We've gotta give credit to space scientists. They come up with great names for things, probably because so many of them like science fiction. One of these delicious names is given to the region of the sky that's blocked from our view by the Milky Way: the Zone of Avoidance.
The Zone of Avoidance (ZoA) covers about 20% of the sky and is opaque to optical light. The ZoA isn't completely impenetrable; astronomers can still observe the region beyond it in x-rays, infrared and radio. But observations in each of those bands have their limitations.
NASA's Chandra X-ray Observatory isn't the best observatory for surveying beyond the ZoA, but is does excel in one way. Its strength is its angular resolution, and that makes it useful for pinpointing individual sources. It's useful for detecting things like active galactic nuclei and galaxy clusters.
There's a strange formation of radio waves beyond the ZoA nicknamed the Treble Clef (VLSSJ0318.9+5755), so-named because it looks kind of like the musical notation symbol. New research published in Astronomy and Astrophysics used the Chandra and SRG/eROSITA to determine what the Treble Clef is and to trace its history. It's titled "The Treble Clef radio phoenix and its old nonthermal filaments," and the lead author is Andrea Botteon of the National Institute for Astrophysics (INAF) in Italy.
"This source, also detected in previous radio observations, was originally believed to be a pulsar, but was later suggested to lie in a heavily obscured galaxy cluster behind the Galactic plane," the authors write. "In this paper, we characterize the main properties of the host cluster (redshift, mass, temperature, X-ray luminosity, and dynamical status) for the first time, using X-ray observations from Chandra and SRG/eROSITA."
Chandra and eROSITA detected extremely hot diffuse gas in the region. The gas is super-heated to millions of degrees and emits x-rays. The researchers also used radio observations from the LOw Frequency ARray (LOFAR), a network of radio telescopes in Europe, to observe the Treble Clef.
This is a LOFAR image of the Treble Clef. Its main filament and other features are labelled. Image Credit: Shimwell et al. 2026.
Along with infrared data from the United Kingdom Infrared Telescope, the researchers created a composite, multispectral image of the Treble Clef. In the composite image below, x-ray data is blue, white is infrared, and radio is red.
The composite image of the Treble Clef is made of x-ray, infrared, and radio data. Image Credit: X-ray: NASA/CXC/INAF-IRA/A. Botteon et al.; Infrared: UKIDSS-GPS ; Radio: ASTRON/LOFAR; Image processing: NASA/CXC/SAO/N. Wolk
This work also confirmed, for the first time, that galaxy clusters were involved in the creation of the Treble Clef. Initial observations of the Treble Clef region go back years, and in those surveys, researchers struggled to understand what they were seeing. Thanks to Chandra and eROSITA, galaxy clusters have been identified. A critical part of identifying a galaxy cluster or clusters is finding the Brightest cluster galaxy (BCG). A BCG is defined as the brightest individual galaxy in a cluster, and they're some of the most massive galaxies in the entire Universe. BCGs generally coincide with the peak x-ray emissions from their clusters.
X-ray images from eROSITA (left) and Chandra (right). In both images, a red star marks the location of the brightest cluster galaxy (BCG). Image Credit: Botteon et al. 2026. A&A.
According to the authors, the Treble Clef has a simpler explanation than other proposed explanations, yet its history is still long and dramatic. "We propose a simplified formation scenario in which the radio phoenix originates from an aged relativistic plasma, which was initially ejected by the BCG and subsequently revived by the cluster merger," they write. The Treble Clef is called a radio phoenix, due to it's re-energized brightness, like a phoenix rising from the ashes.
It started with a supermassive black hole (SMBH) outburst in the center of the BCG. The outburst sent massive clouds of charged particles outwards, along with magnetic fields, at relativistic speeds. Initially, this created a burst of x-ray light that gradually faded as the travelling particles lost energy.
Next, two galaxy clusters suffered a collision. This compressed them, and the compression re-energized the cloud of charged particles. They brightened again, but couldn't regain their prior energy level. Instead of emitting x-rays, they emitted radio waves. The colliding galaxy groups may have also generated shock waves that added to the re-energized crescendo.
The initial merger likely occurred several hundred million years ago, and the clusters are now moving in separate directions.
This schematic illustrates the formation scenario presented in the paper. It shows "two stages of a head-on collision between two clusters, A and B, in which the already aged (or “dead”) nonthermal plasma was injected by the BCG of cluster A prior to core passage. Following core passage, corresponding to the phase in which we currently observe the system (possibly within a few 10^8 yr after first passage, based on the disturbed X-ray morphology and the age of the radio plasma), clusters A and B move away from each other in opposite directions," the authors write. Swept off material is left behind, and in the space between A and B, the gas is stretched and compressed, creating the elongated shape observed in radio. The ICM in the region is turbulent from all this activity, and it creates the complex morphology and Treble Clef shape. Image Credit: Botteon et al. 2026. A&A.
"We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the zone of avoidance," the authors write. "We speculate that its complex morphology is shaped by gas motions generated in the intracluster medium during the ongoing merger, which are also likely responsible for the candidate radio halo tentatively observed in the cluster center."