They map gusty winds in a distant neutron star system

10 Apr. (EUROPE PRESS) –

MIT astronomers have mapped the ‘disco winds’ associated with the Hercules X-1 accretion disk, a system where a neutron star extracts material from a star like the Sun.

The findings, published in the journal ‘Nature Astronomy’may offer clues about how supermassive black holes shape entire galaxies.

An accretion disk is a huge swirl of gas and dust that clumps around a black hole or neutron star like cotton candy as it pulls in material from a nearby star. As it spins, the disk kicks up powerful winds that push and pull on the rotating plasma. These bulk flows can affect the environment around black holes by heating up and ejecting surrounding gas and dust.

At immense scales, ‘disk winds’ may offer clues about how supermassive black holes shape entire galaxies. Astronomers have observed hints of disk winds in many systems, including accreting black holes and neutron stars. But to date they had only glimpsed a very limited view of this phenomenon.

Now, astronomers have observed a broader swath of winds, in Hercules X-1, a system in which a neutron star is extracting material from a Sun-like star. The accretion disk of this neutron star is unique, as it wobbles or precesses as it spins. Taking advantage of this oscillation, astronomers have captured different perspectives of the rotating disk and have created, for the first time, a two-dimensional map of its winds.

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 on the softer end of what accretion disks they can rotate.

If astronomers can detect more wobble 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 in a number of objects and determine how the properties of the wind change, for example, with the mass of a black hole, or with the amount of material it is accreting,” explains Peter Kosec. , a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research — that will help determine how black holes and neutron stars influence our universe.”

Disk winds have been most frequently observed in X-ray binaries, systems in which a black hole or neutron star pulls in material from a less dense object and generates a fiery disk of inspiring matter, along with an outflow wind.

It is not clear exactly how the winds are released from these systems. Some theories propose that the magnetic fields could destroy the disk and expel part of the material towards the outside in the form of wind. Others hold that the radiation from the neutron star it could heat and evaporate the surface of the disk in red-hot gusts of wind.

Clues to the origin of the wind can be deduced from its structure, but the shape and extent of disk winds have been difficult to resolve. Most binary stars produce accretion disks relatively uniformly, like thin gas doughnuts that rotate in a single plane.

Astronomers studying these disks from distant satellites or telescopes can only observe the effects of disk winds within a fixed, narrow range, relative to their spinning disk. Any wind that astronomers manage to detect is therefore a small portion of its larger structure.

“We can only probe the properties of the wind at a single point, and we are completely blind to everything around that point,” says Kosec.

In 2020, he and his colleagues realized that a binary system could offer a broader view of disk winds. Hercules X-1 has been distinguished from most known X-ray binaries by its warped accretion disk, that wobbles as it revolves around the system’s central neutron star.

“The disk wobbles every 35 days, and the winds originate somewhere in the disk and cross our line of sight at different heights above the disk over time,” Kosec explains. “That’s a very unique property of this system. which allows us to better understand its vertical wind properties.”

In the new study, the 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 an X-ray spectrum, that is, the number of X-ray photons reaching our detectors, versus their energy. We measure absorption lines, or the lack of X-ray light at very specific energies. –says Kosec– Based on the relationship between the intensity of the different lines, we can determine the temperature, speed, and amount of plasma inside the wind disk.”

With Hercules X-1’s warped disk, astronomers could see the line of the disk moving up and down as it wobbled and rotated, similar to how a warped disk appears to wobble when viewed edge-on. The effect was such that the researchers were able to observe signs of winds in the disk 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 oscillated and rotated over time, the researchers were able to scan properties such as the temperature and density of winds at different heights relative to its disk and build a map. two-dimensional vertical structure of the wind.

“What we see is wind rising from the disk, at about a 12-degree angle to the disk as it expands into space,” Kosec said. “It’s also getting colder and clumpier, and weaker the higher above the disk”.

The team plans to compare their observations with theoretical simulations of various wind launch mechanisms, to see which might best explain the wind’s origins. Later, they hope to discover more warped and wobbled systems and map their wind structures on the disk. Thus, scientists could have a broader view of disk winds and how they influence their environment, especially on much larger scales.

“How do supermassive black holes affect the shape and structure of galaxies?” says Erin Kara, MIT Assistant Professor of Physics in the class of 1958. “One of the leading hypotheses is that disk winds, thrown from a black hole, they can affect the appearance of galaxies. Now we can get a more detailed picture of how these winds are thrown and what they look like.”