AXIS: The Most Powerful X-Ray Space Telescope

Its name is AXIS, and it could become the most powerful X-ray space telescope ever built. NASA has approved $5 million in funding for a preparatory study and will decide in 2026 whether it can be constructed. If approved, the launch into space is scheduled for 2032.

This journey involves Stefano Marchesi, a researcher at the Department of Physics and Astronomy "Augusto Righi" at the University of Bologna.

"AXIS is a Probe-class mission, with development costs that can reach up to $1 billion," says the University of Bologna scholar. "If approved, it will create a space telescope capable of capturing extremely high-quality images of the universe in X-rays."

AXIS, proposed as the successor to NASA's Chandra space telescope launched in 1999, will retain Chandra's exceptional angular resolution, allowing it to identify sources in close proximity. Additionally, it will feature a significantly larger field of view and an improved ability to collect X-rays, advancing the capabilities of X-ray astronomy. This way, it will be possible to discover sources that are currently too weak to be observed by Chandra or any other existing X-ray telescope.

Marchesi has been working on the project for years. In a study published in 2020, he simulated a series of long-duration observations that AXIS could conduct on specific regions of the sky, particularly highlighting how these campaigns could shed new light on the formation of the first supermassive black holes at the centres of the earliest galaxies in the universe.

“Observational data of this kind are virtually non-existent today, and current models propose widely differing hypotheses about the birth of the first galaxies,” explains the researcher from the University of Bologna. “AXIS could revolutionize our understanding by significantly improving these models, enabling us to precisely characterise what can be described as the dawn of the universe—when, just a few hundred million years after the Big Bang, the first galaxies and their supermassive black holes began to take shape.”

Another area of focus for Stefano Marchesi is "blazars"—supermassive black holes that emit jets of matter at nearly the speed of light, producing light across the entire electromagnetic spectrum. These extragalactic sources are responsible for some of the most energetic and powerful phenomena in the universe.

With an X-ray telescope like AXIS, working in synergy with observations across other wavelengths, it may become possible to identify populations of particularly extreme blazars with as-yet unexplored properties.

"I am particularly interested in how X-ray observations can help predict emission in the Very High Energy (VHE) observational band, the most energetic portion of the gamma-ray spectrum," explains Stefano Marchesi. "We are talking about photons with energies ranging from 20 GigaElectronVolts to 300 TeraElectronVolts, with wavelengths around a billionth of a nanometer—infinitely smaller than visible light, which has wavelengths of several hundred nanometers."

In a recent study, the researcher from Alma Mater suggests that these particularly extreme blazars could be detected in the Very High Energy band thanks to the Cherenkov Telescope Array Observatory (CTAO), the network of Cherenkov telescopes currently under construction in the Canary Islands and Chile, which will be operational by 2035.

"By combining the capabilities of the Cherenkov Telescope with those of AXIS, it will be possible to accurately identify the sources observed by CTAO," adds Marchesi. "Alternatively, we could conduct observational campaigns to look for evidence of simultaneous variability in X-rays and in the VHE band, thus gaining much information on the blazar and the black hole that generates it, or use the properties derived from AXIS observations to select interesting sources for dedicated observations with CTAO."