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An accretion disk is a colossal whirlwind of gas and dust that gathers around a black hole or neutron star like cotton candy as it pulls in material from a nearby star. As the disk spins, it creates powerful winds that push and pull on the sprawling, spinning plasma. These massive outflows can affect the surroundings of black holes by heating and blowing the gas and dust around them.
At immense scales, “disk winds” can offer clues to how supermassive black holes shape entire galaxies. Astronomers have observed signs of disk winds in many systems, including accreting black holes and neutron stars. But to date, they have only ever glimpsed a very narrow vision of this phenomenon.
Now astronomers at MIT have observed a wider band of winds, in Hercules X-1, a system in which a neutron star pushes material away from a sun-like star. The accretion disk of this neutron star is unique in that it wobbles, or “precedes,” as it spins. By taking advantage of this wobble, astronomers captured different perspectives of the spinning disk and created a two-dimensional map of its winds for the first time.
The new map reveals the vertical shape and structure of the wind, as well as its speed – around hundreds of kilometers per second, or about a million miles per hour, which is at the milder end of what the discs of accretion can rotate.
If astronomers can spot more wobbly systems in the future, the team’s mapping technique could help determine how disk winds influence the formation and evolution of star systems, and even entire galaxies.
“In the future, we could map disk winds across a range of objects and determine how the properties of the wind change, for example, with the mass of a black hole or the amount of matter it accumulates.” , explains Peter Kosec, postdoctoral fellow at MIT’s Kavli Institute for Astrophysics and Space Research. “This will help determine how black holes and neutron stars influence our universe.”
Kosec is the lead author of the study, which is published in natural astronomy. His MIT co-authors include Erin Kara, Daniele Rogantini and Claude Canizares, as well as collaborators from several institutions, including the Cambridge Institute of Astronomy, UK.
Disk winds have most often been observed in X-ray binaries – systems in which a black hole or neutron star extracts matter from a less dense object and generates a white-hot disk of spiraling matter , as well as outgoing wind. Exactly how the winds are launched from these systems is unclear. Some theories propose that the magnetic fields could shred the disk and blow some of the material outward as wind. Others postulate that radiation from the neutron star could heat and evaporate the disk’s surface in white-hot bursts.
Clues to a wind’s origins can be inferred from its structure, but the shape and extent of disc winds have been difficult to resolve. Most binaries produce relatively uniformly shaped accretion disks, like thin donuts of gas spinning in a single plane. Astronomers studying these disks from satellites or distant telescopes can only observe the effects of the disk’s winds in a fixed, narrow range, relative to their spinning disk. Any wind that astronomers manage to detect is therefore only a small piece of its larger structure.
“We can only probe the properties of the wind at a single point, and we are completely blind to anything around that point,” Kosec notes.
In 2020, he and his colleagues realized that a binary system could provide a broader view of record winds. Hercules X-1 stood out from most known X-ray binaries for its distorted accretion disk, which wobbles as it orbits the system’s central neutron star.
“The disk really oscillates over time every 35 days, and the winds come from somewhere in the disk and cross our line of sight at different heights above the disk over time,” Kosec says. “This is a very unique property of this system that allows us to better understand its vertical wind properties.”
A distorted oscillation
In the new study, the researchers observed Hercule X-1 using two X-ray telescopes, the European Space Agency’s XMM Newton and NASA’s Chandra Observatory.
“What we are measuring is an X-ray spectrum, which means the amount of X-ray photons that arrive at our detectors, relative to their energy. We are measuring absorption lines, or the lack of X-ray light at very high energies. specific,” says Kosec. “From the ratio of the strength of the different lines, we can determine the temperature, speed and amount of plasma in the wind of the disc.”
With the distorted disk of Hercules X-1, astronomers could see the line of the disk move up and down as it wobbled and spun, similar to how a distorted recording appears to oscillate when viewed by the edge. The effect was such that researchers could observe signs of disk winds at changing heights relative to the disk, rather than at a single fixed height above a uniformly rotating disk.
By measuring X-ray emissions and absorption lines as the disk wobbled and rotated over time, the researchers were able to analyze properties such as temperature and wind density at different heights from its disk and build a two-dimensional wind map. vertical structure.
“What we see is the wind rising off the disk, at an angle of about 12 degrees to the disk as it expands into space,” Kosec explains. “It also gets colder and lumpier, and weaker at higher heights above the disc.”
The team plans to compare their observations with theoretical simulations of various wind-initiating mechanisms, to see which could best explain the origins of the wind. Further out, they hope to discover more distorted and oscillating systems, and map their disc wind structures. Then, scientists could get a broader view of the disk’s winds and how these flows influence their surroundings, especially at much larger scales.
“How do supermassive black holes affect the shape and structure of galaxies? asks Erin Kara, a 1958 career development assistant professor of physics at MIT. “One of the main hypotheses is that disk winds, launched from a black hole, can affect the appearance of galaxies. We can now get a more detailed picture of how these winds are launched and what They look like.”
Peter Kosec, Vertical Wind Structure in an X-ray Binary Revealed by a Precessing Accretion Disc, natural astronomy (2023). DOI: 10.1038/s41550-023-01929-7. www.nature.com/articles/s41550-023-01929-7
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