As matter swirls violently down toward the event horizon of a black hole, it forms an accretion disk, heating plasma to millions of degrees and blasting X-rays into space. But what exactly does the accretion disk of a black hole look like? A sandwich?
Well, maybe. The Imaging X-ray Polarimetry Explorer, launched last December as a collaboration between NASA and the Italian Space Agency, detects the polarization of X-rays, which lets astronomers understand how they reflect off the accretion disk as they’re created.
The results of its first observation of a mass-accreting black hole, published last week in Science, have started to winnow down the models that have sprung up since the last time anyone observed the polarization of these extreme X-rays, more than 40 years ago.
WHAT’S NEW — In 1971, Cygnus X-1 became the first black hole to be observed from Earth. While black holes had long been predicted theoretically, Cygnus X-1 became the first to be positively identified. Within four years, NASA launched the first — and until IXPE, also last — satellite to try to measure the polarization of its X-rays, the Eighth Orbiting Solar Observatory, or OSO-8.
In the almost 50 years since OSO-8 flew, astrophysicists have relied on the measurements taken in the mid-seventies as a baseline for understanding how X-rays emerge from black holes. But, speaking with Inverse, lead author Henric Krawczynski of Washington University in St. Louis notes “there are a lot of new technology to enable these new measurements to now get equally significant results in one hundredth of the time.”
In the past two decades, a new technique known as gas electron multipliers has allowed for much more precise and fast observations. Compared to OSO-8, IXPE is hundreds of times more sensitive and precise at detecting the polarization of X-rays.
Allows them to model the shape of the accretion disk around the black hole. As matter in the disk speeds closer and closer to the event horizon, the increased density of charged particles means they deflect off one another, braking and turning some of their incredible kinetic energy into radiation–a feeling that, luckily, there’s a compound German word astrophysicists use to describe: bremsstrahlung (“braking radiation”).
As plasma undergoes bremsstrahlung in the final 2000 kilometers above the event horizon, the X-rays it emits scatter off other material close to the black hole, carrying information about the shape of the region across 7,300 light years to Earth. Still, Krawczynski notes, “you need to detect a crazy amount of photons” to get the kind of accuracy IXPE has.
In this case, the polarization of X-rays IXPE picked up from Cygnus X-1 indicate two things about the shape of the plasma spiraling into the black hole.
First, it confirms earlier observations that black holes spit out two jets of plasma perpendicular to the accretion disk. Second, the observations are consistent with a model referred to as a “hot corona sandwich” – where plasma acts as the bread above and below a layer of matter – or one where matter sandwiches a narrow layer of plasma.
“A lot of studies about space-time of black holes is based on measuring the deflection of this hot gas and cold gas,” explains Krawczyniski. Because of this, mapping out the topology of the disk surrounding a black hole is important to understanding the ways it deflects both light and X-rays.
At this stage, IXPE’s results are about refining the ways that other astronomers and astrophysicists use black holes as a tool. Krawczyniski says, “Right now, it’s kind of like finding out how the tool you use to study spacetime actually looks.” Observations from IXPE over the coming years will refine black holes as an observational tool.
But Krawczyniski also hopes that in the near future, the satellite will be able to observe the shape of space-time itself at Cygnus X-1. Cygnus X-1 shifts unpredictably between “hard” and “soft” states — “hard” meaning the X-rays have high energy, “soft” meaning they tend to have lower. Cygnus shifts between states as the disk gets closer to the event horizon and the plasma is ejected; right now, it’s in the hard state.
“In some months,” Krawczyniski says, “hopefully they will go into the soft stage and we will see only the X-rays from the accretion disk… and then you can constrain the spacetime itself.”
WHAT’S NEXT — IXPE is planned for a two-to-five year mission, but that only means it will be taking about 50 different observations of objects in the night sky. Because “you get good signals after a week for the brightest, so once you get to the weaker ones you have to look a lot longer,” explains Krawczyniski, “you have to see how many objects you can catch.”
This will mean trying to balance observing new objects with returning to previously observed ones for deeper observation, to see how they change over time and better narrow down their properties. Krawczyniski explains the team has already collaborated on picking up different aspects of the same black hole – “we have these multi-wavelength campaigns with five satellites, three optical telescopes, and everybody looking at the same object.”