Hercules X-1 is an X-ray binary system in which a neutron star is drawing material away from a Sun-like star. This neutron star’s accretion disk is unique in that it wobbles as it rotates. By taking advantage of this wobble, astronomers have captured varying perspectives of the rotating disk and created a 2D map of its winds.
Kosec et al. mapped the winds associated with the accretion disk around Hercules X-1, a binary system located 6,600 parsecs (21,526 light-years) away in the constellation of Hercules. Image credit: Jose-Luis Olivares, MIT / D. Klochkov, ESA.
Despite their existence being known for decades, the driving mechanism of accretion disk winds is still poorly understood.
In supermassive black holes, these outflows can even be powerful enough to dictate the evolution of the entire host galaxy, and yet so far astronomers do not understand whether they are launched by radiation pressure, magnetic forces, thermal irradiation or a combination thereof.
Clues to a wind’s origins may be deduced from its structure, but the shape and extent of disk winds has been difficult to resolve.
Most binaries produce accretion disks that are relatively even in shape, like thin donuts of gas that spins in a single plane.
Astronomers who study these disks from far-off satellites or telescopes can only observe the effects of disk winds within a fixed and narrow range, relative to their rotating disk.
Any wind that astronomers manage to detect is therefore a small sliver of its larger structure.
“We can only probe the wind properties at a single point, and we’re completely blind to everything around that point,” said Dr. Peter Kosec, an astronomer at MIT’s Kavli Institute for Astrophysics and Space Research.
In 2020, Dr. Kosec and his colleagues realized that Hercules X-1, a nearly edge-on X-ray binary with a warped accretion disk precessing with a period of about 35 days, could offer a wider view of disk winds.
“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,” Dr. Kosec said.
“That’s a very unique property of this system which allows us to better understand its vertical wind properties.”
In their new study, the astronomers observed Hercules X-1 using two X-ray telescopes: ESA’s XMM-Newton Space Observatory and NASA’s Chandra X-ray Observatory.
“What we measure is an X-ray spectrum, which means the amount of X-ray photons that arrive at our detectors, versus their energy,” Dr. Kosec said.
“We measure the absorption lines, or the lack of X-ray light at very specific energies.”
“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.”
With Hercules X-1’s warped disk, the authors were able to see the line of the disk moving up and down as it wobbled and rotated, similar to the way a warped record appears to oscillate when seen from edge-on.
The effect was such that they could observe signs of disk winds at changing heights with respect to the disk, rather than at a single, fixed height above a uniformly rotating disk.
By measuring X-ray emissions and the absorption lines as the disk wobbled and rotated over time, the researchers could scan properties such as the temperature and density of winds at various heights with respect to its disk and construct a two-dimensional map of the wind’s vertical structure.
“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,” Dr. Kosec said.
“It’s also getting colder and more clumpy, and weaker at greater heights above the disk.”
The team now plans to compare their observations with theoretical simulations of various wind-launching mechanisms, to see which could best explain the wind’s origins.
Further out, the scientists hope to discover more warped and wobbling systems, and map their disk wind structures.
Then, they could have a broader view of disk winds, and how such outflows influence their surroundings — particularly at much larger scales.
“How do supermassive black holes affect the shape and structure of galaxies?” said Dr. Erin Kara, an astronomer 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.”
The findings were published in the journal Nature Astronomy.
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P. Kosec et al. Vertical wind structure in an X-ray binary revealed by a precessing accretion disk. Nat Astron, published online April 10, 2023; doi: 10.1038/s41550-023-01929-7
