Four years ago, astronomers spotted a distant supermassive black hole consuming an entire star. The star strayed too near the massive black hole, and the intense gravitational pull prevented its escape.
This was an instance of a Tidal Disruption Event (TDE), and four years on, the energy output resulting from this occurrence continues to intensify.
The specific tidal transient event has been designated AT2018hyz, where ‘AT’ stands for Astronomical Transient, ‘2018’ marks the year of its initial detection, and ‘hyz’ is the sequential identifier for that year. It was first pinpointed in 2018 as part of the All-Sky Automated Survey for Supernovae (ASASS-SN), but radio emissions only appeared and were recorded in 2022.
Observations confirming the ongoing surge in energy emission are detailed in a fresh study featured in The Astrophysical Journal. The lead author for this work is Yvette Cendes of the University of Oregon.
“We present findings from contemporary radio monitoring of the Tidal Disruption Event (TDE) AT2018hyz, which was first identified in the radio spectrum 972 days post-disruption, following several unsuccessful attempts during earlier searches,” the authors state.
The new data collected by the researchers spans the period from approximately 1370 to 2160 days after the disruption. “We observed that the light curves continue to rise across all frequencies throughout this timeframe,” they report.
While this event was initially captured in optical light back in 2018, at that time, it was merely another example of a TDE. Several years later, Cendes’s team re-examined AT2018hyz and discovered it was emitting substantial energy in the form of radio waves.
They published a paper in 2022 regarding this peculiar TDE phenomenon (Treatment Destruction Effect). This publication highlighted the rising emissions, noting, “Such a steep rise cannot be accounted for by any reasonable outflow scenario initiated at the time of the disruption, and instead indicates a delayed launch.”
In the current paper, Cendes and her collaborators state that the energy radiated by the supermassive black hole has dramatically increased over the intervening years. In fact, it is now 50 times brighter than it was when first detected.
Two hypothetical frameworks aim to account for this rise in radio luminosity. One is termed a “delayed spherical outflow.”
Under this model, the outflow was launched roughly 620 days after the stellar destruction. “The physical evolution of the radius of the spherical outflow confirms that it was launched with a significant delay, about 1.7 years, relative to the optical detection,” the researchers write.
The alternative scenario involves an astrophysical jet, one that is significantly off-axis and moving at relativistic speeds.
“Radio emission from an off-axis jet will be suppressed in the early stages due to relativistic beaming, but will eventually increase rapidly as the jet slows down and widens,” the authors explain.
The research suggests that the torrent of radio waves emanating from the supermassive black hole will keep ascending until it peaks around 2027.
“This is truly unusual,” comments Cendes. “It’s hard for me to imagine anything like this growing for so long.”
When the researchers calculated the black hole’s energetic output, another surprise awaited them. It is approximately equivalent to the energy released by a Gamma-Ray Burst (GRB). Given that GRBs represent the most luminous and powerful explosions in the cosmos, this supermassive black hole (SMBH) event ranks among the most energetic witnessed to date.
For amusement, the authors drew a comparison to the Death Star from “Star Wars.” Science fiction fans have previously estimated the Death Star’s energy output, and based on those figures, the authors’ calculations indicate the supermassive black hole is emitting at least a trillion times more energy than a fully operational Death Star. The actual figure could potentially be as high as 100 trillion times that of the fictional weapon of mass destruction.
However, these calculations are derived from a vast distance. Only ongoing observations can verify their accuracy.
This discovery prompts an important question: Do other black holes and TDEs throughout the universe exhibit a similar upward trend in radiation? We currently lack an answer because this topic has not been thoroughly investigated.
“If an explosion occurred, why would you expect to see something years after the blast when nothing was visible before?” Cendes asks, noting that securing observation time on the world’s most powerful telescopes is fiercely competitive. Now that they have identified one supermassive black hole with such anomalous luminosity, their proposals to search for others will carry more scientific weight.
It is worth noting that this is not the sole TDE event exhibiting a delayed radio signal. However, its luminosity is exceptionally high compared to others.
“We find that AT2018hyz represents a unique tidal disruption event even within the population of radio-delayed TDEs, and future observations should allow us to distinguish between these scenarios,” the authors conclude.
Cendes and her team intend to continue monitoring AT2018hyz across multiple wavelengths. This ongoing work is expected to enable them to “track the ongoing evolution of the outflow and the par-nuclear environment,” the authors finish.
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