On April 27, at the Space Science Forum of the 2024 Zhongguancun Forum Annual Meeting, the Einstein Probe (EP) satellite mission led by the Chinese Academy of Sciences released the first batch of scientific detection images in orbit.
The Einstein Probe satellite was launched into orbit on January 9, 2024, as one of the series of space science satellite missions implemented under the second phase of the Chinese Academy of Sciences Space Science Pioneer Project. The satellite, led by China, involved the European Space Agency, the Max Planck Institute for Extraterrestrial Physics in Germany, and the French Space Agency in international cooperation to discover and explore X-ray transient sources and eruptive celestial bodies in the universe. It aims to issue alerts to guide other astronomical devices for subsequent follow-up observations.
Yuan Weimin, the chief scientist of the Einstein Probe and a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, introduced that since the satellite was launched into orbit, the Wide-field X-ray Telescope (WXT) and the Follow-up X-ray Telescope (FXT) have obtained multiple sets of X-ray scientific observation data of cosmic bodies during the in-orbit testing and instrument calibration period.
"We have detected 2100 known X-ray sources, 17 new transient sources, 168 stellar flares, and issued more than 10 global telegrams, guiding international optical and radio telescopes including the largest Keck Telescope and space X-ray observatories to conduct follow-up observations," Yuan Weimin said.
Capturing the cosmic "fireworks," the first batch of 11 images released
The first batch of images includes a total of 11 images, such as observation images of the center of the Milky Way, transient source images, Crab Nebula observation images, and observation images of the elliptical galaxy Messier 87 (M87).
Yuan Weimin explained that the observation image of the center of the Milky Way shows the observation image of the center of the Milky Way directed by the Wide-field X-ray Telescope, with an exposure time of about 40,000 seconds. Almost all bright X-ray celestial bodies can be distinguished in the image, and the purple cross-shaped spots are imaged by the Lobster Eye X-ray Telescope, while the blue cloud-like structures come from the foreground radiation of the Milky Way's hot gas.
Observation image of the Wide-field X-ray Telescope (WXT) pointing towards the center of the Milky Way. X-ray data copyright EP Science Center.
The transient source image shows a gamma-ray burst candidate (EP240219a), one of the earliest transient sources discovered by the Wide-field X-ray Telescope. It is also the first transient source published on the astronomer telegram by the Einstein Probe satellite team. This gamma-ray burst appeared suddenly in the field of view of the Wide-field X-ray Telescope on February 19, 2024, and disappeared approximately 100 seconds later.
The first reported transient source by the Wide-field X-ray Telescope (WXT). X-ray data copyright EP Science Center.
Image of the Crab Nebula, showing the observation made by the X-ray telescope on February 28, 2024. The Crab Nebula is a famous supernova remnant, with its precursor star exploding in 1054, observed and meticulously documented by Chinese astronomers during the Song Dynasty.
Observation image of the elliptical galaxy Messier 87 taken by the Follow-up X-ray Telescope (FXT), showing the bright giant elliptical galaxy in the Virgo galaxy cluster observed in the X-ray energy range. Messier 87 contains a large amount of diffuse hot gas, with a massive black hole at its center producing jets that bring out cold gas from the central region, forming a jet-like structure. This structure is clearly visible in the image captured by the Follow-up X-ray Telescope. Additionally, the image also clearly shows the brightness step change of a cold front at the outer periphery of Messier 87.
The Follow-up X-ray Telescope (FXT) observed images of the elliptical galaxy M87. X-ray data courtesy of the EP Science Center.
"The Einstein Probe satellite is designed to capture these fleeting cosmic 'fireworks,' issue alerts, and guide other astronomical instruments for follow-up observations," said Yuan Weimin.
Real-time alerts are issued to guide global tracking observations.
"Our initial images released are detection images from orbit, not scientific results, but they demonstrate that the Einstein Probe satellite can search for transient sources with high sensitivity," said Zhang Chen, Assistant Chief Scientist of the Einstein Probe.
Research on transient sources has always been a hot topic in astronomy, as it is associated with various extreme celestial activities. For instance, around dying galaxies, black holes, or dense stellar bodies where the gravitational field is extremely strong, intense activities occur, accelerating surrounding matter to very hot states and emitting a large amount of X-rays, containing many physical processes. Studying these extreme phenomena will greatly advance physics.
"One of the core scientific goals of the Einstein Probe is to systematically discover cosmic X-ray transient sources and variable celestial bodies with the highest detection sensitivity, monitoring celestial activities," Yuan Weimin said.
Yuan Weimin introduced that the new transient sources detected by the Einstein Probe have different origins, including tidal disruption events of stars, gamma-ray bursts, new magnetic cataclysmic variables, and new X-ray binaries. The observational results have received high recognition and attention from international peers, providing important guidance for coordinated observations by ground and space telescopes worldwide.
Zhang Chen told Science China that the Einstein Probe satellite will release observational messages in real-time on astronomical information sharing platforms such as astronomer telegrams. Currently, satellite observation platforms like the Swift satellite, the Chandra X-ray Observatory, the Newton satellite, as well as ground-based observation platforms like the Keck Telescope, have conducted tracking observations following the messages released by the Einstein Probe satellite. Additionally, the Einstein Probe satellite is also conducting joint observations with domestic satellites such as the "Wise Eye" satellite and the "Huairou-1" satellite.
"We need to utilize all available observational resources to conduct observations and research, striving to understand these transient source phenomena more clearly," Zhang Chen said.
Yuan Weimin introduced that on April 25, after 107 days since the launch of the Einstein Probe satellite, the first paper written by the satellite's scientific team has been submitted, focusing on the high-redshift gamma-ray burst EP240315a.
Illustration of the Einstein Probe satellite. Image provided by the National Space Science Center of the Chinese Academy of Sciences.
About to complete in-orbit testing
Yuan Weimin introduced that the current test results indicate that the satellite and payload functional performance indicators have met or exceeded the design requirements. In the next phase, the satellite will continue to carry out and complete in-orbit testing according to the established plan.
Zhang Shuangnan, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, commented that the Einstein Probe satellite is "a pride of China." It has achieved "broad, fast, accurate, and deep" capabilities, enabling extensive sky surveys. It can detect transient sources with high sensitivity, providing precise information on each eruptive phenomenon. Its observational depth surpasses that of similar international telescopes by several orders of magnitude, promising to unveil the various mysteries of cosmic changes profoundly.
Kipar Nandra, Director of the Max Planck Institute for Extraterrestrial Physics in Germany, believes that the Einstein Probe satellite is highly successful with strong detection capabilities. So far, it has discovered many different transient sources, which are areas awaiting further exploration in the future. "The whole world is paying attention to this mission and the great scientific discoveries it can make."
"We are currently conducting sky surveys according to the established observation plan, looking forward to seeing more transient sources. At the same time, we have started accepting scientists' observation applications and conducting targeted observations. In the next two months, we will also perform calibration observations. After the calibration observations are completed, we will deliver the satellite and carry out scientific observations according to the established plan," said Zhang Chen.
Recently, the Einstein satellite has been fully dedicated to searching for gravitational wave events. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States discovered a suspected merger event between a black hole and a neutron star. Currently, all major telescopes worldwide are searching for this event, hoping to observe new physical phenomena.
"We will strengthen international cooperation and data sharing efforts, detecting the fleeting 'fireworks' in the universe, making significant contributions to high-energy transient astronomy observations and research," Yuan Weimin said.
