New wide-field X-ray mission will revolutionise search for transient high-energy events

The X-ray satellite “Einstein Probe” of the Chinese Academy of Sciences (CAS) was launched successfully from the Xichang Satellite Launch Center in China on a Long March-2C rocket on January 9th, 2024. Equipped with cutting-edge X-ray mirrors and detectors, with major contributions from the Max Planck Institute for Extraterrestrial Physics (MPE), the spacecraft will start a new era in high-energy time-domain astrophysics, with a particular focus on highly variable and short-lived phenomena associated with black holes and neutron stars in our Milky Way and distant galaxies.

The primary scientific goal of the Einstein Probe is to explore the transient and variable X-ray sky, capturing powerful bursts of high-energy light emanating from objects such as merging neutron stars and black holes. Unlike conventional X-ray telescopes, Einstein Probe’s unique design allows it to monitor almost a tenth of the sky simultaneously, discovering new sources as they light up in X-rays and enabling in-depth studies of known and new celestial phenomena over extended periods.

“So far, the exploration of the transient sky in X-rays has been limited to the few brightest objects,” says Arne Rau, astrophysicist at MPE and member of the Einstein Probe Science Management Committee. “With Einstein Probe’s design and sensitivity, I am eagerly looking forward to studying a much larger number of events, including the signatures of stars that are unfortunate enough to be disrupted by the supermassive black holes at the centres of galaxies.”

Unlike the stars we can see with our naked eyes, most cosmic objects that shine in X-rays are highly variable. They continuously brighten and dim, sometimes shining brightly for just a brief period before disappearing completely. 

“As the Einstein Probe can monitor the whole sky with high cadence, it is complementary to other X-ray missions such as eROSITA,” says Peter Friedrich, who led the optics contribution from MPE. “It provides a crucial role by filling a wavelength gap in time-domain observations.” 

X-rays are associated with high-energy events such as collisions between neutron stars, supernova explosions, stars being disrupted and consumed by massive black holes, or energetic particles spewing out from hot gas circling these exotic and mysterious objects. Einstein Probe will improve our understanding of these cosmic events by discovering new sources and monitoring the variability of objects shining in X-rays all over the sky.

Even more excitingly, it will also help to advance our understanding of the sources which produce gravitational waves. When two ultra-dense, massive objects merge, such as two neutron stars or black holes, they create a gravitational wave signal, which has been detected on Earth several times. However, it is often difficult to pinpoint the location of the source – which could most easily be spotted in X-rays if a burst of light accompanies this cosmic crash. Einstein Probe’s capability to routinely spot new X-ray sources, to react quickly, and to point in the direction of detections made by the ground-based gravitational wave experiments will enable scientists to study these short-lived events and identify their origin promptly.

Central to the success of Einstein Probe are its state-of-the-art instruments—the Wide-field X-ray Telescope (WXT) and the Follow-up X-ray Telescope (FXT), the latter closely following the design of the MPE’s eROSITA X-ray telescope. MPE contributed the eROSITA  flight spare mirror assembly of the FXT, and worked with ESA and industrial partners to produce the other.

The WXT uses an innovative “lobster-eye” optical design, allowing Einstein Probe to survey  3600 square degrees, covering nearly one-tenth of the celestial sphere in one shot. This capability is crucial for monitoring the entire night sky every 4.5 hours, providing a comprehensive view of the X-ray Universe. When the WXT finds a new X-ray source, the satellite can rotate within seconds and point the much more sensitive FXT on it for a more detailed study. MPE performed calibration of both WXT and FXT telescopes in its PANTER facility.

MPE also contributed the FXT’s state-of-the-art detectors, for which the MPG Halbleiterlabor supplied the sensors. “We developed the CCD detector modules based on our experience with the successful eROSITA mission,” says Norbert Meidinger, who was responsible for the detectors at MPE. “Their outstanding time resolution of 50ms in the standard observation mode and even 2ms in window mode combined with a state-of-the-art energy resolution close to the theoretical limit are crucial to achieve the science goals of the Einstein Probe mission.”

Einstein Probe is an X-ray mission of the Chinese Academy of Sciences (CAS) in collaboration with the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics (MPE). In return for contributing to the development of this mission, MPE will have direct access to 10% of the data generated by Einstein Probe’s observations.

“It was a great team effort to adapt our eROSITA hardware to the specifications of the new mission – under very difficult conditions during the pandemic,” adds Peter Friedrich. “We are very proud to be a part of this exciting project and look forward to many discoveries and science opportunities in the variable X-ray sky.”