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At 16:00 Beijing time on January 18, 2024, the Event Horizon Telescope (EHT) Collaboration released a new image of the supermassive black hole (M87*) at the center of the elliptical galaxy Messier 87. This image, captured in April 2018, marks another independent imaging of M87* following the first-ever black hole image taken in 2017.
The newly released image shows a ring-shaped structure similar to the one captured in 2017. The deep shadow at the center of the bright ring is the "black hole shadow" predicted by general relativity. In comparison to the 2017 image, the brightest part of the ring is deflected by approximately 30 degrees counterclockwise, possibly due to changes in turbulent materials around the black hole. This achievement has been published in Astronomy & Astrophysics.
Figure 1: A new image of M87* captured in April 2018 (right) released by the Event Horizon Telescope Collaboration. Similar to the 2017 image (left, released on April 10, 2019), the 2018 black hole image shows a "donut" shape with a dark center and bright periphery of the same size. A subtle distinction lies in the position of the brightest part. In the 2018 image, the brightest part appears in the 5 o'clock direction, deviating approximately 30 degrees counterclockwise compared to the 2017 image. Image Credit: EHT Collaboration.
"A fundamental requirement of science is to be able to reproduce results," said Dr. Keiichi Asada, an associate research fellow at Academia Sinica Institute for Astronomy and Astrophysics in Taiwan, China." Confirmation of the ring in a completely new data set is a huge milestone for our collaboration and a strong indication that we are looking at a black hole shadow and the material orbiting around it."
The new era of directly imaging a black hole began with the extensive analysis of observational data from M87* in 2017. It opened a new window for us not only to study black hole astrophysics more intuitively but also to verify general relativity at the level of fundamental physics. Theoretical models suggest that the state of the material around M87* in 2017 was not relevant to that in 2018. Therefore, multiple observations of M87* will provide independent constraints on the plasma and magnetic field structure around the black hole, which will help us distinguish complex astrophysical processes from general relativistic effects.
EHT is in continuous development to achieve this exciting new scientific goal. The Greenland Telescope, situated deep within the Arctic Circle, joined the EHT observations five months after its construction, which enhanced the spatial sampling of the EHT array in the north-south direction and significantly improved the reliability of black hole imaging. Moreover, the Large Millimeter Telescope (LMT) participated in EHT observations for the first time with its complete 50-meter aperture, greatly boosting observation sensitivity. The EHT array also doubled the data recording bandwidth compared to the 2017 observations. Repeated observations and array upgrades are crucial for demonstrating result reliability and enhancing result credibility.
Figure 2: Distribution of telescopes participating in the 2018 EHT observations. Image Credit: EHT Collaboration.
The image of M87* captured in 2018 is quite similar to the one captured in 2017 —both displaying a bright ring of the same size, with a darker central region, and one side of the ring appearing brighter than the other. Since the mass and distance of M87* rarely increase significantly over a lifetime, the diameter of the ring predicted by the general relativity should remain constant from year to year. Comparing the measured diameter in 2017 with that in 2018, it remains consistently stable, providing strong support for the conclusion that M87* aligns with the description of general relativity.
"The biggest change, that the brightness peak shifted around the ring, is actually something we predicted when we published the first results in 2019," said Dr. Britt Jeter, a postdoctoral fellow at Academia Sinica Institute for Astronomy and Astrophysics in Taiwan, China. "While general relativity says the ring size should stay pretty fixed, the emission from the turbulent, messy accretion disk around the black hole will cause the brightest part of the ring to wobble around a common center. The amount of wobble we see over time is something we can use to test our theories for the magnetic field and plasma environment around the black hole."
"In the work published in Nature in September 2023, we reported the periodic oscillation of black hole jets at the center of the M87 galaxies which can be tied to the 'frame-dragging' effect caused by the spin of the central black hole. During this precession, the corresponding changes in the radiation around the black hole at the micro-arcsecond scale need to be investigated more deeply through subsequent multi-band observations over a longer timescale," said Yuzhu Cui, a postdoc in Zhejiang Lab.
To release the new M87* image observed in 2018, Yuzhu Cui contributed to linking and discussing the structural variations on the event horizon scale (micro-arcsecond scale) with the jet precession on the milli-arcsecond scale. "It has been six years from conducting the observation in 2018 to releasing the final images in 2024. This achievement is a strong evidence of the power of the international collaboration thanks to the tremendous efforts made by the members in the EHT Collaboration. At the same time, we are continuously observing M87* and SgrA*, the nearest supermassive black hole to Earth, hoping to further study the dynamic properties of black holes and learn more about our universe."
In addition to the years 2017 and 2018, EHT has successfully conducted observations in 2021 and 2022, with the forthcoming observations in the first half of 2024. The EHT array undergoes continuous improvement every year, involving the addition of new telescopes, hardware enhancements, and the incorporation of new observation bands. Concurrently, the EHT Collaboration is diligently engaged in in-depth analysis of these observational data. China is also actively advancing the construction of facilities and technological development related to submillimeter radio telescopes, which will significantly contribute to uncovering more mysteries of black holes. We anticipate more new results and discoveries in the future.
About Event Horizon Telescope (EHT) Collaboration
The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, and North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. The individual telescopes involved are the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment Telescope (APEX), the IRAM 30-meter telescope, the Northern Extended Millimeter Array (NOEMA), James Clerk Maxwell Telescope (JCMT), Large Millimeter Telescope (LMT), Submillimeter Array (SMA), Submillimeter Telescope (SMT), South Pole Telescope (SPT), ARO 12m Radio Telescope, and Greenland Telescope (GLT). Data were correlated at the Max-Planck Institute for Radio Astronomy (MPIfR) and the MIT Haystack Observatory. The postprocessing was done within the collaboration by an international team at different institutions.