One of the leading hypotheses behind how supermassive black holes affect the shape and structure of galaxies is that disk winds, launched from a black hole, can affect how galaxies look. The recent observations of disk winds from the neutron star’s accretion disk in Hercules X-1 have provided a more precise understanding of the mechanism that launches these winds and their appearance.
Researchers have created a 2D representation of the turbulent winds present in a distant system consisting of a neutron star. This mapping of the “disk wind” could provide insights into how galaxies are formed.
In a black hole or neutron star system, an accretion disk forms as gas and dust are drawn in from a neighboring star, creating a massive vortex. As the disk rotates, it generates strong gusts of wind that interact with the expansive, rotating plasma, exerting force through push-and-pull motions. These immense outflows have the potential to influence the environment surrounding black holes by heating and dispelling gas and dust in the vicinity.
Astronomers have been studying “disk winds” at large scales to gain insight into how supermassive black holes influence the formation of galaxies. Despite observing signs of disk winds in various systems, such as black holes and neutron stars, the phenomenon has only been viewed from a limited perspective.
However, MIT astronomers recently observed a broader range of disk winds in Hercules X-1, a system where a neutron star is drawing material from a star similar to the sun. This neutron star’s accretion disk is unique as it wobbles, or “precesses,” while rotating. By utilizing this wobble, the astronomers were able to capture different viewpoints of the rotating disk and produce a two-dimensional map of its winds – marking the first time such a map has been created.
By utilizing the wobbling of the neutron star’s accretion disk in Hercules X-1, MIT astronomers have created a two-dimensional map of its winds that exposes its vertical shape, structure, and velocity. The velocity, which is in the range of hundreds of kilometers per second, is relatively mild compared to the maximum spin-up of accretion disks. If additional systems exhibiting this wobble can be discovered, this mapping method could assist astronomers in understanding how disk winds impact the creation and development of stellar systems and galaxies.
“In the future, we could map disk winds in a range of objects and determine how wind properties change, for instance, with the mass of a black hole, or with how much material it is accreting,” adds lead author Peter Kosec, a postdoc in 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 predominantly detected in X-ray binaries, which are systems where a black hole or neutron star draws in material from a less dense object, resulting in a white-hot disk of matter spiraling inward and an outflowing wind. The specific mechanism that launches winds from these systems remains unclear, with various theories being proposed. According to some theories, magnetic fields may tear apart the disk, expelling some of the material outward as wind. In contrast, others suggest that the neutron star’s radiation could result in the disk’s surface heating and evaporating in powerful bursts.
The structure of a wind may provide hints about its origins, but determining the shape and scope of disk winds has posed a challenge. Accretion disks produced by most binaries are generally uniform in shape, resembling thin gas donuts that rotate in a single plane. From remote satellites or telescopes, astronomers studying these disks can only examine the effects of disk winds within a limited and fixed range in comparison to the disk’s rotation. Consequently, any wind astronomers may identify represents only a small portion of the entire structure.
“We can only probe the wind properties at a single point, and we’re completely blind to everything around that point,” Kosec adds.
In 2020, the researchers recognized that a specific binary system could provide a more extensive perspective of disk winds. Hercules X-1 is unique among most X-ray binaries due to its distorted accretion disk, which wobbles while revolving around the central neutron star in the system.
“The disk is really wobbling over time every 35 days, and the winds are originating somewhere in the disk and crossing our line of sight at different heights above the disk with time,” explains the authro. “That’s a very unique property of this system which allows us to better understand its vertical wind properties.”
For their latest research, they employed two X-ray telescopes, namely the XMM-Newton from the European Space Agency and NASA’s Chandra Observatory, to study Hercules X-1.
“What we measure is an X-ray spectrum, which means the amount of X-ray photons that arrive at our detectors, versus their energy. We measure the absorption lines, or the lack of X-ray light at very specific energies,” Kosec adds. “From the ratio of how strong the different lines are, we can determine the temperature, velocity, and the amount of plasma within the disk wind.”
Astronomers were able to visualize the movement of Hercules X-1’s warped disk by observing its line moving up and down while wobbling and rotating. This phenomenon is similar to how a warped record appears to oscillate when viewed from the edge. This effect allowed the researchers to observe changes in the height of disk winds relative to the disk rather than at a fixed height above a uniformly rotating disk.
Over time, by monitoring X-ray emissions and absorption lines during the disk’s wobbling and rotation, they were able to investigate wind properties, such as density and temperature, at different heights relative to the disk. Based on this data, they developed a two-dimensional map that displayed the vertical structure of the wind.
“What we see is that the wind rises from the disk, at an angle of about 12 degrees with respect to the disk as it expands in space,” Kosec adds. “It’s also getting colder and more clumpy, and weaker at greater heights above the disk.”
The research team intends to compare their findings with theoretical simulations of different mechanisms that launch winds to determine which theory best explains the origin of the wind. In the future, the team aims to discover additional systems that exhibit warping and wobbling behavior and map their disk wind structures. By doing so, astronomers could obtain a more comprehensive understanding of disk winds and how they influence their surroundings, particularly on a much larger scale.
“How do supermassive black holes affect the shape and structure of galaxies?” poses Erin Kara, the Class of 1958 Career Development Assistant Professor of Physics at MIT. “One of the leading hypotheses is that disk winds, launched from a black hole, can affect how galaxies look. Now we can get a more detailed picture of how these winds are launched, and what they look like.”
Source: MIT
Image Credit: Jose-Luis Olivares, MIT. Based on an image of Hercules X-1 by D. Klochkov, European Space Agency.