Just outside Hiroya Yamaguchi’s office is a whiteboard full of exploded stars, schematics of spaceships and spectral lines. A4 format prints cover almost all the free space, except for a small corner where he sometimes scribbles with white chalk. Right now Yamaguchi, an associate professor at the Japan Institute of Space and Astronautical Sciences, is standing in front of this board facing me.
He gives me a crash course on the X-Ray Imaging and Spectroscopy Mission, or XRISM, a partnership between NASA, the Japan Aerospace Exploration Agency (JAXA), and the European Space Agency (ESA). The first thing I learn is that I’ve been saying the name of the telescope wrong all along. Fortunately, I was mostly repeating the incorrect “ex-riz-um” in my head. It’s actually pronounced “criz-um.”
The second is that this space telescope was launched on September 6, 2023, and it carried the heaviest weight of all: Expectation.
Related: JAXA, NASA unveil first images from XRISM X-ray Space Telescope
JAXA’s two previous X-ray telescopes, Suzaku and Hitomi, had problems after launch. Suzaku’s spectrometer failed after launch, but was able to complete a ten-year imaging mission. Hitomi was disastrous: After the first light image was taken, the spacecraft spun out of control and broke apart. XRISM is working well so far, Yamaguchi says, and has already provided scientists with a wealth of data since first light in January — including some discoveries no one expected to find.
“There are so many surprises,” laughs Yamaguchi, looking around at the various prints taped to the board.
However, there is a bit of a problem.
First, the good news: The telescope’s main instrument, a soft X-ray spectrometer known as Resolve, is performing as expected. Somewhat worse news: The shutter door covering the Resolve did not open. Multiple attempts to open the door – or “slider” – failed. Despite reports suggesting that JAXA and NASA do decided to “operate the spacecraft as is for at least 18 months”Yamaguchi told me that “it hasn’t been officially decided.”
A NASA spokesperson confirmed: “NASA and JAXA continue to have discussions about the best way forward to operate XRISM; The main option now is to gather science over the next 18 months before making another attempt to open the gate, but the agencies will continue to evaluate alternatives.”
With the door closed there was an interesting “What if?” situation arises for mission specialists and X-ray astronomers. On the one hand, the spacecraft is doing great, showing that it’s capable of delivering a ton of new, exciting data. Attempting to open the door risks damaging the spacecraft. On the other hand, opening the door could fundamentally change our understanding of the universe.
Solve for “X”
X-rays provide a way to probe some of the most energetically active phenomena in the universe—but because Earth’s atmosphere blocks X-rays, space telescopes are a prerequisite.
“We are revealing the composition of the universe,” Aurora Simionescu, an astrophysicist at the Netherlands Institute for Space Research, tells me. “That’s what an x-ray does.
There are more than a dozen X-ray telescopes currently in space, with NASA’s Chandra Observatory, one of its so-called Large Observatories, perhaps the best known for incredible views provides the x-ray universe. With its ability to see the most detailed X-ray spectra to date, XRISM hopes to claim a similar legacy. However, Yamaguchi points out that although Chandra and XRISM observe the same part of the electromagnetic spectrum, they are meant to do so in different ways. This refers to the instrumentation on board.
Resolve is what is known as a microcalorimetric spectrometer. The detector converts X-rays into heat, measuring minute changes in temperature—we’re talking millikelvin changes—to determine the number and energy of observed X-rays coming from a particular region of space. Energy is measured in electron volts (eV).
The device must therefore be cooled to just a few degrees above absolute zero. This is even cooler than the cosmic microwave background, which is a remnant of radiation from the beginning of time. So far, this radiation is scattered throughout our universe hidden from the human eye because it’s absolutely freezing. “You’re basically almost 30 times cooler than the coldest part of the universe,” says Simionescu. The extreme cooling effect is achieved by chemical and mechanical means.
Chandra uses a different style of X-ray detector that contains an array of charge coupled devices, or CCDs. This converts the X-ray photons into electrons, rather than heat.
Measuring energy is particularly useful because you can plot the number of X-rays that reach your telescope as a function of their energy level—creating what researchers call a “spectrum.” XRISM’s Resolve has an advantage here. It is capable of measuring energies about 20 to 30 times higher than Chandra and with greater resolution. “This allows XRISM to study much more detail about the atomic physics and velocity structure of X-ray sources,” says Patrick Slane, director of the Chandra X-ray Center.
However, Chandra has its own advantages. It’s also constructed with the highest-quality X-ray mirrors ever made, Slane says, meaning its image quality far surpasses XRISM. The key here is that the mirrors give Chandra an angular resolution of 0.5 arcseconds, essentially allowing Chandra to distinguish between objects in the sky that are close together. Compare this to XRISM, which has an angular resolution of 1.7 arcsecminutes.
With this technical feat, Slane says, Chandra can pick out point X-ray sources about 200 times easier than XRISM. In practice, this makes NASA’s telescope extremely useful for focusing on these point sources—distant, smaller targets like neutron stars, planets, and comets. XRISM is good for “extended” targets such as diffuse gas between and within galaxies.
Which finally brings us to the XRISM shutter: The closed door effectively blocks low-energy X-rays from reaching the detector. From now on, the telescope continues to probe the high-energy X-ray universe because those wavelengths are unaffected by the gate dilemma—in fact, Yamaguchi and Simionescu both say it’s already producing fantastic results at higher energies.
But if the door closes for good, scientists will have to contend with parts of our universe remaining inaccessible… at least until the next X-ray telescope arrives, which will probably be the Athena mission in the mid-2030s.
XRISGate-gate
The slide was designed to maintain a near-vacuum inside the telescope’s cryostat—essentially a refrigerator that ensures its instruments stay extremely cold—while XRISM was stationed on Earth.
Once the telescope is in orbit, maintaining this kind of vacuum is not a problem. In space, space itself creates a vacuum. For this reason, the gate valve was designed to open in two steps after activation using a set of controls. In short, the actuators would move back to allow the doors – made of beryllium window and steel mesh – to open. It did not happen.
JAXA tried to tweak the opening of the instrument on three separate occasions, but it didn’t budge. Another attempt would be much riskier, potentially requiring the spacecraft to be heated from extremely cold temperatures and shaken. Target? Force release of actuators. This is a risk that space agencies operating XRISM will have to consider. With the shutter closed, they are already transmitting data. And it’s very good data.
“The most beautiful thing is when you look at the data and it doesn’t look at all like what you expected – and that’s what’s happening with the current XRISM data,” says Simionescu.
Still, it’s a tough break for Simionesco. She is particularly interested in studying X-rays from “galactic atmospheres” – the stuff XRISM was built to look at with the shutter open. With the gate closed, this part of the x-ray universe remains locked. He fully agrees with the decision not to risk trying to open the gate – at least for now. But that doesn’t mean it isn’t painful to know what it could be.
“I’m absolutely gutted that we can’t see below 2 keV,” says Simionescu.
And what could lie below?
Some space-based X-ray telescopes, such as ESA’s XMM-Newton, can see lower-energy X-rays, down to below 2 keV. For example, he observed the Coma cluster, which contains over 1000 galaxies, at energies as low as 0.3 keV. And XRISM’s other tool, Xtend, is also capable of reaching lower energies. But these are also CCD detectors and not so useful for obtaining spectra.
Outside of XRISM, there is no Earth-orbiting X-ray telescope with the ability to look through “stretched” low-energy objects with high resolution, which is particularly important for Simionescu’s work.
During an online call, he shares a wide-angle X-ray image of M87, the first black hole humans imaged in visible light. The picture was snapped Chandra in 2019.
“This is my favorite subject in the world,” he enthuses.
The space around that black hole is a vortex. Simionescu’s cursor bounces across the sky as he points to a large jet emanating from the black hole, as well as regions of dense gas and a long filament stretching light-years into space. He describes a plot of the spectra observed by Chandra on M87 – all below 2 keV – and notes how it’s all a “mumbo jumbo” of emission lines from oxygen, neon, nickel and other gases.
With the gate open, that would change.
“You could tell what the composition of the gas is, how it’s moving, how it’s being pushed out by the black hole — all information you can’t get right now,” he says.
It is interesting to consider the leap forward with XRISM against the backdrop of uncertainty surrounding NASA’s Chandra mission.
Unfortunately, the field of X-ray astronomy could be without Chandra in the near future. The space telescope’s 25-year operation is facing extreme budget cuts in 2024. Astronomers say the proposed budget would lead to the cancellation of the mission.
“If Chandra were to be cancelled, we would lose a tremendous resource for all of modern astrophysics,” says Slane.
This would be an ignominious end to the Great Observatory, which remains invaluable for future discoveries, including working in tandem with XRISM. If JAXA unlocks its doors, Chandra will be an important follow-up to the XRISM observations.
Meanwhile, the ghosts of Suzaku and Hitomi remain until the next attempt to open the door. Currently, the field of X-ray astronomy is excited about what is to come. The worst case scenario isn’t that bad, depending on how you look at it.
“We are recording fantastic data that no one has been able to take before,” says Simionescu. “All the spectra are absolutely spectacular.