The Wide-Field X-ray Telescope (WXT) is able to scan half the sky within 5 hours and systematically search for new X-ray sources that indicate high-energy transient phenomena in the vicinity of black holes and neutron stars.  The Follow-up Telescope (FXT) carries out follow-up observations of the most exciting events with deeper images. The Max Planck Institute for Extraterrestrial Physics (MPE) contributed significantly to the satellite project led by the Chinese Academy of Sciences (CAS), in particular to the FXT instrument, which is based on the design of the eROSITA telescope and opens up a further field of application for the X-ray imaging technology developed at MPE.

The first images of the Einstein Probe satellite were presented to the public after the 7th Joint Workshop of the Einstein Probe Consortium, which took place in Beijing from 24 to 26 April 2024. Since the launch, the satellite and its instruments have been put into operation, and functionalities and characteristics have been measured and calibrated.

"The WXT has a unique combination of field of view and high sensitivity, and is designed to make new discoveries in the variable X-ray sky. With these first data, we now know that this promise can be realised," says Arne Rau, astrophysicist at MPE and member of the Einstein Probe Science Management Committee. Calibration has now been completed for 10 of the 12 WXT modules. With a positional accuracy of ~2 arcminutes, an angular resolution of 4-5 arcminutes and a light collection area of about 3 cm2 (at an energy of 1 keV), the instrument not only fulfils the scientific requirements, but even exceeds them in some areas. The performance of the WXT optics in orbit was well predicted by the previous calibration on the ground in the MPE's PANTER facility.
Tests of the FXT in Earth orbit have similarly confirmed the performance of the two units, which were modelled on the design of MPE's eROSITA X-ray telescope. For the Einstein Probe FXT, MPE supplied the replacement mirror module from eROSITA and worked with ESA and industry partners to provide the second mirror module. "It is very pleasing to see that both mirror modules of the FXT in orbit show the performance in terms of resolution and collecting area that we measured in the extensive X-ray optical tests on the ground," says Peter Friedrich, who led MPE's optical contribution to Einstein Probe. "Personally, I am also pleased that the eROSITA replacement mirror module has now also found an appropriate scientific use.

The MPE also contributed the state-of-the-art pnCCD detector modules for both FXT units. As with eROSITA, these are based on a sensor technology developed at the Max Planck Society's Semiconductor Laboratory. "The FXT CCD detector modules developed at MPE for spatially and time-resolved spectroscopy of X-ray photons have fulfilled the high expectations we had of them before the satellite was launched. We are delighted to have made a decisive contribution to the success of the Einstein Probe mission," adds Norbert Meidinger, who was responsible for the detectors at MPE. The mirror-detector combination offers a source positioning accuracy of <10 arcseconds and a light collection area of ~300 cm2 at 1.25 keV, together with excellent performance in terms of energy and time resolution. Like the WXT, the FXT has met and in some areas exceeded its scientific requirements.

Even at this early stage of testing, the Einstein Probe satellite has already made new discoveries. "WXT started finding new variable sources almost as soon as it was switched on. We have since discovered more than a dozen new X-ray transients and over a hundred starbursts in our galaxy," adds Rau. These discoveries were quickly reported in more than 20 telegrams to the global astronomical community, leading to worldwide follow-up observations with ground- and space-based telescopes, including MPE's own GROND instrument at the 2.2 metre MPG telescope in La Silla. These initial results confirm the potential of Einstein Probe to discover new sources and possibly new astrophysical phenomena, in particular high-energy events associated with black holes, neutron stars and their mergers.

The project is the first major collaboration between MPE and CAS in space, combining unique technologies from China and Europe for a common scientific goal. "Einstein Probe utilises the instrument developments we have made for eROSITA to open up new scientific opportunities," says Paul Nandra, Director of the MPE High Energy Group, adding: "The collaboration has worked perfectly - we are already looking forward to the next one!"

In the coming months, the satellite will continue its calibration activities in orbit before starting regular operations around mid-June 2024.

Einstein Probe is a scientific mission in space led by the Chinese Academy of Sciences (CAS) in collaboration with MPE, the European Space Agency (ESA) and the French space agency Centre National D'Etudes Spatiales (CNES).

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Contact:
Hannelore Hämmerle
Max-Planck-Institut für extraterrestrische Physik
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85748 Garching
E-Mail: pr@mpe.mpg.de
Internet: www.mpe.mpg.de