A telescope on the International Space Station made an incredible high-resolution measurement of the x-rays resulting from a black hole sucking up matter that could have important implications for astronomers’ understanding of these mysterious objects.
Scientists know that black holes emit high-energy x-rays when they eat up matter, but how and from where has been a matter of discussion. The ISS’s Neutron star Interior Composition Explore, or NICER, has allowed scientists to observe these x-rays like never before. This observation could help scientists better understand not just black holes a few times the mass of the Sun like the one observed here, but perhaps the billion-solar-mass behemoths at galactic centres as well.
“There has been a debate about how black holes evolve,” Erin Kara, postdoctoral fellow at the University of Maryland and NASA Goddard Space Flight Center, told Gizmodo. “We see them go into these crazy outbursts when they have a disk of material falling into them... What’s responsible for the outburst is something that’s been debated since black holes were discovered.”
Black holes are regions of space so massive and compact that beyond a certain point, called the event horizon, no matter or energy (including visible light) can escape their gravitational pull. But when they suck up matter from a nearby companion star, they show complex structure. An “accretion disk” of matter shredded by the black hole’s gravity orbits it like Saturn’s rings, and hot gas, called a corona, sits above the ring in the region of the black hole’s pole. These mass-eating events are typically accompanied by blasts of high-energy x-rays from the corona, transitioning into lower-energy x-rays originating from the accretion disk.
But how does the eating process progress? Does the disk begin further away, and then move closer to the event horizon? Or is it instead the corona that moves inward while the disk remains close to the black hole?
Understanding this process required high-resolution measurements of the x-rays—not only did scientists need to know the energy of the x-rays, but also the exact time that they arrived to the microsecond or less. Scientists are ultimately looking for what they call “reverberation lags” or “light echoes.” Light echoes are essentially high-energy x-rays from the corona, some of which hit the accretion disk resulting in lower-energy x-rays. Scientists are interested in measuring the time between the initial flash and the lower-energy echoes.
Scientists observed the light echo from a black hole called MAXI J1820+070 with NICER. The data revealed that the lag was six to 20 times shorter than previous measurements, according to the paper published today in Nature. This might simply have been because NICER can measure the timing with better precision than other instruments.
“To measure these light echoes to half a millisecond is incredible,” said Kara. Just imagine—scientists could measure the time between the x-rays and their echoes to 300-nanosecond resolution for a black hole around 10,000 light-years away.
The scientists were able to turn the measurement into an inference about the accretion process. The shorter echo implied that the accretion disk reached much closer to the event horizon than previously thought, meaning that it’s the corona that gets smaller, rather than the accretion disk that moves closer, over time.
“It’s a cool measurement that resolves a lot of tensions we saw in slightly lower resolution measurements,” Daryl Haggard, assistant professor of physics at McGill University, told Gizmodo.
But the paper has its limitations. Haggard cautioned that this was just one source. “This could be a particular behaviour of this accreting black hole system,” she said. “That’s always a problem when you have a single source. We’d like to see the same behaviour observed in outbursts in other stellar-mass black holes.” And there might be other interpretations that match the data, as well.
Still, the paper could have important implications. It would resolve an inconsistency between the largest supermassive black holes and these smaller stellar-mass black holes. Supermassive black holes’ accretion disks extend nearly the whole way to the central black hole, and the previous measurements had implied that smaller black holes’ disks didn’t—a confusing discrepancy. The new results demonstrate that perhaps they just needed to look with better equipment.
It’s a neat observation and shows that at least one small black hole is more similar to supermassive black holes than previously thought. And it’s pretty cool that it came from a telescope on the International Space Station.