On the afternoon of April 27, the parallel forum of the 2024 Zhongguancun Forum Annual Conference with the theme of "Space Science Exploration and Discovery" - "Space Science Forum" was successfully held, and the forum released the first batch of in-orbit detection images of EP satellites.
The payload of the Einstein Probe (EP) satellite consists of two X-ray telescopes: a lobster-eye soft X-ray monitor (WXT) with a large field of view (3600 square degrees) and a Wolter-I follow-up X-ray telescope (FXT) with a field of view of about 1°×1°. The FXT payload, also known as "Fengxingtian", is a China-led international cooperation space X-ray telescope, undertaken by the Institute of High Energy Physics of the Chinese Academy of Sciences, with the participation of the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences, and the participation of the European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics (MPE) of Germany through international cooperation. As one of the two scientific payloads of the EP satellite, FXT has demonstrated excellent X-ray imaging performance for the first time, combining high energy resolution and high time resolution.
The first batch of FXT on-orbit scientific exploration images is 6 images, and the following two observation images of supernova remnant Crab and giant elliptical galaxy M87 are interpreted in detail.
Crab interpretation
On July 4, 1054 A.D., the "guest star" suddenly landed in the sky near the Tianguan Star in the constellation Taurus, which was unusually bright - this is the supernova of the Crab Nebula defined by astronomers later, and it is also the "remains" of this "Tianguan Guest Star" after the eruption, which has evolved for thousands of years to form the crab nebula that is now observed by humans. The Crab Nebula emits intense radiation in the X-ray band, causing many X-ray astronomical telescopes to produce a severe photon accumulation effect, saturating the brightness of the image and distorting it.
The FXT project team has carried out innovative development of the detector, and specially designed two scientific observation modes for the bright object source: windowed and time-varying scientific observation mode. Figure 1 shows the open window observation image of FXT at 0.3~10keV with an exposure of 2600 seconds. The image clearly shows the whole picture of the Crab Nebula, and the bright spot in the center of the image is the young and energetic Crab pulsar, which is a pulsar cloud generated by the interaction of high-energy particles emitted by the pulsar and the surrounding matter.
Fig.1 Crab nebula
Crab pulsars are powerful sources of electromagnetic radiation that rotate rapidly with a period of 33.8 milliseconds and radiate X-ray photons in a narrow beam that are picked up by FXT, creating the pulse profile we see. FXT uses a time-varying mode to obtain the precise pulse period, phase, and profile of a pulsar.
Fig. 2 Outline of a pulsar in the Crab Nebula
Figure 2 shows the pulse profile of a Crab pulsar observed by FXT in time-varying mode, with the abscissa as the phase and the ordinate as the number of photons.
M87 interpretation
Fig.3 Image of the giant elliptical galaxy M87 observed by FXT
M87 is a bright, giant elliptical galaxy in the Virgo Cluster. Fig. 3 shows the M87 image directly obtained by FXT using the full-frame scientific observation mode, with an energy range of 0.3~10 keV and an exposure time of 39 ks. Unlike optical images, FXT observed a large amount of diffuse hot gas in M87 in the X-ray energy range. The central bright spot of M87 comes from the radiation of the active galactic nucleus (AGN) at the center of the galaxy, and also extends outward from two arm-like structures, which are due to the cold gas jet generated by the AGN activity, which envelops out the cold gas, and the cold gas density is high, which enhances the radiation and finally forms a bright arm-like structure. In addition, in the periphery of M87, a step change in surface brightness is clearly observed, forming a "cold front". In Figure 3, a schematic diagram of the field of view of the main international Wolter-I X-ray telescopes is also placed, with the field of view of the Chandra telescope of the United States on the upper left, the field of view of the Suzaku telescope of Japan on the lower left, and the field of view of the XMM-Newton telescope in Europe on the upper right. Compared to these telescopes, the FXT's field of view is a complete whole, with no gaps in the field of view because there are no multiple CCDs stitched together, and the FXT's field of view is much larger, with an unobstructed view of the "cold front" structure around the M87 and even X-ray radiation from the gas of the Virgo Cluster further outer.
Preliminary test results show that the functional performance indicators of the FXT telescope meet or exceed the design requirements. After seven years, the results are inseparable from the dedication and efforts of every member of the FXT team. Figure 4 shows a group photo of the leaders of the Institute of High Energy Physics and some members of the FXT team at the Xichang Satellite Launch Center on the eve of the EP satellite launch.
Figure 4 Group photo of the EP-FXT team at the Xichang Satellite Launch Center
In the next phase, FXT will continue to carry out and complete the detailed calibration in orbit according to the established plan. After the official delivery, FXT will become another powerful tool for space X-ray astronomical observation.