In a small subset of FR II galaxies, the black hole seems to switch its jets on and off repeatedly over millions of years. Each burst of activity leaves behind its own pair of hotspots. Older episodes sit farther from the galaxy's core, because their material has had more time to travel outward. Newer episodes form closer in.

Galaxies showing two episodes are called double-double radio galaxies, and about 192 examples are known. Triple-doubles, with three episodes, are far rarer: only seven confirmed or candidate examples exist. Until this finding, no galaxy had ever shown four confirmed episodes.

The team had to rule out a rival explanation: that these might simply be random "knots" — dense clumps — within one continuously active jet, rather than relics of separate historical outbursts. The distinction matters because knots in a single jet should all show nearly identical ages, while truly separate episodes should show clearly increasing ages with distance.

How do you date a jet?

The researchers used two independent clocks.

First, they measured each hotspot pair's physical distance from the core and, assuming a typical jet speed, calculated its "kinematic age." Second, they measured how much the radio emission's "color" — its distribution across radio frequencies — had aged and steepened over time, giving a "spectral age."

The results lined up. The four hotspot pairs show spectral ages ranging from 4.5 to 20.5 million years, and age clearly increases with distance from the core. That is exactly the pattern expected for sequential jet episodes, not random knots.

A statistical symmetry test settled the question. "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. In plain English: the four-episodes interpretation is about three million times more likely than the knots explanation.

"... we conclude that J023721.13−010528.5 exhibits four distinct episodes of jet activity, i.e., a quadruple-double radio galaxy (QDRG)," they add.

What are the caveats?

The finding is preliminary in one respect: the paper is a preprint and has not yet passed peer review.

The data quality also has a known weakness. Observation quality is noticeably poorer on the southern, fainter side of the source — hotspot S3 in particular — which produces larger uncertainties and less clean age trends there compared with the northern jet. The northern side carries the strongest evidence.

Why is the galaxy S-shaped?

The team noticed something else in the geometry. The axes of successive hotspot pairs rotate counterclockwise by increasing amounts: −7° for the N2–S2 pair, −16° for N3–S3, and −24° for N4–S4.

That progressive twist suggests the central black hole's spin axis is slowly wobbling over millions of years. The researchers say this progressive rotation is consistent with the galaxy's jets showing an S-shaped overall structure.

This source is a giant among giants. It stretches roughly 3.9 million light-years across — about 23 quadrillion miles — making it the second-largest S-shaped radio source known.

The researchers suggest that giant, S-shaped radio galaxies could be the best hunting ground for finding more of these rare, multi-episode systems. If the quadruple-double class is real, this object is likely only the first of several waiting in existing radio surveys.