Einstein Probe: China launches powerful space X-ray observatory - South China Morning Post

First planned a decade ago, the Einstein Probe is led by researchers of the Chinese Academy of Sciences and has major instrument contributions from the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics in Germany.

The mission’s chief scientist, Yuan Weimin from the National Astronomical Observatories in Beijing, said the probe’s detection ability was more than an order of magnitude higher than that of similar telescopes in the world.

“It can detect more distant and weaker signals, see them more clearly, and locate them more accurately,” Yuan told China Science Daily.

Erik Kuulkers, the ESA’s project scientist for the mission, said he was very excited for the launch, which he had been working on since 2018 with Chinese and German colleagues.

“Thanks to its uniquely wide field of view, the Einstein Probe will see more bursts of X-rays than other satellites in the past. It will transform our way of how we view the transient universe,” Kuulkers said on Monday.

X-rays are produced in the universe when matter is heated to millions of degrees. They are often emitted under extreme physical conditions, such as very high magnetic fields or gravity, or during cosmic explosions.

However, observing X-rays is challenging. Many X-ray sources are not permanent – they seem to appear in the sky for a short time and then disappear, making them hard to detect.

X-rays are readily absorbed by the Earth’s atmosphere, so scientists must build telescopes that are launched into orbit. These telescopes require specialised mirrors to reflect and collect X-rays since they are highly penetrating.

The Einstein Probe will target the X-ray universe. Illustration: EP team/National Astronomical Observatory of China

Since the 1960s, more than 50 telescopes have been put into space to study cosmic X-rays. Most were used to accurately measure specific sources, but could not see much else because of their limited view. Some could quickly scan the sky but see only the brightest sources.

In 1980, Roger Angel from the University of Arizona proposed a novel design for X-ray telescopes to have both a wide field-of-view and decent resolution. Angel was inspired by crustaceans such as lobster and shrimp that developed eyes with unique structures to live in murky environments under water.

A lobster eye consists of many tiny square-shaped tubes, all pointing to the same spherical centre. The structure allows light from all directions to reflect inside the tubes and converge on the retina to give the lobster an unlimited field of view.

China tests ‘lobster eye’ telescope to capture images of the universe

Angel’s idea remained a formidable engineering challenge for a long time until micro-processing technologies matured and a technique known as micropore optics became possible in recent years.

The Wide-field X-ray Telescope on board the Einstein Probe comprises 12 modules, each containing more than 30 million square micropores. Each pore measures 40 micrometres along the side and is coated with an ultrathin layer of iridium to increase reflectivity.

The surface of the pores must be extremely flat and smooth, affording less than 1 nanometre of error, according to Yuan. It took his team a decade to develop the lobster-eye-like modules.

Thanks to its design, the Einstein Probe can observe an area of sky more than 10,000 full moons’ size at any given time. In contrast, existing large X-ray telescopes, such as Nasa’s Chandra X-ray Observatory, can only image a section of sky smaller than one full moon at a time.

It means the Einstein Probe could observe almost the entire night sky in just three orbits around Earth, or under five hours.

In December 2020, a technology demonstration mission named the Lobster Eye Imager for Astronomy was launched to test the Einstein Probe’s key technologies.

Using a 53kg (117lb) instrument similar to one of the 12 modules on the Wide-field X-ray Telescope, the mission proved that the technologies worked and that observation precision exceeded expectations.

“My congratulations to the team which has done such great work to develop the lobster-eye technology,” Angel said before the launch.

He said that given the “beautiful” test imaging results already of some of the most famous X-ray sources in the sky, “we can expect the Einstein Probe to be very successful”.

Congratulations to the team which has done such great work to develop the lobster-eye technology

Roger Angel, astronomer

During five years of operation, the probe will systematically survey to detect X-rays from all kinds of cosmic objects, from black holes to neutron stars, supernovae and even the emissions of comets in our solar system.

It will be especially useful in revealing supermassive black holes that have been dormant and impossible to detect. However, if a nearby star comes too close, the black hole will wake up, start pulling the star apart and sucking in materials, making it shine in X-ray light.

The Einstein Probe is also planned to join ground-based telescopes in the search for gravitational waves – ripples of space-time created by massive objects in the distant universe, such as two neutron stars colliding. It will look for X-ray emissions from such events and help locate the massive objects.

As the probe’s international partners, the ESA and the Max Planck Institute for Extraterrestrial Physics developed the Follow-up X-ray Telescope, which comprises a pair of conventional X-ray focusing telescopes.

Also, the ESA’s ground stations will help download data from the observatory throughout the mission. In return, its scientists will get access to 10 per cent of the observation data, according to the agency’s website.

Kuulkers said there were differences in the culture and way of working between the Chinese and European scientists, but they learned from each other throughout the project.

“We succeeded to get agreement to maximise the mission’s science output, and to get the right return to the different parties in terms of data and scientific publication,” he said.

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Later this year, China will launch the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor), a two-decade collaborative project between the national space agencies of China and France to study gamma-ray bursts produced by distant explosions of stars.

The SVOM is also scheduled to be launched by a Long March-2C rocket from the Xichang Satellite Launch Centre.

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