in the journal Astronomy & Astrophysics.
A. Botteon et al. 2026. “The treble clef radio phoenix and its old non-thermal filament.” A&A, in press. arXiv:2607.06659.
Source: www.sci.news