Astronomers discover a radio galaxy with evidence of four separate jet outbursts
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Astronomers have identified what may be the first example of a new class of radio galaxy, one whose central black hole appears to have launched powerful jets on four separate occasions. High-resolution observations with the upgraded Giant Metrewave Radio Telescope (GMRT), combined with data from MeerKAT and the Very Large Array, reveal four pairs of radio structures arranged symmetrically around the central galaxy.
A paper outlining this discovery was posted to the arXiv preprint server on Sept. 8.
One after another
Radio galaxies are a type of active galactic nucleus in which a central supermassive black hole launches powerful jets of particles, visible in radio waves. The most powerful class, known as FR II, produces bright “hotspots” where these jets slam into surrounding gas, forming structures that can span millions of light-years, making them some of the largest objects in the universe.
In a small subset of these FR II galaxies, the central black hole appears to switch its jets on more than once over time. Each burst of activity leaves behind its own pair of hotspots, with older episodes sitting farther from the galaxy’s core because they’ve had more time to travel outward and newer episodes forming closer in.
Galaxies with two such episodes are known as double-double radio galaxies. About 192 known examples of this subclass exist. Triple-double radio galaxies show three separate episodes. They are rarer, with only seven known or candidate examples. Until now, no galaxy had ever been found with four confirmed episodes of jet activity.
In this study, the team led by Pavan Vijay Khadekar of the Indian Institute of Science Education and Research, Pune, studied a radio source known as J023721.13−010528.5, hosted by a massive elliptical galaxy at redshift 0.372.
Age and distance
By combining radio observations from three powerful telescopes across multiple radio frequencies, the team identified four distinct pairs of hotspots labeled N1–N4 and S1–S4, moving outward from the core.
They calculated each hotspot’s physical distance from the core and, assuming a typical jet speed, estimated the “kinematic ages” of each hotspot pair. By calculating how much the radio “color” had aged and steepened over time, researchers also derived the “spectral ages” of these pairs.
To rule out an alternative explanation—that these might be random “knots” in one continuously active jet rather than truly separate historical episodes—they also ran independent tests. The knots in a single active jet should all show nearly identical ages; separate episodes should show clearly increasing ages with distance.
The four hotspot pairs show spectral ages ranging from 4.5 to 20.5 million years. The team also found that age clearly increases with distance from the core, exactly the pattern expected for sequential jet episodes rather than random knots. That means the radio emission became progressively older from the inner structures toward the outer ones.
Researchers note that the observation quality is noticeably poorer on the southern/fainter side (S3 in particular), leading to larger uncertainties and less clean trends on that side of the source compared to the northern jet.
‘Quadruple-double’
Next, they ran a statistical symmetry test, which found the “four separate episodes” explanation is about 3 million times more likely than the “random knots” explanation. “We estimated that the probability that these are 3 inner pairs of hotspots is ∼3 × 106 times more than that of the knots-in-the-jet model,” the team writes in the paper.
“… we conclude that J023721.13−010528.5 exhibits four distinct episodes of jet activity, i.e., a quadruple-double radio galaxy (QDRG),” they add.
The team also found that the axes of successive hotspot pairs are rotating counterclockwise by increasing amounts, with a shift of −7° for N2-S2, −16° for N3-S3 and −24° for N4-S4. This indicates the central black hole’s spin axis is slowly “wobbling” over time.
This “progressive rotation,” researchers say, is consistent with the galaxy’s jets also showing an S-shaped morphology. Stretching roughly 3.9 million light-years across (23 quadrillion miles), this is the second-largest S-shaped radio source known. Researchers note that giant, S-shaped radio galaxies could be the best hunting ground for finding more of these rare, multi-episode systems.
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Publication details
Pavan Vijay Khadekar et al, J023721.13$-$010528.5: A Giant S-shaped Radio Galaxy Showing Four Episodes of Jet Activity, arXiv (2026). DOI: 10.48550/arxiv.2609.08506
Journal information:
arXiv
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Astronomers discover a radio galaxy with evidence of four separate jet outbursts (2026, September 27)
retrieved 27 September 2026
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