An invisible explosion rivaling a billion suns vanished in space—until its trace reached Earth
Gamma-ray bursts (GRBs) may be the most powerful explosions in the entire universe, but they can sometimes go completely unnoticed. Despite being able to release as much energy in seconds as the Sun could in its 10-billion-year lifetime, GRBs could still be ‘invisible.’ This is because they are narrow, relativistic jets, and not spherical explosions. One of these jets needs to be pointed directly at Earth in order to be able to see the bright gamma-ray flash. If not, it remains a hidden explosion, one of which astronomers believe they have detected.
A new study accepted in The Astrophysical Journal reports what may be the clearest example yet of such a hidden explosion. It explores what’s left to analyze if the initial blast of a gamma-ray burst isn’t noticed. GRBs are caused when giant stars collapse into black holes or when compact objects merge, launching jets in multiple directions. In case these jets are directed away from Earth, what remains visible is a slowly fading afterglow, especially in radio wavelengths. These echoes or “orphan afterglows” can be seen years later, as the blast wave plows into surrounding space or gas. However, it is important to note that such afterglows are not routine and hard to detect.
One such strange radio signal is the ASKAP J005512–255834, detected by the Australian SKA Pathfinder (ASKAP), a 36-antenna radio telescope in Western Australia. The signal was first seen on February 4, 2022, while conducting wide-field surveys, but was absent from data taken about 250 days earlier. This radio afterglow brightened rapidly and then began to fade away slowly. At its peak, this source released 10³² watts of radio energy, comparable to the total radio emission of billions of suns.
According to the study, most long-lived radio transients either evolve quickly or flare repeatedly, but this source did neither. Instead, it behaved like the echo of a single explosion, consistent with an orphan afterglow. Astronomers have been familiar with this concept for decades, but it’s virtually impossible to spot them without the initial flash. So, a massive area of sky has to be scanned, over thousands of square degrees, like what was just done using the ASKAP.
The ASKAP J005512–255834 was bright only in radio wavelengths, and not in visible or X-ray ones. This behavior is consistent with an off-axis GRB where the jet is not initially pointed toward Earth. It becomes visible in radio waves, as it slows and spreads. The lack of an X-ray or optical detection supports this theory but other explanations have not been ruled out just yet.
The radio transient is associated with a small but bright galaxy located about 1.7 billion light-years away. It has an irregular structure where stars are being actively formed. Massive cosmic explosions and other such events usually occur in these environments. The study suggests that the blast isn’t aligned with the galaxy’s central nucleus, but rather within a compact star-forming region, possibly consistent with a nuclear star cluster.
Keeping the idea of orphan afterglows aside, the researchers also considered other possibilities that would explain the ASKAP J005512–255834. With stars, pulsars, and supernovae ruled out, one of the leading alternative explanations exhibiting this radio behavior is a tidal disruption event, such as a star being torn apart by an intermediate-mass black hole. This class of black holes sits between stellar-mass and supermassive, and has been extremely rare at radio wavelengths. If confirmed, that discovery would be just as huge as finding a hidden explosion as powerful as a billion suns, too.
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