Something lit up the radio sky in early 2022, and scientists are still not entirely sure what it was. A team of astronomers using the Australian SKA Pathfinder telescope, better known as ASKAP, has published findings in The Astrophysical Journal describing one of the most unusual and luminous radio transients ever recorded. The object, catalogued as ASKAP J005512.2-255834, sits roughly 543 megaparsecs away, which translates to about 1.77 billion light-years from Earth. It has been glowing in radio frequencies for more than 1,000 days, it brightened by a factor of at least 20 in less than 250 days, and after all that time and all that analysis, the researchers behind the study still cannot say with certainty what caused it. The two leading candidates are both extraordinarily rare events, and one of them has never been detected before through radio observations alone.
The discovery itself came about almost by accident. The team was searching archival ASKAP data for orphan afterglows, the radio echoes of gamma-ray bursts whose initial high-energy flash was never detected on Earth because the jet was pointed in a different direction. They were focused on a region of sky associated with a gravitational wave event designated GW190814. The transient they found turned out to be entirely unrelated to that event, sitting at a redshift that placed it far outside the gravitational wave distance range, but what they stumbled upon was arguably far more interesting than what they were looking for. ASKAP J0055-2558, as researchers abbreviated it, was absent from earlier observations of that part of the sky, appeared suddenly at 3 gigahertz in February 2022, and has been declining slowly but detectably ever since.
What makes this source so difficult to classify is the combination of properties it displays. Its peak radio luminosity sits around 10 to the power of 39 ergs per second, which places it far beyond anything explained by an ordinary stellar explosion. Every known class of supernova is simply too faint and too short-lived to account for what is being observed here. Fast blue optical transients, which represent another class of energetic stellar death, typically fade in under 1,000 days and do so rapidly. Thermal tidal disruption events, where a star gets torn apart by a black hole and briefly flares in radio, also fall well short of the observed luminosity. Whatever produced ASKAP J0055-2558, it released an enormous amount of energy and has been doing so for an unusually long time.
The host galaxy raises questions the radio data alone cannot answer.. Using spectroscopy from the Keck Observatory and the ANU 2.3 metre telescope in Australia, the team confirmed that the transient is almost certainly associated with a low-mass, star-forming galaxy catalogued as 2dFGRS TGS143Z140, sitting at a redshift of 0.116. The probability of the spatial coincidence being a chance alignment is less than two percent. The galaxy itself is relatively small, with a stellar mass of roughly 1.55 billion solar masses, and it shows signs of a recent merger or interaction, with prominent tidal tails extending outward and multiple H II regions scattered through its outer arms. Crucially, the transient is not located near the galaxy’s core. It sits about 8.7 kiloparsecs from the galactic centre, well out in the outskirts, coincident with the end of one of those tidal tails.
That offset location is one of the most important clues in the entire investigation, and it is what has led researchers to favour two specific explanations over all others. The first is an orphan long gamma-ray burst afterglow. Gamma-ray bursts are among the most energetic explosions in the universe, produced when a massive star collapses or when two compact objects merge. They fire relativistic jets of material outward at nearly the speed of light. If the jet happens to be pointing away from Earth at the time of the burst, no gamma-ray signal is ever detected, but as the jet slows down and spreads laterally it eventually becomes visible in the radio. The radio emission can persist for years, brightening gradually before fading, and its luminosity and spectral behaviour can be consistent with what is seen from ASKAP J0055-2558. The transient’s location in a star-forming region of a low-mass galaxy also fits the known preference of long gamma-ray bursts for exactly those kinds of environments.
The second explanation is more exotic and in some ways more compelling. The team suggests this could be a tidal disruption event involving an intermediate-mass black hole located away from the galaxy’s nucleus. Tidal disruption events occur when a star wanders too close to a black hole and gets shredded by tidal forces, releasing a burst of energy as the debris falls inward. These events are most commonly associated with the supermassive black holes found at the centres of large galaxies, but a rarer class involves intermediate-mass black holes, objects in the range of a few thousand to a few million solar masses that can lurk inside dense star clusters, dwarf galaxy cores, or the outskirts of larger systems. The deep optical imaging obtained with the Magellan telescope reveals what appears to be a faint compact stellar system right at the position of the radio transient, with an estimated stellar mass of somewhere between one million and a hundred million solar masses. That kind of structure is consistent with a nuclear star cluster or an ultra-compact dwarf galaxy, both of which are known environments capable of hosting intermediate-mass black holes.
The physics of the radio emission itself supports an energetic, mildly relativistic outflow. Equipartition analysis of the radio spectra places the blast wave velocity between 20 and 90 percent of the speed of light, depending on the assumed geometry of the outflow, and the total kinetic energy ranges from 10 to the power of 50 to 10 to the power of 53 ergs. Those numbers place the event in a region of energy-velocity space that is sparsely populated but consistent with the late-time behaviour of long gamma-ray burst afterglows and jetted tidal disruption events. The electron density around the source decreases with distance in a way that resembles a stellar wind environment, which is expected for both gamma-ray burst progenitors and tidal disruption events near compact stellar objects. X-ray follow-up with the Swift satellite detected nothing, which is consistent with both candidate explanations at such a late stage of evolution.
Distinguishing between these two scenarios turned out to be one of the central frustrations of the study. By the time ASKAP J0055-2558 was discovered and followed up across multiple radio frequencies and optical wavelengths, the event was already in its late-time Newtonian phase, meaning the jet had long since decelerated and widened to the point where on-axis and off-axis behaviour look essentially identical. Early X-ray coverage might have been decisive, since jetted tidal disruption events tend to produce luminous, long-lived X-ray emission while orphan afterglows are typically faint. Uninterrupted optical monitoring around the time of the initial brightening would also have helped enormously, but there was a gap of nearly seven months in the ZTF survey data covering precisely that period. A prompt optical spectrum of the transient itself would have provided additional diagnostic information. The lesson is a familiar one in time-domain astronomy: the first few days and weeks after a transient appears often contain the information that makes everything else interpretable, and that window, once missed, cannot be recovered.
What can be said with confidence is that ASKAP J0055-2558 represents a genuinely rare object. If it is an orphan gamma-ray burst afterglow detected purely through radio observations, it would be only the second convincing such detection in history. If it is a tidal disruption event associated with an intermediate-mass black hole at a significant offset from its host galaxy’s nucleus, it would be the first event of that kind ever identified through radio observations alone. Either outcome is significant, and both point toward a growing population of energetic, long-lived extragalactic radio transients that wide-field surveys are only now beginning to uncover at the rate their existence demands.
Source:
Gulati, A., Murphy, T., Kaplan, D. L., et al. (2026). ASKAP J005512.2−255834: A Luminous, Long-lived Radio Transient at z=0.1—an Orphan Afterglow or an Off-nuclear TDE from an IMBH? The Astrophysical Journal, 1000, 118. https://doi.org/10.3847/1538-4357/ae4351
