article · Astronomy and Astrophysics
Observations from the second Event Horizon Telescope campaign in April 2018 confirm the persistent presence of an asymmetric ring around the supermassive black hole in the galaxy M 87. Utilizing an upgraded global radio array with wider frequency coverage and added baseline coverage from the Greenland telescope, the measurements yielded a median ring diameter of 43.3 microarcseconds. This measurement aligns closely with observations recorded in 2017. Although the diameter remains stable, the location of the brightest segment of the ring shifted by approximately 30 degrees compared to the previous year. This structural stability supports the interpretation of a shadow generated by gravitationally lensed emission surrounding a rotating Kerr black hole with a mass of roughly 6.5 billion solar masses. Furthermore, the shift in brightness asymmetry aligns the black hole spin axis more closely with its large-scale jet.
Capturing repeated images of a supermassive black hole verifies that the observed ring is a permanent feature rather than a temporary phenomenon. Confirming its consistent size provides vital empirical validation for theoretical physics and general relativity, while detecting changes in brightness helps researchers understand how matter swirls into these extreme cosmic objects over time.
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In April 2019, the Event Horizon Telescope (EHT) Collaboration reported the first-ever event-horizon-scale images of a black hole, resolving the central compact radio source in the giant elliptical galaxy M 87. These images reveal a ring with a southerly brightness distribution and a diameter of ∼42 μas, consistent with the predicted size and shape of a shadow produced by the gravitationally lensed emission around a supermassive black hole. These results were obtained as part of the April 2017 EHT observation campaign, using a global very long baseline interferometric radio array operating at a wavelength of 1.3 mm. Here, we present results based on the second EHT observing campaign, taking place in April 2018 with an improved array, wider frequency coverage, and increased bandwidth. In particular, the additional baselines provided by the Greenland telescope improved the coverage of the array. Multiyear EHT observations provide independent snapshots of the horizon-scale emission, allowing us to confirm the persistence, size, and shape of the black hole shadow, and constrain the intrinsic structural variability of the accretion flow. We have confirmed the presence of an asymmetric ring structure, brighter in the southwest, with a median diameter of 43.3 −3.1 +1.5 μas. The diameter of the 2018 ring is remarkably consistent with the diameter obtained from the previous 2017 observations. On the other hand, the position angle of the brightness asymmetry in 2018 is shifted by about 30° relative to 2017. The perennial persistence of the ring and its diameter robustly support the interpretation that the ring is formed by lensed emission surrounding a Kerr black hole with a mass ∼6.5 × 10 9 M ⊙ . The significant change in the ring brightness asymmetry implies a spin axis that is more consistent with the position angle of the large-scale jet.
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DOI: 10.1051/0004-6361/202347932
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