Small black holes are surprisingly messy eaters, Japanese spacecraft discovers: 'Being surprised is good'

Small black holes making a meal out of companion stars are surprisingly messy eaters, astronomers using a Japanese space telescope have discovered.

Scientists have known for some time that when supermassive black holes — those with masses millions or billions of times that of the sun — devour matter, such as surrounding gas and dust or even an unfortunate passing star, most of that material gets "spit out" rather than accreted into the black hole.

The new research shows, however, that even much smaller stellar-mass black holes, with masses just a few times that of the sun, struggle to keep their cosmic meals down. This surprising discovery could have implications for how black holes, particularly supermassive black holes, influence the evolution of their host galaxies.

"Being surprised is good," team leader and University of Michigan researcher Jon Miller said. "Seeing expectations proven naive means progress. Having entirely new avenues to chase down is tremendous. Astronomy is full of surprises and never boring."

Stellar mass black hole is a hot mess

The team used the X-Ray Imaging and Spectroscopy Mission (XRISM), a spacecraft operated by the Japan Aerospace Exploration Agency (JAXA), in partnership with NASA and the European Space Agency, to study a system called 4U 1630-472.

4U 1630-472, which is about 26,000 light-years away from Earth, is an "X-ray binary," consisting of a stellar mass black hole and a sun-like star.

The immense gravitational influence of the black hole is stripping material away from the companion star. However, because this stolen matter has angular momentum, it can't fall straight to the black hole; instead, it forms a swirling, flattened cloud called an accretion disk around the black hole. Material then falls from this disk onto the black hole during feeding periods.

The gravitational influence of the black hole generates incredible tidal forces in the accretion disk, causing friction and heating its matter to temperatures as great as 18 million degrees Fahrenheit (around 10 million degrees Celsius). This results in the gas and dust glowing brightly in X-rays.

Emissions from 4U 1630-472 vary dramatically in brightness. At times, the system has around the same brightness as the sun; this is believed to correspond to periods in which the black hole is "quiescent," or not feeding. However, around every two Earth years, the brightness of the system increases by around 10,000 times over the course of just seven days or so. This brightening coincides with the system having an outburst.

On Feb. 16, 2024, XRISM saw 4U 1630-472 at the tail end of an outburst, allowing the researchers to perform some novel black hole science.

This revealed that, even as the X-ray emissions of 4U 1630-472 were fading, the system was still ejecting material at an incredible 20 million mph (32 million kph) — about 3% the speed of light, and 15,000 times faster than the top speed of an F-16 jet fighter!

"We got to see a range of gas flow rates that we never get to observe with massive black holes at the centers of galaxies," Miller said. "The time scale to see something like that from the black hole in the Milky Way would be hundreds of millions of years. So one of the reasons we study these smaller ones is to get a clue as to how gas flows onto very massive black holes could change over the evolution of a galaxy."

What Miller and colleagues had expected to see was matter moving in a more controlled, less chaotic way as the flow of matter slowed, which the scientist explained using an everyday analogy.

"You expect to spill a lot if you try to pour a bucket of water into a cup, but not when you are pouring a cup of water into a bucket," Miller said. "Black holes seem to spill in both extremes. There was still mass being thrown around instead of accreted directly into the black hole."

The team aims to continue investigating feeding black holes and their messy eating habits with XRISM, but recent NASA budget cuts could threaten such exploration of these systems.

"We're not done with what NASA calls the mission's prime phase, which is a two-year period where you find examples of groundbreaking science that you can do. Then you spend the rest of the mission diving into those areas once you've unearthed them," Miller said. "The money that's required to do these missions is almost all up front — it's the building, technology development and the launch. Operating year-to-year is a small fraction of the total."

As this is a JAXA-led mission, there is a chance the Japanese space agency could continue operating XRISM alone, but Miller acknowledges this would be far from ideal for science.

"There's a community of hundreds, maybe as many as a couple thousand scientists in the U.S. who stand to benefit from XRISM," Miller concluded. "We're all trying very hard to use the hell out of it right now and hoping that it isn't our last chance."

The team's research was published in the July edition of The Astrophysical Journal Letters.