NASA and JAXA Detect Interstellar X-Rays With 36-Pixel Sensor

The X-Ray Imaging and Spectroscopy Mission (XRISM) satellite, a collaborative effort between the Japan Aerospace Exploration Agency (JAXA), NASA, and other agencies, has delivered its first images. XRISM was sent up to explore high-energy processes and plasma conditions at various points of interest throughout the universe.

Its primary instrument, Resolve, captures images in the “soft” X-ray spectrum in the range of 0.3 to 13 keV (4.1 nm to 0.095 nm), where photons have up to 5,000 times the energy of visible light photons. These X-rays are generated in some of the hottest regions, largest structures, and highest-gravity objects in the universe, such as supermassive black holes, supernovas, and galactic clusters.

 

Supernova remnant N132D in the Large Magellanic Cloud. Image used courtesy of NASA

 

The observatory was lifted to orbit as a ride-share payload on September 7, 2023, on an H-IIA rocket from the Tanegashima Space Center in Japan for Japan’s SLIM mission. It resides in a geocentric circular orbit at 341 miles (550 km) altitude. It has two instruments: the XRISM, including the Resolve detector with a field of view of 3 arcminutes, and Xtend, a four CCD X-ray imager with a 38-arcminute field.

 

The Resolve Micro-Calorimeter Spectrometer

The Resolve detector is a micro-calorimeter spectrometer. As a spectrometer, it can detect the wavelength and, thus, the composition of the image. Chemical elements emit X-ray photons at a specific energy level just as they emit visible light at specific frequencies. Note that visible light and X-rays can both be categorized by wavelength in the electromagnetic spectrum. Super-small-wavelength, high-energy particles like X-rays are generally measured in electron volts (eV) or keV rather than nanometers.

 

Silicon, sulfur, argon, calcium, and iron detected by Resolve. Image used courtesy of NASA

 

The “micro-calorimeter” part of the moniker refers to the sensor’s ability to detect the energy of each X-ray photon in addition to its spectrum. When an X-ray photon hits the detector, multiple electrons are released as opposed to a single electron released from a visible photon in an optical detector. This causes the temperature to rise proportional to the particle's energy level.

 

How XRISM Focuses and Detects X-Rays

The XRISM instrument uses 203 concentric rings of highly polished, gold-coated metal. The rings have an incident angle to the incoming X-rays of about 1 degree. All of the inner surfaces of the rings are made reflective so the X-rays can be double-reflected for focusing. 

 

Polished and tuned concentric rings for focusing X-rays. Image used courtesy of JAXA

 

Once focused, the X-ray photons hit the 6 x 6-pixel array of the Resolve detector. Using the Doppler effect to observe changes in wavelength and energy level, the detector can determine relative motion. In addition to detecting composition, data can tell the speed and direction of motion, providing a 3D model of the object. For example, when imaging the expanding shell of a supernova, the instrument can tell the difference between the near half and far half of the supernova.

 

Construction and operation of the Resolve 36-pixel sensor. Image used courtesy of JAXA

 

Since the signals are so faint, the micro-calorimeter needs to be very cold to detect them. The detector is cooled with liquid helium to near absolute zero (-459.58°F, -273.1°C). The included helium supply is planned for a three-year lifespan. Past that point, it may be possible to extend the mission by employing onboard mechanical coolers.

XRISM is not the first use of the Resolve detector. A similar instrument was launched in February 2016 but failed only a month later due to a software issue with the stability control system. That mission returned a single observation from the Perseus Galaxy Cluster, which validated the instrument’s abilities.