On April 27, 2024, the Space Science Forum of the Zhongguancun Forum Annual Meeting was held in Beijing. The Einstein Probe (EP) satellite mission, led by the Chinese Academy of Sciences, released the first batch of on-orbit detection images. The satellite carries the world's first wide-field X-ray imaging telescope (WXT) and a follow-up X-ray telescope (FXT). The WXT was jointly developed by the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences (referred to as Shanghai Tech Physics) and the National Astronomical Observatory of the Chinese Academy of Sciences (referred to as National Astronomical Observatory), with the key component of its micro-pore optics (MPO) provided by North Night Vision.
An observation image pointing towards the center of the Milky Way Galaxy by the WXT. Image source: X-ray data copyright EP Science Center.
"Spanning 7 years in the engineering phase alone, the team has gradually turned concepts into reality, and now this telescope has finally 'opened its eyes,'" said Sun Shengli, a member of the Chinese Academy of Sciences, researcher at the Shanghai Institute of Technology, and head of the WXT payload. "This process is very exciting and uplifting."
Why the need for the Einstein Probe?
The EP satellite is one of the series of space science satellite missions initiated and implemented by the Chinese Academy of Sciences as part of the second phase of space science. Led by China, the European Space Agency (ESA), the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany, and the French Space Agency (CNES) are participating in the satellite development in the form of international cooperation. The mission aims to discover and explore X-ray transient and eruptive celestial bodies in the universe, issue alerts to guide other astronomical devices for subsequent follow-up observations. WXT is currently the X-ray telescope with the highest detection sensitivity and spatial resolution internationally, conducting large-field, high-sensitivity, rapid-time-domain sky surveys in the soft X-ray monitoring window.
Illustration of the Einstein Probe satellite. Image source: Shanghai Institute of Technology
While there are already numerous X-ray satellites in space, such as the Chandra X-ray Observatory, NuSTAR, and INTEGRAL, why do we still need a space telescope like this?
Analogy by Sun Shengli using a microscope: A regular electron microscope can see the structure of molecules but cannot see how the structure changes. Similarly, previous space telescopes could study the state of celestial bodies but couldn't observe their dynamic processes clearly. To understand the evolution of the universe, we need deeper observational capabilities and a broader range of observations, which is the main focus of high-energy transient astronomy.
The main research objects of transient astronomy are transient sources and violent eruptive celestial bodies. Transient sources refer to celestial bodies that appear and disappear quickly, while violent eruptive celestial bodies are those whose brightness suddenly increases by orders of magnitude in a short period.
Zhang Chen, a researcher at the National Astronomical Observatory and assistant chief scientist of the EP satellite, explained: "When events like black hole tidal disruption of stars or gamma-ray bursts occur, they emit X-rays. By studying X-rays, we can explore the physical events that occur in very high-temperature, high-energy processes."
However, in the vast universe, the occurrence of such sudden events is random, making it difficult for existing space telescopes to observe them.
The EP satellite's main feature is its high sensitivity and large field of view. The Wide-field X-ray Telescope (WXT) searches for and locates transient sources, while the Focusing X-ray Telescope (FXT) precisely measures these sources. The EP satellite has a field of view of 3850 square degrees, covering about one-eleventh of the sky, filling the gap in the international community for large-field, all-sky monitoring devices in the soft X-ray band.
With the highest detection sensitivity internationally and the broadest monitoring range in cosmic space, it will discover transient celestial bodies, monitor their activities, discover and explore dormant black holes of various scales, detect electromagnetic counterparts of gravitational wave sources, and accurately locate them.
A "star" honed over a decade
The project traces back to 2013. On November 15 of that year, the scientific demonstration meeting for the "Einstein Probe Satellite" project under the Chinese Academy of Sciences' Space Science Pilot Program was held. It was successfully launched on January 9, 2024, marking a journey that was not exactly a decade in the making.
Looking back at the development process, this team, mainly composed of individuals born in the 1980s, overcame numerous challenges and breakthroughs in key technologies.
The first challenge was focusing X-rays. Due to the high energy of X-ray photons, their high penetration and easy interaction with atoms make it difficult to focus them through refraction or reflection. Traditional methods like linear optics and focusing imaging were inadequate for the precise spatial positioning and large field of view required by transient astronomy.
In 1979, inspired by the principle of lobster eyes for full-sky imaging, Roger Angel of the University of Arizona proposed a biomimetic X-ray imaging optical configuration. This optical system meets astronomers' demands for a large field of view and represents the next generation of X-ray transient astronomy equipment. However, due to its high technical difficulty, this concept remained unrealized for decades after its proposal.
In 2010, the National Astronomical Observatory began exploring micro-pore lobster eye X-ray imaging technology. "Initially, we planned to purchase similar equipment from abroad, but the prices were high, and there were technological and economic barriers for China," noted Zhang Chen.
After years of technological innovation, the National Astronomical Observatory, in collaboration with Northern Night Vision, successfully developed lobster-eye X-ray optical components based on Micro Pore Optics (MPO) technology. Leveraging the rich experience in satellite optical payload development at the Shanghai Institute of Technology, they finally integrated and developed the complete WXT. The key components of this device were independently developed in China.
Sun Xiaojin, Deputy Researcher at the Shanghai Institute of Technology and Chief Designer of the WXT payload, explained that the typical lobster-eye MPO lens is a curved surface measuring 42.5mm x 42.5mm, with a curvature radius of 750mm. Each lens contains nearly one million small squares, with each square measuring 40μm x 40μm and a wall thickness of 8μm. The smoothness reaches 0.83nm, imposing stringent requirements on the key components' acquisition and system integration and adjustment.
Another major challenge was finding photon detectors for the WXT focal plane. Since the 1990s, X-ray telescopes have generally used Charge-Coupled Devices (CCDs) as focal plane detectors. However, China currently lacks the ability to produce scientific-grade CCDs that meet astronomical requirements, and imported CCDs are prohibitively expensive. Additionally, conventional gas detectors may fail due to impacts from micrometeoroids and cosmic dust in space.
The team innovatively applied back-illuminated Complementary Metal-Oxide-Semiconductor (CMOS) detectors to detect X-ray photons. While CMOS has been widely used in digital cameras and smartphone cameras, this was the first time globally that large-array CMOS detectors were applied in astronomical detection. "The CMOS detectors we use feature high performance, sensitivity, and consistency, with each detector area measuring 6cm x 6cm," noted Ling Zhixing. "This is a significant innovation, as no other project has used such a large-scale silicon-based imaging detector to date."
WXT dedicated back-illuminated CMOS film detector. Image source: Shanghai Institute of Technology.
This ultra-high-precision "wide-angle camera" generates a staggering amount of data. Meeting the demands for speed and performance of the telescope within limited processing chip resources and low-power environments has been a major challenge for the team. The peak data rate of the telescope is 25.3G, equivalent to generating 25 1G movies per second.
To ensure data reliability, it is generally necessary to store all raw data and then transmit it to the ground for further analysis. However, the raw data volume of EP is too large to be transmitted in real-time through the link between the satellite and the ground. Therefore, on-orbit data processing must first be conducted on the satellite, followed by transmitting the effective information to the ground science operations center.
"EP is like searching for astronomical events in the vast universe, finding the most effective information in massive data, it's like finding a needle in a haystack process," explained Xue Yulong, senior engineer and WXT payload software manager at the Shanghai Institute of Technology.
The team worked tirelessly for three consecutive months, often until 3 a.m. each day, to ultimately achieve a balance between high performance, low resources, and low power consumption. The processed data rate was eventually reduced to 5.127Mb, over forty thousand times less than the original data volume.
"Electronic hardware design is a meticulous and patient process. Any negligence or mistake during the research and development process can lead to significant losses," said Yan Ailiang, senior engineer and WXT payload detector manager at the Shanghai Institute of Technology. Through formal guarantees in the development process and collaborative team efforts, the product was successfully developed, produced, launched, and observed in orbit without any errors.
A model of open collaboration
Ling Zhixing emphasized that the EP satellite project's transition from theory to reality is a result of sincere cooperation. "This is multidimensional cooperation, including long-term collaboration between domestic and foreign institutions, leveraging the strengths of various units within the Chinese Academy of Sciences, joint efforts of research institutes and enterprises, in-depth communication between scientists and engineers, and collaboration among members of different generations."
On July 27, 2022, an experimental module of WXT (EP-WXT Pathfinder, later named Leia) was launched aboard the Space New Technology Test Satellite (SATech-01). The instrument's observation field reaches 340 square degrees, making it the world's first wide-field X-ray focusing imaging telescope. Initial in-orbit test results showed that within a single observation (approximately 13 minutes), multiple X-ray sources from different directions could be simultaneously detected, along with recording the temporal variations in X-ray radiation intensity and the X-ray spectra of celestial bodies. Following the completion of in-orbit testing, Leia has already achieved a series of scientific accomplishments.
At the time, X-ray observation experts from the University of Leicester in the UK, Professors Paul O'Brien and Richard Willingale, stated, "For decades, we have been anticipating a truly wide-field soft X-ray telescope, and Leia's successful operation is exhilarating. This technology will bring transformative advancements to monitoring the X-ray sky, demonstrating the enormous scientific potential of the EP satellite."
With the release of the first batch of in-orbit observation images from the EP satellite, temporal astronomy is poised for a significant new development. In the next phase, EP will continue its planned in-orbit testing, strengthen international cooperation and data sharing efforts, detect transient "fireworks" in the universe, and make significant contributions to high-energy temporal astronomy observation and research.
