If they will ever be built, X-ray interferometers would have a ~10,000 times better spatial resolution than even the most advanced current X-ray telescope. They would be able to image black holes with a telescope system that fits on a single spacecraft. And compared to the current radio images of black holes, this would be in a wavelength range that carries extra information about the violent processes around these objects. At SRON we are working on the first steps towards an X-ray interferometry space mission.
Combining telescopes
By combining multiple telescopes you can obtain a spatial resolution as if the area between them were covered by one enormous dish. Such a setup is called an interferometer. Some sources in the sky are so small or far away that interferometry is the only option to resolve any spatial information. For example, the famous image of the supermassive black hole (SMBH) in galaxy M87 was taken by a combination of radio telescopes covering the entire globe. With X-rays, we would see more interesting features, as X-rays are directly emitted by hot gas surrounding an SMBH.
Because X-rays are blocked by earth’s atmosphere, you would have to build an X-ray interferometer in space. Moreover, because X-rays have a wavelength that is 10 billion times smaller than the radio waves with which the M87 SMBH was observed, the size of the interferometer can shrink by the same factor. Such an interferometer would therefore fit on a single spacecraft, without losing the spatial resolution.
Figure 1. The observational revolution that XRI will bring. a. Actual image of M87 by NASA’s Chandra X-ray observatory. b. Simulation of an observation with 0.1 mas XRI resolution of the M87 SMBH. c. Impression of a reverberation process around a distant SMBH. d. The famous X-ray binary Cygnus X-1. e. The surface of a red dwarf star.
Current X-ray telescopes
Figure 1a shows an image of M87 made with NASA’s Chandra X-ray observatory. A single Chandra pixel contains some 5000 x 5000 X-ray interferometer pixels of 0.1 micro arcseconds. That resolution allows to resolve the shadow of the SMBH in the centre of M87 (figure 1b) and violent processes within a few lightyears from SMBHs in more distant galaxies (figure 1c). Also, stellar sized compact objects in our own Milky Way can be studied, for example the Cygnus X-1 binary system, which contains a small black hole and a large, heavy star. The simulation in figure 1d shows details such as the accretion disk around the black hole (right) and its reflection and shadow on the surface of the giant star (left).
Basic principle
The basic physical principle behind X-ray interferometry is that of a Michelson interferometer, which creates two identical paths for an incoming photon (left to right, green and red beams, Figure 2a). A photon behaves as a wave along its path through the instrument, and manifests itself as a particle when it hits the detector on the left. Multiple photon detections build up a pattern of bright fringes at the detector.
Figure 2. The principle of X-ray interferometry (left) and the scheme of the compact ‘telephoto’ design by Willingale (right). The sketches are not to scale.
The first X-ray fringes were produced with an interferometer akin to the schematic in figure 2a. That design however does not lead to a compact and practical space instrument. The telephoto Willingale design sketched in figure 2b implies a length reduction by a factor ~500.
SRON develops an X-ray interferometry testbed
SRON is currently developing an X-ray interferometry testbed in collaboration with the Precision and Microsystems Engineering group at the Technical University of Delft, aiming to demonstrate the compact Willingale design with mirror technology from the Dutch company Cosine, as a first step in the direction of an X-ray interferometry space mission. Together with the University of Amsterdam, SRON is developing an end-to-end simulator, to study all the aspects of making a scientific observation with a space X-ray interferometer.
X-ray interferometry received a recommendation for the ESA technology development in the context of the Voyage 2050 program. SRON will start a study of how to point a X-ray interferometer in space—together with the Italian Istituto Nazionale di Fisica Nucleare—for the AOCS & Pointing Division of ESA.
