Runaway black hole defies growth limit, expands at record rate

Astronomy has a way of surprising you. When you believe the universe has yielded its biggest secrets, distant giants and faint signals come along to pose even more questions.

Two recent discoveries drive this point home: a possible link between fast radio bursts and faint gamma-ray flashes, and a black hole that is growing quicker than anyone thought.

Fast radio bursts, or FRBs, are among the most enigmatic events in the night sky. First detected in 2007, the bursts last only a few milliseconds, yet during that brief time they can release as much energy as the Sun puts out in weeks. Some occur once, while others repeat sporadically.

One such repeater, FRB 20240114A, was especially intriguing to a group of scientists led by Claudio Ighina. In combing through data from NASA's Fermi Gamma-ray Space Telescope, his team made an unexpected find: a faint gamma-ray signal that appeared in the same part of the sky at nearly the same time as the radio burst.

Our analysis suggests the faint gamma-ray emission is not noise, but could instead be a faint GRB linked with the FRB," the researchers said in their study.

If that connection holds, it would be a breakthrough. Magnetars — neutron stars with unimaginably strong magnetic fields — have been long suspected by astronomers as the perpetrator of most FRBs. Magnetars are already observed to emit both radio and high-energy light, but a gamma-ray companion has always remained elusive. Until now, at least.

The gamma-ray burst from FRB 20240114A was faint — so faint it almost got lost in the background noise. Yet its statistical significance was 4.6 sigma, just below the 5-sigma threshold physicists use to declare a discovery. That value is significant because it means the event had an extremely low chance of being random.

Its energy flux was a paltry 6.1 × 10⁻⁷ erg/cm²/s, making it one of the weakest gamma-ray bursts on record. It also only lasted for a fraction of a second. Weak does not necessarily mean insignificant, though. If magnetars are producing weak gamma-ray bursts alongside FRBs, then there could be many subtle signals that have been missed.

Finding weak gamma-ray counterparts to FRBs could reveal the missing pieces to how these intense cosmic events occur," the authors wrote. By picking up even the faintest bursts, scientists can potentially verify how energy is released in the most extreme conditions possible.

The case for linking FRBs and faint gamma-ray bursts will be built up step by step. Gamma-ray telescopes don't pin down positions as precisely as radio observatories, so it's possible the two bursts came from different sources. To be sure, researchers need more examples of FRBs with weak GRBs. If the same signals repeat again and again, the chance of coincidence vanishes.

The broader payoff would be huge. FRBs and GRBs are the universe's most violent events. If magnetars are the perpetrators, astronomers now have a new window on the behavior of matter and energy when squeezed into states beyond the human imagination. As Ighina and colleagues put it, the faint gamma-ray burst whispers are just as important as the universe's loudest fireworks.

While faint signals are keeping astronomers busy, other discoveries roar with brute force. One involves a black hole so massive and so hungry that it's forcing scientists to rethink how these objects grow and change.

This 12.8-billion-light-year-away black hole tips the scales at about a billion solar masses. Its remarkable aspect is how fast it's expanding. As gauged by NASA's Chandra X-ray Observatory, it's accreting at up to 2.4 times the so-called Eddington rate, the speed limit that's supposed to prevent black holes from accreting more than gravity can deliver.

It was a bit surprising to see this black hole growing by leaps and bounds," said Luca Ighina of the Center for Astrophysics | Harvard & Smithsonian, who led the research.

The Eddington limit is due to the fact that when matter falls into a black hole, it gets hot and radiates. The radiation creates a pressure in the backward direction which usually counteracts the pull of gravity. The growth should then slow down after the limit is reached.

However, for quasar RACS J0320-35, the black hole seems to have defied that universal law. Chandra's X-ray spectrum detected a signature that matched theoretical simulations of black holes accreting material at a rate exceeding the Eddington rate. Optical and infrared observations corroborated the result.

To reach such a size in under a billion years, astronomers previously thought black holes had to start their lives with very massive "seed" masses — at least 10,000 Suns. However, if RACS J0320-35 has been growing at more than the Eddington rate, then it might have started much smaller, perhaps from the collapse of a single massive star.

“By knowing the mass of the black hole and working out how quickly it’s growing, we’re able to work backward to estimate how massive it could have been at birth,” said co-author Alberto Moretti of INAF-Osservatorio Astronomico di Brera.

To further the mystery, RACS J0320-35 propels jets of material racing outward at near light speed. That's not normal for quasars, and it suggests that the fast growth of the black hole could be energizing the jets. For researchers, this is a chance to connect two of the universe's most dramatic processes: black hole feeding and jet formation.

How did the universe create the first generation of black holes?" said co-author Thomas Connor of the Center for Astrophysics. "This is one of the biggest questions in astrophysics and this one object is allowing us to go after the answer."

Both results — the faint gamma-ray signal linked to a fast radio burst and the runaway growth of a distant black hole — offer scientists new potent tools for exploring the extremes of physics.

By studying FRBs and faint GRBs together, astronomers can finally nail down the role of magnetars and decipher energy release mechanisms in the densest stellar remnants. The rapid growth of RACS J0320-35 challenges predictions of how quickly black holes can grow, offering fresh insights into the dawn of the universe's first giants.

For humanity, these finds do more than contribute to cosmic trivia. They concentrate our understanding of how stars are born and die, how matter behaves under unimaginable pressures, and how galaxies evolve. They also drive technology, forcing the creation of telescopes and methods that look deeper, sharper, and faster than before.

Research findings are available online in the journal The Astrophysical Journal Letters.

Like these kind of feel good stories? Get The Brighter Side of News' newsletter.