Scientists map strong winds in a distant neutron star system


April 10, 2023

(Nanowerk NewsAn accretion disk is a colossal swirl of gas and dust that gathers around a black hole or neutron star like cotton candy as it pulls material from a nearby star. As the disk rotates, it emits a strong wind that pushes and pulls the rotating and rotating plasma. These massive flows can influence the black hole’s surroundings by heating and blowing the surrounding gas and dust.

On very large scales, “disc winds” can provide clues about how supermassive black holes formed entire galaxies. Astronomers have observed signs of disc winds in many systems, including accretion black holes and neutron stars. But until now, they had only glimpsed a very narrow view of this phenomenon.

Now, MIT astronomers have observed an even wider swath of wind, in Hercules X-1, a system in which a neutron star pulls material from a sun-like star. This neutron star’s accretion disk is unique in that it wobbles, or “precesses,” as it rotates. By taking advantage of this wobble, astronomers have captured multiple perspectives of a rotating disk and created a two-dimensional map of its winds, for the first time.

The new map reveals the shape and vertical structure of the wind, as well as its speed – around hundreds of kilometers per second, or about a million miles per hour, which is on the lighter end of what the accretion disk can rotate. disc winds associated with the accretion disk around a neutron star MIT astronomers mapped a “disc wind” associated with the accretion disk around Hercules X-1, a system in which a neutron star pulls material away from a sun-like star, represented as a teal ball. These findings may offer clues to how supermassive black holes form entire galaxies. (Image: Jose-Luis Olivares, MIT. Based on Hercules X-1 image by D. Klochkov, European Space Agency)

If astronomers can spot more wobble systems in the future, the team’s mapping techniques could help determine how disc winds influence the formation and evolution of star systems, and even entire galaxies.

“In the future, we can map the disc winds across different objects and determine how the wind’s properties change, for example, by the mass of the black hole, or by how much matter has accumulated,” said Peter Kosec, postdoc at the Kavli Institute for Astrophysical and Space Research. MIT. “That will help determine how black holes and neutron stars affect our universe.”

Kosec is the lead author of a study that appears in Natural Astronomy (“Vertical wind structure in an X-ray binary revealed by an earlier accretion disk”). His MIT co-authors include Erin Kara, Daniele Rogantini, and Claude Canizares, along with collaborators from various institutions, including the Institute of Astronomy in Cambridge, England.

Fixed vision

Disc winds are most often observed in X-ray binaries – systems in which a black hole or neutron star pulls material from a less dense object and produces a disk of hot inspired matter, along with an escaping wind. Exactly how wind is launched from this system is unclear. Some theories suggest that the magnetic field could damage the disk and blow some of the material out as wind. Others argue that the radiation of a neutron star can heat up and vaporize the surface of the disk in a hot blast.

Clues about the origin of the winds can be deduced from their structure, but the shape and range of disc winds are difficult to determine. Most binaries produce an accretion disk that is relatively flat, like a thin doughnut of gas rotating in one plane. Astronomers studying the disk from distant satellites or telescopes can only observe the effects of the disk’s winds within a fixed, narrow range relative to the rotating disk. Therefore, any winds that astronomers have managed to detect are only a small part of the larger structure.

“We can only investigate wind properties at one point, and we are completely blind to everything around that point,” notes Kosec.

In 2020, he and his colleagues realized that a single binary system could offer a wider view of the wind disc. Hercules X-1 stands out from the most famous X-ray binaries because of its warped accretion disk, which wobbles as it rotates around the system’s central neutron star.

“The disc actually wobbles from time to time every 35 days, and the wind originates somewhere on the disc and crosses our line of sight at different heights above the disc over time,” explains Kosec. “It’s a very unique property of this system that allows us to better understand its vertical wind properties.”

A curved sway

In the new study, researchers observed Hercules X-1 using two X-ray telescopes – the European Space Agency’s XMM Newton and NASA’s Chandra Observatory.

“What we measure is the X-ray spectrum, which means the number of X-ray photons that arrive at our detector, versus their energy. We measure the absorption lines, or lack of X-ray light at very specific energies,” said Kosec. “From the ratio of how strong the different lines are, we can determine the temperature, speed, and amount of plasma within the wind disk.”

With the Hercules X-1’s bent disk, astronomers can see the outline of the disk move up and down as it wobbles and rotates, similar to the way the bent disk appears to oscillate when viewed from the edge up. The effect is such that researchers can observe signatures of disc winds at varying heights with respect to the disk, rather than at one fixed height above a uniformly rotating disc.

By measuring the X-ray emission and absorption lines as the disk wobbles and rotates over time, the researchers can scan properties such as temperature and wind density at various heights with respect to the disk and create a two-dimensional map of the winds. vertical structure.

“What we see is that wind rises from the disk, at an angle of about 12 degrees to the disk as it expands in space,” said Kosec. “It also gets colder and lumpier, and weaker at higher altitudes on the dish.”

The team plans to compare their observations with theoretical simulations of various wind launch mechanisms, to see which best explains the origin of the wind. Further afield, they hope to find systems that are more twisted and wobbly, and to map their wind disc structure. Then, scientists can have a much broader view of disc winds, and how those outflows affect their environment — especially on a much larger scale.

“How do supermassive black holes affect the shape and structure of galaxies?” pose of Erin Kara, Assistant Career Development Class of 1958 Professor of Physics at MIT. “One of the leading hypotheses is that disc winds, launched from black holes, can affect the appearance of galaxies. Now we can get a more detailed picture of how these winds are launched, and what they look like.”


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