The Black Hole That Won’t Stop Getting Brighter

When a star gets too close to a supermassive black hole, the ending is usually swift and predictable. The black hole shreds the star, there’s a brilliant flash of light, and within a few months, the show is over. Fade to black.

Well, one black hole (AT2018hyz) didn’t get the memo.

In October 2018, astronomers watched a black hole roughly 200 million light-years away tear apart an unfortunate star. The initial fireworks were visible to optical telescopes, and then, as expected, things quieted down. For nearly three years, radio telescopes found nothing. Then in late 2021, something strange started happening. The system began emitting radio waves. And instead of fading away like it was supposed to, it kept getting brighter. And brighter. And brighter still.

Now, almost six years after that star’s destruction, AT2018hyz is still intensifying. At last measurement in September 2024, it had reached a radio luminosity of roughly 10⁴⁰ ergs per second, rivaling the brightest black hole-powered tidal disruption events ever observed (though still about three times dimmer than the record-holder Sw J1644+57). Scientists have two wildly different explanations for what they’re seeing, and they can’t yet prove which one is right.

A Cosmic Explosion That Won’t Stop

Yvette Cendes, an astrophysicist at the University of Oregon, has been tracking AT2018hyz since 2021 using some of the world’s most powerful radio telescopes. Her team has watched the radio emissions grow roughly 24-fold over three years of observations. By comparison, most cosmic explosions brighten quickly and then fade over weeks or months. This one just keeps climbing.

The behavior, published in The Astrophysical Journal, resembles a firework that refuses to fizzle out, instead growing steadily bigger and brighter long after it should have faded. The energy output now rivals some of the most extreme black hole behaviors astronomers have documented.

What makes AT2018hyz particularly puzzling is the waiting game. For nearly three years after destroying the star, the black hole was essentially silent at radio wavelengths. Then, around late 2021, something switched on. The radio emissions appeared suddenly and have been climbing ever since, with no signs of slowing down even in the most recent observations.

Two Theories, Both Extreme

Scientists are wrestling with two very different explanations, and the truth is, both require pretty extreme conditions.

The first possibility: About 620 days after destroying the star (roughly 1.7 years) the black hole launched a massive spherical blast of material. This wasn’t just any outburst. The material would be traveling at about 30% the speed of light (roughly 90,000 kilometers per second) and carrying 2 to 40 times more energy than similar outflows from other black holes. As this expanding shell crashes through gas and dust around the black hole, it creates shock waves that produce the radio waves we’re detecting.

Why would a black hole wait nearly two years to launch such a powerful outflow? That’s the million-dollar question. Maybe something changed in how material was falling into the black hole. Maybe the outflow ran into a particularly dense cloud of gas. Scientists don’t have a definitive answer yet.

The second possibility is even more dramatic: What if AT2018hyz launched a powerful jet right when it destroyed the star (similar to the famous Sw J1644+57) but we’re viewing it almost completely edge-on?

Think of a lighthouse. If you’re standing directly in the beam, it’s blindingly bright. But if you’re standing off to the side, you barely see anything at first. As the beam spreads out over time, more light reaches you, and it appears to get brighter even though the lighthouse itself hasn’t changed.

In this scenario, the jet would be highly relativistic, with a Lorentz factor of about 8, meaning it’s moving at nearly the speed of light. It would carry roughly 100 times more energy than the spherical outflow. We just couldn’t see most of it at first because we’re viewing it from such an unfavorable angle, somewhere between 80 and 90 degrees off the main beam. As the jet decelerates and spreads sideways over time, more of its emission becomes visible to us.

Both explanations fit the data scientists have collected so far, adding to the mystery.

What Happens Next

The only way to know for sure is to keep watching until the brightness finally peaks and starts to decline. Based on the competing models, that moment of truth could come fairly soon, or it might not happen for several more years.

If AT2018hyz is a delayed spherical outflow, the radio emissions should peak relatively soon across most frequencies. But if it’s an off-axis jet, the brightness might not peak until around 2027 at some frequencies, and potentially not until 2030 or beyond at others.

Cendes and her colleagues are planning continued observations through 2025 and beyond. They’re also hoping to use a technique called very long baseline interferometry, which could potentially image the structure of the outflow directly and reveal whether it’s truly spherical or shows signs of being jet-like.

About 40% of these stellar destruction events now appear to produce delayed radio emission: turning on hundreds or thousands of days after the initial flash. Scientists are still trying to understand what triggers these late-blooming outbursts. AT2018hyz represents the most extreme example found so far, both in terms of how bright it’s become and how long it’s continued to intensify.

In summation, nearly six years after a star made the fatal mistake of wandering too close to a supermassive black hole, the aftermath is still unfolding. And astronomers will be watching closely as this cosmic mystery continues to brighten the radio sky.

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