At observatories around the world, telescopes have been stitching together their data to create pictures as a single Earth-sized telescope. From the South Pole to the Arctic in Greenland, nearly a dozen observatories have collaborated over the years to form the Event Horizon Telescope (EHT). By combining the radio signals captured simultaneously at each telescope, the EHT effectively emulates a single telescope the size of Earth. 

Applying this strategy to astronomical imaging, the EHT collaboration has allowed astronomers to capture the first observation of black holes with enough resolution to clearly picture their ‘event horizon’ — the region where light is unable to escape the immense gravity of a black hole. 

In 2017, the EHT collaboration yielded the first-ever image of a supermassive black hole at the center of the Messier 87 galaxy (M87*), which is located roughly 55 million light-years from Earth. This achievement was not only major news within scholarly circles, but also made headlines in the mainstream media. 

Since the initial photo, astrophysicists have continued their work with the EHT by imaging a second black hole, Sagittarius A*, located at the center of the Milky Way and furthered our understanding of black holes by comparing real-world data with simulations and models. 

U of T’s von Fellenberg and M87*

Sebastiano von Fellenberg, a U of T postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA), recently led the data calibration for the M87* black hole, using data collected from the EHT. By removing atmospheric effects and accounting for the differences between each telescope, he stabilized the signal from the black hole over time, ultimately improving the clarity of the image. 

Alongside numerous researchers, von Fellenberg helped determine that the bright ring around M87*, made up of plasma orbiting the black hole, has changed between three observations made with the EHT in 2017, 2018, and 2021. The paper discussing these observations appears in Astronomy & Astrophysics, published in September. 

By comparing the observations made across these timescales, the EHT team members were able to identify a change in the brightness profile — the pattern of brightness — in the ring around the black hole, which aligned with previous models for black holes like M87*. Unexpectedly, the direction in which the light waves travelled — or ‘light polarization’ — within the black hole was changing significantly, which introduced new research pathways for astrophysicists. 

While discussing the polarization results in an interview with The Varsity, von Fellenberg said, “We have two options: either something is fundamentally broken with general relativity, which I think is highly unlikely… [or] there may also be something in the foreground.” 

With regard to the possible findings in the foreground, von Fellenberg mentioned three primary ways to explain the phenomena that the EHT had observed.

One, that a “change in the underlying magnetic field structure” of the black hole’s accretion disk — the swirling disk of gas and debris around the black hole — could cause the change in light polarization. There could also be effects due to a “change in the degree of Faraday rotation along the line of sight” as the light from the black hole passes through plasma on its way to Earth. Additionally, their observations could have “evolving contributions from different emission regions,” meaning the visible light included some combination of the ring and a jet of light launched from the black hole. Some combination of all three explanations is also possible. 

In order to determine what exactly is behind the changing polarization observed around M87*, the EHT collaboration is continuing to probe M87*. One of the next challenges they hope to tackle is the imaging of the jet of light emitted out of M87* with the EHT. While this jet has been visualized before, the EHT provides added resolution at new frequencies to enhance the image quality. 

To achieve this, the EHT is expanding the number of observatories to their telescope, such as the Kitt Peak observatory in Arizona and NOEMA in the French Alps. These new additions will hopefully further enlighten us about the strange and curious physics of black holes.