Recent findings from a global space mission are validating years of theories surrounding the areas around supermassive black holes.
What’s even more thrilling than the data itself is the entry of the long-awaited satellite, the X-Ray Imaging and Spectroscopy Mission, or XRISM, which is starting to uncover unique insights.
“We now have the ideal tool to accurately depict the largely uncharted dimensions surrounding supermassive black holes,” stated Jon Miller, an astronomy professor at the University of Michigan, regarding XRISM.
“We are beginning to glimpse what that environment truly resembles.”
The Japanese Aerospace Exploration Agency (JAXA), in collaboration with NASA and the European Space Agency, announced these new findings, which were also featured in The Astrophysical Journal Letters.
Miller served as the lead author for this research. Together with over 100 international co-authors, he studied active galactic nuclei, which include supermassive black holes and their extreme environments.
To achieve this, they utilized XRISM’s exceptional capability to collect and analyze X-ray spectra emitted by various cosmic events.
“It’s incredibly exciting to obtain X-ray spectra at such an unprecedented high resolution, especially for the hottest plasmas in the universe,” said Lia Corrales, an assistant professor of astronomy at U-M and a co-author of both XRISM studies.
“The spectra provide an abundance of information; thus, we will likely spend many years interpreting the initial datasets.”
Twisted Accretion Disks
Aficionados of space exploration may be aware that the Chandra X-ray Observatory, known as NASA’s flagship X-ray telescope, recently marked its 25th anniversary in operation.
However, it’s less commonly known that, during these 25 years, a collaborative effort involving scientists, engineers, and officials from various space agencies has been striving to deploy other sophisticated X-ray missions to complement the data provided by Chandra.
XRISM is now fulfilling that mission.
Using their dataset, Miller, Corrales, and their team have solidified a theory regarding the structures known as accretion disks found near supermassive black holes in active galactic nuclei.
These disks can be likened to vinyl records composed of gas and other loose particles from a galaxy, spinning due to the immense gravity of the black holes at their centers. By examining these accretion disks, researchers can gain insights into the events occurring around the black hole and their impact on the lifecycle of the host galaxy.
By investigating a galaxy called NGC 4151, located over 50 million light-years away, the XRISM team confirmed that the disk’s shape is more complex than previously thought.
“What we’re observing is that the disk isn’t flat. It exhibits a twist or warp,” explained Miller. “It also appears to be thicker towards the edges.”
While hints of this complex geometry have surfaced in data over the past 25 years, XRISM’s findings provide the most compelling direct evidence to date.
“We had some suggestions,” Miller remarked. “But it would be akin to a forensic analyst stating that we lacked sufficient evidence to convict anyone based on what we had.”
The team also discovered that the accretion disk is losing a significant amount of gas. Scientists have their theories on the fate of this material, and Miller notes that XRISM will facilitate more definitive discoveries.
“It has been challenging to determine the fate of that gas,” he said. “Obtaining direct evidence is the crucial work that XRISM enables.”
Furthermore, XRISM is not only prompting researchers to rethink existing theories but also allows them to explore previously hidden areas of space.
A Missing Component
Despite their intense gravitational fields from which not even light can escape, black holes are significant sources of electromagnetic radiation that we can detect.
For example, the Event Horizon Telescope, a network of ground-based instruments sensitive to radiation emitted as radio waves, has enabled astronomers to observe the very edge of two different black holes.
There are numerous other instruments on Earth and in space that capture various forms of radiation, including X-rays and infrared light, providing extensive, galaxy-scale observations around black holes.
Yet, high-resolution tools for studying what occurs between these two scales, from the immediate vicinity of the black hole to its host galaxy, have been lacking. This area houses accretion disks and other fascinating celestial formations.
If we divide the expansive view of a black hole by its close-up view, we approach a figure near 100,000. For physicists, each zero represents an order of magnitude, indicating a significant gap in coverage that spans five orders of magnitude.
“Understanding how gas is absorbed by black holes, how it is lost, and how black holes influence their host galaxies hinges on these orders of magnitude,” Miller remarked.
XRISM now provides researchers the ability to investigate these scales by detecting X-rays emitted by iron around black holes and using the “S” from its name for spectroscopy.
Instead of capturing X-ray light to form an image, XRISM’s spectroscopy instrument measures the energy of individual X-ray photons. Researchers can then quantify how many photons with specific energies were detected across a range of energies, or spectrum.
By gathering, analyzing, and comparing spectra from different areas near a black hole, researchers gain deeper insights into the ongoing processes.
“We often say that spectra place the ‘physics’ in ‘astrophysics,’” Miller said.
Although several other X-ray spectroscopy instruments are operational, XRISM’s features the most advanced capabilities, relying on a microcalorimeter known as “Resolve.” This device converts incident X-ray energy into heat instead of a conventional electrical signal.
“Resolve is enabling us to characterize the multi-structured and multi-temperature environment surrounding supermassive black holes in ways that were previously impossible,” Corrales stated.
According to Miller, XRISM offers researchers a tenfold improvement in energy resolution compared to previous instruments. Scientists have awaited such a device for 25 years, though this has not stemmed from a lack of attempts.
Persistence Pays Off
Years before its launch in 1999, Chandra was originally envisioned as the Advanced X-Ray Astrophysics Facility, intended as a single mission equipped with cutting-edge technology for both X-ray imaging and spectroscopy.
However, due to high costs, it was split into the Chandra telescope and a separate spectroscopy mission named Astro-E, which JAXA developed. Unfortunately, Astro-E was lost during its launch in February 2000.
Recognizing the necessity of the tool, JAXA, NASA, and the European Space Agency collaborated to redesign and re-launch the Astro-E mission approximately five years later, this time under the name Suzaku, inspired by a mythical phoenix-like creature.
“Suzaku successfully entered orbit,
However, there was a leak in its cryogenic system, causing all of its coolant to escape into space. Consequently, the main scientific instrument was unable to gather any actual data,” Miller explained. “There was another camera onboard that focused on X-rays, and it performed excellently for around 10 years.”
Just a few months after the conclusion of the Suzaku mission, the space agencies initiated a third project to deliver the X-ray spectroscopy needed by the scientific community. This mission launched as Astro-H in February 2016 and was later renamed Hitomi after reaching orbit and successfully deploying its solar panels.
Miller had traveled to Florida for a meeting regarding Hitomi when the mission faced a catastrophic failure. A navigation error caused Hitomi to spiral out of control.
“It was spinning so rapidly that the solar panels detached,” Miller recalled.
Less than 40 days post-launch, the space agencies lost all communication with Hitomi.
“You could actually go out to the beach in Florida at night and see it tumble through the sky,” Miller described. “It flickered in a remarkably unique manner.”
Before its demise, the Hitomi mission succeeded in obtaining what Miller described as one and a half scientific observations. This amount of data was significant enough to change researchers’ perspectives on galaxy clusters, which are made up of hundreds or even thousands of galaxies, he noted.
Therefore, a significant expectation rested on XRISM when it was launched in September 2023. Early feedback indicates that XRISM is well-prepared to fulfill its mission. Miller, along with a few global colleagues, was among the first to analyze the data that would contribute to their new findings.
“It was quite late in Japan, at an unusual hour in Europe, and we were all on Zoom. Everyone struggled to find the right words,” Miller shared. “It was truly awe-inspiring.”
Miller’s original doctoral research was intended to analyze data from the Astro-E mission, making him invested in this pursuit for over half his life and nearly his entire scientific career.
Throughout this period, Hitomi and other successful missions like Chandra have provided invaluable data, allowing him and other researchers to deepen their understanding of the universe. However, the scientists were aware that to achieve the breakthroughs they desired, they would need a tool similar to the X-ray calorimeter onboard XRISM.
“There were challenging phases, but we continually received indications of what might be achievable,” Miller stated. “It’s nearly impossible to replicate these conditions in experiments conducted on Earth, and we’ve been eager to uncover many specifics of how they function. I believe we’re finally on the brink of making significant advancements.”
