AT 2020afjz (TSS2020a): The First Fast Extragalactic Transient Discovered by TESS

The universe is filled with cosmic explosions that happen in the blink of an eye, yet for decades, our telescopes have been too slow to catch them in the act. While astronomers have long studied supernovae and other stellar deaths that unfold over days or weeks, the fastest events—those that flare and fade in mere hours—have remained largely invisible. These fleeting moments are powered by extreme physics, often involving jets of matter moving at nearly the speed of light, but without a telescope that can watch the sky continuously and take pictures very quickly, these events slip by unnoticed. The challenge has been finding a way to spot these hour-long flashes before they vanish, a task that requires a unique combination of wide vision and rapid attention.

A team of astronomers has now captured one of these elusive events, marking a significant shift in how we observe the cosmos. Using data from the Transiting Exoplanet Survey Satellite, known as TESS, researchers discovered a bright flash of light in a distant galaxy that appeared, peaked, and faded entirely within a single night. This event, designated AT 2020afjz, is the first time such a fast, extragalactic explosion has been discovered without a prior warning from a gamma-ray detector. Instead of waiting for a satellite to trigger an alert, the team used a new software pipeline to sift through TESS data and find the flash on its own. The result is a detailed record of an explosion that lasted only about two and a half hours at its brightest, offering a rare, high-definition look at the birth and death of a relativistic jet.

The discovery began in November 2020, when the TESS satellite was scanning a patch of sky in the southern hemisphere. TESS is designed to look for planets by watching stars for tiny dips in brightness, but its cameras are also capable of catching sudden brightenings. The researchers developed a system to process the satellite's full-frame images, looking for anything that changed rapidly. They found a source that rose to its peak brightness in about an hour and then faded away, remaining visible for roughly ten hours before disappearing into the background. The light curve, which tracks the brightness over time, showed a slow rise and a quick decline, a pattern that differs from the typical behavior of most known cosmic explosions.

To understand what caused this flash, the team first had to locate its home. By comparing the position of the flash with deep images from other telescopes, they pinpointed its origin to a faint, distant galaxy. This galaxy is part of a pair of interacting galaxies, meaning the two are close enough to be pulling on each other's gravity. The flash occurred near one of these galaxies, which appears to be a small, star-forming system. The distance to this galaxy is vast; the light we see left it when the universe was much younger, roughly corresponding to a redshift of 0.67. The fact that the event happened in a galaxy where stars are still being born supports the idea that it was caused by the death of a massive star, a scenario that often leads to powerful jets of energy.

The nature of the explosion itself remains a puzzle with two possible solutions, both of which fit the data equally well. The researchers modeled the light curve using physics that describe how jets of matter interact with the space around them. They found that the event could be explained in one of two ways. The first possibility is that the explosion was an "orphan afterglow," meaning the jet was fired in a direction that was not pointing directly at Earth. In this scenario, we are seeing the side of the jet rather than its core, which would explain why the light rose slowly and why no gamma-ray burst was detected. The second possibility is that the jet was pointing toward us, but it was "dirty," meaning it was clogged with too much matter to produce a bright flash of gamma rays. In this case, the explosion would be a "dirty fireball," where the energy is released slowly as a bright optical flash without the usual high-energy signature.

A critical piece of the puzzle is missing: the team cannot confirm whether gamma rays were actually emitted. The satellite designed to detect these high-energy bursts, Fermi-GBM, was blocked by the Earth at the exact moment the explosion occurred. Because of this gap in coverage, the researchers cannot definitively say if the event was a gamma-ray burst that happened to be hidden from view, or a gamma-ray burst that never happened at all. This uncertainty means they cannot choose between the "orphan" and "dirty fireball" explanations. However, both scenarios point to the same conclusion: the event was driven by a relativistic engine, likely the collapse of a massive star, but it behaved differently than the typical explosions we are used to seeing.

The significance of this discovery extends beyond the specific event. It proves that TESS, a telescope primarily built to find planets, can also serve as a powerful tool for finding fast, rare explosions in the universe. By using the new software pipeline to analyze the satellite's data, the team has shown that we can now detect these hour-long transients without needing a gamma-ray trigger. This opens the door to finding a whole new population of cosmic events that have been hiding in plain sight. While the team cannot yet say exactly how many of these events exist, the discovery of AT 2020afjz suggests that they are out there, waiting to be found. As the software continues to scan more data from TESS, astronomers expect to uncover more of these fleeting moments, providing a clearer picture of the most extreme physics in the universe.

The study also highlights the importance of timing and precision. The flash lasted for only 2.4 hours above half its maximum brightness, a window so short that it would have been missed by telescopes that take pictures only once a day. The ability to resolve the rise and fall of the event with such detail allowed the researchers to model the physics of the explosion with a high degree of confidence. They determined that the explosion occurred in a dense environment, likely a cloud of gas and dust, which influenced how the light traveled to us. This density suggests that the explosion happened in a region where stars are actively forming, consistent with the location of the host galaxy.

Despite the lack of gamma-ray data, the event provides a benchmark for understanding these fast transients. It shows that not all relativistic explosions look the same, and that some can be detected purely through their optical light. The researchers plan to apply their methods to future data from TESS, hoping to find more examples and eventually piece together the full story of these gamma-ray quiet events. For now, AT 2020afjz stands as a testament to the power of persistent observation and the value of looking at the sky with fresh eyes, revealing a hidden corner of the universe that was previously out of reach.