Just outside Hiroya Yamaguchi’s office is a chalkboard full of exploded stars, spaceship diagrams, and spectral lines. The A4 printouts take up almost all the free space, except for a small corner where he sometimes scribbles with white chalk. Yamaguchi, now an associate professor at the Japan Institute of Aeronautics and Astronautics, stands in front of this blackboard, 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 learned was that I had been saying the name of the telescope wrong all this time. Luckily I was mostly repeating the wrong phrase “exi-riz-um” in my head. It’s actually pronounced “criz-um.”
Secondly, the fact that this space telescope was launched on September 6, 2023 carries the heaviest burden: Expectation.
Relating to: JAXA and NASA reveal first images from XRISM X-ray space telescope
JAXA’s two previous X-ray telescopes, Suzaku and Hitomi, both experienced problems post-launch. Suzaku’s spectrometer malfunctioned after launch but managed to fulfill a decade-long imaging mission. Hitomi was a disaster: After taking its first light image, the spacecraft went into an uncontrolled spin and broke up. Yamaguchi says XRISM has performed well so far, providing scientists with plenty of data since early January, including some discoveries no one expected to find.
“There are so many surprises,” laughs Yamaguchi as he glances at the various printouts 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 working as expected. Some more bad news: The aperture door covering Resolve did not open. Multiple attempts to open the door or “gate valve” were unsuccessful. Despite reports suggesting JAXA and NASA are doing this decided to “operate the spacecraft as is for at least 18 months”Yamaguchi told me that “no official decision has been made.”
A NASA spokesperson said, “NASA and JAXA continue to have ongoing discussions about the best way to operate XRISM; the current, leading option is to collect science over the next 18 months before making another attempt to open the gate valve, but the agencies will do so.” said. “Continue to evaluate alternatives.”
With the door closed, it’s an engaging “What If?” For mission experts and X-ray astronomers, the situation speaks for itself. On the one hand, the spacecraft works perfectly and shows that it has the ability to deliver a wealth of new, exciting data. Trying to open the door risks damaging the spacecraft. On the other hand, opening the door can fundamentally change our understanding of the universe.
Solve ‘X’
X-rays provide a way to investigate some of the most energetic events in the universe; but since Earth’s atmosphere blocks X-rays, space-based telescopes are a prerequisite.
“We are unraveling the structure of the universe,” astrophysicist Aurora Simionescu of the Netherlands Institute for Space Research tells me. “That’s what X-rays do.”
There are currently more than a dozen X-ray telescopes in space; these include NASA’s Chandra observatory, called the Great Observatories; Perhaps the best known of these is incredible views It enabled the X-ray universe. XRISM hopes to create a similar legacy with its ability to see the most detailed X-ray spectra ever. But Yamaguchi emphasizes that although Chandra and XRISM observe the same part of the electromagnetic spectrum, they must do so in different ways. This depends on the instrumentation on board.
Resolve is a device known as a microcalorimeter spectrometer. The detection apparatus converts X-rays into heat and measures small changes in temperature (we’re talking changes in millikelvins) to determine the number and energy of X-rays observed coming from a particular region of space. Energy is measured in electron volts (eV).
Therefore, the device only needs to be cooled to a few degrees above absolute zero. This is even colder than the cosmic microwave background radiation, which is radiation left over from the beginning of time. This radiation is scattered throughout our universe, but hidden from human eyes Because it’s definitely cold. “You’re basically almost 30 times colder than the coldest part of space,” Simionescu says. The supercooling effect is achieved by chemical and mechanical means.
Chandra uses a different style of X-ray detector that includes an array of charge-coupled devices, or CCDs. This converts X-ray photons into electrons instead of heat.
Measuring energy is especially useful because you can plot the number of X-rays reaching your telescope against their energy levels; You can create what researchers call a “spectrum.” XRISM’s Resolve has an advantage in this case. It can measure energies 20 to 30 times higher and with higher resolution than what Chandra can measure. “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.
But Chandra also has its advantages. Slane says it’s also built with the highest quality X-ray mirrors ever produced; This means that the imaging quality far exceeds that of XRISM. The key here is that the mirrors give Chandra an angular resolution of 0.5 arcseconds; This allows Chandra to distinguish objects that are close together in the sky. Compare this to XRISM which has an angular resolution of 1.7 arcsminute.
Thanks to this engineering feat, Slane says Chandra can pick out X-ray point sources about 200 times more easily 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 gas spreading between and within galaxies.
This finally brings us to XRISM’s gate valve: The closed door effectively blocks low-energy X-rays from reaching the detector. As of now, the telescope continues to explore the high-energy X-ray universe because these wavelengths are not affected by the gate dilemma; In fact, both Yamaguchi and Simionescu say that the telescope gives fantastic results at higher energies.
But if the door is completely jammed, scientists will have to deal with parts of our universe remaining inaccessible… at least until another X-ray telescope comes along, which will likely be the Athena mission in the mid-2030s.
XRISMgate gate
The gate valve is designed to maintain near-vacuum in the telescope’s cryostat (essentially the refrigerator that keeps the instruments ultra-cold) while XRISM is deployed on Earth.
Maintaining such a gap is not a problem once the telescope reaches orbit. Space itself creates the void in space. Therefore, the gate valve was designed to be opened by twisting it in a two-step process via a series of actuators after launch. In short, actuators would slide backwards to allow the beryllium window and steel mesh door to be flipped open. This did not happen.
JAXA tried to expertly open the device on three different occasions, but did not budge. The next attempt would be much riskier, potentially requiring the spacecraft to heat up and shake from extremely low temperatures. Aim? Displacement of actuators by applying force. This is a risk that space agencies operating XRISM must weigh. They are already entering data banking when the gate valve is closed. And this is very good data.
“The great thing when you look at the data is that nothing looks like you expect, and this is happening with the current XRISM data,” Simionescu says.
Still, it’s a difficult time for Simionescu. He is particularly interested in studying X-rays from “galactic atmospheres”; Things XRISM is designed to look at with an open valve. When the door is closed, that part of the X-ray universe remains locked. He fully agrees with the decision not to risk opening the door, at least for now. But that doesn’t mean it isn’t painful to know what could happen.
“I’m definitely very sorry that we can’t see below 2 keV,” says Simionescu.
So what could happen?
Some X-ray space telescopes, such as ESA’s XMM-Newton, can see low-energy X-rays below 2 keV. For example, the Coma Cluster, which contains more than 1,000 galaxies, has been observed at energies as low as 0.3 keV. XRISM’s other instrument, Xtend, can also reach lower energies. However, these are also CCD detectors and are not as useful for obtaining spectrum.
Other than XRISM, there is no X-ray telescope orbiting Earth capable of looking at “extended” objects at low energies with high resolution, which was particularly important for Simionescu’s work.
He shares a wide-field X-ray image of M87 during an online conversation. first black hole imaged by humans in visible light. The picture was: Taken by Chandra In 2019.
“This is my favorite object in the world,” he says excitedly.
The area surrounding this black hole is a vortex. Simionescu’s cursor bounces across the sky, pointing out the massive jet emerging from the black hole, as well as areas of dense gas and a long filament stretching light-years into the universe. He describes a plot of the spectra observed by Chandra in M87 (all below 2 keV) and points out that it is a “jumbo of nonsense” consisting of emission lines from oxygen, neon, nickel and other gases.
This would change when the door opened.
“You can tell what the composition of the gas is, how it moves, how it gets 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 amid the uncertainty surrounding NASA’s Chandra mission.
Unfortunately, the field of X-ray astronomy may be without Chandra in the near future. The space telescope’s 25-year work will face extreme budget cuts in 2024. Astronomers say The proposed budget would result in the cancellation of the mission.
“If Chandra is canceled, we would lose a tremendous resource for all of modern astrophysics,” says Slane.
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This would be an ignominious end for the Great Observatory, which remains invaluable to future exploration, including working with XRISM. If JAXA unblocks its gate, Chandra will be an important tool for tracking XRISM’s observations.
Meanwhile, Suzaku and Hitomi’s ghosts will linger until the next attempt to open the door. For now, the field of X-ray astronomy is excited about the future. The worst case scenario isn’t that bad, depending on how you look at it.
“We are getting extraordinary data that no one has been able to get before,” says Simionescu. “The entire spectrum is absolutely gorgeous.”