The X-ray Imaging and Spectroscopy Mission (XRISM), a Japan Aerospace Exploration Agency (JAXA) and NASA collaboration with European Space Agency (ESA) participation, was built to study the most extreme environments in the cosmos. From space, the satellite collects X-rays to probe the universe's hottest regions, largest structures and strongest gravity wells.
In four recent papers published in Nature and the Astrophysical Journal Letters, the XRISM team - including researchers from Lawrence Livermore National Laboratory (LLNL) - examined the winds coming from a quasar and a neutron star binary system, the gas sloshing in a galaxy cluster, and an astrophysical object shrouded in secrecy.
Understanding these dynamic phenomena and other cosmic mysteries sheds light on how the universe formed and evolved.
"These objects act as cosmic laboratories, helping us to explore extreme conditions and the largest structures in the universe," said LLNL physicist Natalie Hell. "Our recent results stem from looking at these objects with this amazing new X-ray spectroscopic capability."
Building and testing Resolve
The core instrument on XRISM, pronounced "crism," is a high-resolution X-ray microcalorimeter spectrometer. Named Resolve, the instrument was developed by an international consortium of researchers led by NASA's Goddard Space Flight Center and Japan's Institute of Space and Astronautical Science.
Resolve measures the tiny temperature changes created when a single X-ray photon hits its detector. By pooling thousands or millions of those photons, the instrument collects spectra - cosmic barcodes that show what colors (or X-ray energies) of light an object is emitting or absorbing. During its lifetime, XRISM will capture much more detailed spectra for certain objects than ever before, shedding light on properties like their temperature, composition and dynamics.
Scientists at LLNL supported Resolve's development and led the calibration effort as members of its instrument team. This included work at synchrotron facilities and spacecraft test facilities in the U.S. and Japan.
As part of the calibration, LLNL scientists and technicians deployed an Electron Beam Ion Trap (EBIT) to the Cryogenic Research and Integration Facility at Goddard, providing a critical capability for the project.
EBIT technology was invented by LLNL researchers in the mid-1980s. It uses a beam of electrons to trap ions in place and pelt them with electrons that strip off even more electrons. The resultant highly charged ions emit photons with energies similar to the X-rays from astronomical objects.
With those ions, the LLNL team was able to accurately calibrate Resolve before sending the instrument to space.
"The EBIT we delivered started out as a test platform at LLNL," said LLNL physicist Greg Brown. "When we saw a key gap in the tools needed to calibrate Resolve on the ground, our team upgraded and adapted the apparatus to provide the necessary calibration capabilities."
There's something in the wind
Now that XRISM is operational and taking data, the LLNL team has pivoted to analysis.
One of XRISM's first observations was a quasar, an object among the brightest and most energetic in the universe. Quasars are powered by a supermassive black hole at the center of a galaxy that actively consumes the matter around it. As a result, the quasar accumulates a superheated disk and spews out material in the form of powerful winds.
Previous observations of quasars detected winds at speeds considerably lower than the speed of light, and their spectral resolution was not high enough to determine the wind's physical structure or location.
In the case of the quasar PDS 456, new and enhanced spectra from XRISM, however, untangle the chaotic winds. Previous measurements showed a single, broad absorption feature in the spectra - picture a singular, deep trough. XRISM brought more detail to that feature and showed that it actually consists of several distinct and narrow features combined.
Those separate absorption features indicate five discrete clumps of wind material flowing at 20-30 percent of the speed of light along our line of sight to the quasar.
"The team estimated that the wind carries significantly more energy than previous models predicted - expelling up to 300 solar masses of gas per year," said Brown.
Because they found five clumps of wind along a single sightline, the XRISM science team expects there could be millions of clumps in the wind region around the black hole. If so, the environment would mimic the number of wind pockets that exist in Earth's atmosphere.
In another study published in Nature, the XRISM team examined the outflow coming from an accreting neutron star in a binary system. In contrast to the quasar, the wind in this system moved much slower than anticipated. The outflow was also smoother, without the numerous clumps. It was, however, very dense.
Taken together, these findings raise questions about how exactly accretion disks affect wind behavior. It is possible that the incredibly fast, clumpy wind from the quasar could be created in a completely different way than the slower, smooth outflow from the neutron star binary.
A cosmic thermostat
XRISM also turned its detector toward the Centaurus galaxy cluster, a structure of hundreds of galaxies bound together by gravity with huge quantities of hot ionized gas contained between them. This intracluster gas should cool down over time as its heat dissipates into the surrounding space, but other studies have not found that to be the case. Something else is at play.
"Astronomers call this the 'cooling flow problem,' and it's been discussed for decades," said LLNL physicist Megan Eckart. "Gas at the center of galaxy clusters is expected to cool and sink towards the core, but observations consistently show much less cool gas than models predict. A leading theory is that a powerful black hole heats the gas, offsetting the cooling process, but many parts of this story remain unconfirmed."
With spectra from XRISM, the team identified sloshing gas as the culprit. By redistributing the cooled gas, the sloshing motion continuously stirs the pot, preventing excessive accumulation of cooled material at the center and balancing cluster temperature.
That same stirring motion should in theory also disperse any heat injected by, for example, active galactic nuclei containing quasars. However, in this case, the spectra didn't show that effect - perhaps because active galactic nuclei have little influence in this particular cluster.
Essential EBIT expertise
In another study based on XRISM's findings, the team dove into Cygnus X-3. The mysterious object is shrouded in dust, blocking any visible light and presenting an intriguing puzzle.
Cygnus X-3 is a binary system, likely hosting a massive star and a black hole that orbit each other every 4.8 hours. The star's strong winds strip it of its surface material, which the black hole sweeps up and heats. This emits X-rays that XRISM can detect - but the gas in the vicinity also absorbs those X-rays. And gas that's moving will shift the emission and absorption signatures depending on its velocity and direction.
The result is a complicated and overlapping tangle of a spectrum. Imagine a barcode with overlapping lines that scientists must separate to determine what's really going on.
This is where LLNL's EBIT expertise comes back into play. By measuring the corresponding emission lines of relevant elements in the laboratory to high accuracy and under known conditions, the researchers can unravel the XRISM spectra.
"Many of the emission and absorption features from relevant ions are complex and difficult to accurately model, even for the simple ions with only a few electrons," said Hell. "Our measurements in the EBIT laboratory provide the benchmark astronomers need."
With this work, the scientists found two main components of the gas and wind moving in the Cygnus X-3 system: a smooth, large-scale background wind and a dense, turbulent region close to the black hole candidate. The latter suggests that the black hole is carving out a wake as it orbits through the star's ejected gas.
A XRISMatic future
The LLNL team emphasized that XRISM has just begun its mission and will continue to provide valuable data on energetic objects throughout the universe. Laboratory researchers are currently looking into observations of several different sources.
Eckart is part of a team investigating a sudden outburst of matter near the supermassive black hole NGC 3783, with speeds reaching up to 20 percent of the speed of light captured in XRISM's longest continuous observation to-date.
Hell is continuing studies on Cygnus X-3 and other X-ray binary systems, while Brown is contributing to research on W49B, an unusual supernova remnant formed by an explosion around 1000 years ago.
"I'm grateful and excited to see Lab researchers using this new capability to push the boundaries of high-energy astrophysics. Our group's work is closely tied to uncovering the physical processes that are going on in some of the largest, most energetic objects in the universe," said Brown. "Those processes are encoded in the spectra we measure with Resolve. All we have to do is decode the spectra to reveal the exciting physics taking place."
