4 questions about XRISM, the mission that targets extreme events in the universe

The most violent cataclysms of the cosmos. This, in short, is what the new Japanese XRISM space telescope will look at. Launch end of August 2023.

XRISM, what is it?

XRISM is the name given to a scientific mission supervised by the Japanese space agency (Jaxa). Purpose of XRISM, whose acronym stands for X-ray Imaging and Spectroscopy Mission ? Deploy a telescope in low Earth orbit to observe high and very high energy phenomena in the cosmos. In particular, it carries two instruments.

The first of these instruments, called Xtend, is an extended imager specialized in the detection of cosmic sources which emit X-rays. The second, called Resolve, is a spectrometer responsible for measuring the temperature and the dynamics of objects emitting X-rays, by capturing the photons received. In short, one detects, the other measures.

This mission is taking place with the support of its American counterpart (Nasa). The European Space Agency (ESA) is also partthrough the provision of equipment and scientific advice, which will allow it to recover 8% of the available observation time of XRISM, once the observatory is in place.

In particular, ESA chose “test” objectives to calibrate XRISM instruments before the start of its career, in order to verify its performance. It also delivered an optical telescope to help the spacecraft always know where it is pointing, helped in the design of Resolve, and orientation aids, based on the Earth’s magnetic field.

Artist’s impression of the XRISM telescope. // Source: NASA’s Goddard Space Flight Center Conceptual Image Lab

What will XRISM track?

Observation of the Universe is done by looking at what is happening in the cosmos. There is what can be observed via visible light, which is in fact a very small part of the electromagnetic spectrum, and there is everything that is in the domain of the invisible: radio waves, infrared, ultraviolet, but also signals related to high and very high energies: X-rays and gamma rays.

This is where XRISM will position itself, targeting X-rays. This is not the first time that the Japanese space agency has mobilized in this segment. Several past missions were related to high and very high energies: Taiyo (1975), Hakucho (1979), Hinotori (1981), Tenma (1983), Ginga (1987), Yohkoh (1991), Asca (1993), Suzaku (2005 ) and Hitomi (2016).

It turns out that signals related to high and very high energies come from the most intense events in the universe. Here, it is necessary to understand star explosion (a supernova), supermassive black hole or even solar eruption. Events that can only be seen in space, because the Earth’s atmosphere acts as a filter, preventing this radiation from reaching the surface.

supernova star A
It goes boom, boom, BOOM. // Source: ASA, ESA, CSA, STScI, Webb ERO Production Team

During its career, XRISM will target so-called “soft” X-rays, that is to say rays carrying relatively less energy than so-called “hard” X-rays. The instruments on board the observatory are designed to work with an energy ranging from 0.3 to 12 electronvolts (keV). They are considered soft up to 10 keV and hard beyond.

One of the challenges of these observatories is to succeed in capturing signals whose lengths are very short (the more intense they are, the shorter they are). They are shorter than the average distance between the atoms of a block of glass, planetary scientist Pierre Brisson recalled in 2018. It is therefore impossible to use the usual mirrors of a telescope.

Instead of a mirror whose angle would be oriented perpendicular to the source, through which the X-rays would pass, another set of mirrors was developed. This is the principle of the Wolter telescope. The mirrors are in alignment with the X-radiation, in order to capture it along a grazing trajectory, before gradually directing the radiation towards the focus of the instrument.

What questions should XRISM help solve?

Answers could emerge to certain questions that astronomers are asking. The ESA lists a few: how did galaxy clusters form and evolve? What does the structure of spacetime look like under intense gravity? How do massive black holes affect star formation?

Astronomer Yaël Nazé also noted the interest of the scientific community for XRISM in order to know the properties of matter when plunging into a black hole, or when it is violently ejected at the event horizon. Or, what happens during and after a supernova, or to determine the mechanisms linked to stellar eruptions.

On a technological level, XRISM will also be a very practical gateway for ESA, in view of its future Athena mission, which is also dedicated to high-energy astrophysics. This should be launched at the end of the 2030s. XRISM will serve as feedback, particularly concerning the Resolve instrument, an equivalent of which will be deployed on Athena.

When does the tracking of violent events in the universe begin?

XRISM is scheduled to take off on August 26, 2023, at 2:34 a.m. (Paris time). The telescope will be carried by a Japanese H-IIA rocket from the Tanegashima Space Center in Japan. While deployment to low Earth orbit won’t take more than a few days, it will take several weeks to calibrate XRISM and let it take its first shots.

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