The Event Horizon Telescope collaboration has released the image from its second major campaign, confirming the existence of a persistent black hole shadow and a potentially turbulent environment.
The EHT team delighted the world in 2019 with the release of the first image of a black hole’s silhouette, that of M87* (pronounced em-87-star) in the elliptical galaxy M87 in Virgo. This shadow is created by light trying — sometimes in vain — to escape the black hole’s gravity. Photons from the hot gas surrounding the black hole are diverted around the object or even briefly trapped in a loop, together tracing out a ring. This ring girds the central darkness, where light has succumbed to gravity and been swallowed.
The supermassive black hole M87* is so large that light would take 1½ days to cross it. Yet at its distance of 55 million light-years, the shadow’s apparent size is comparable to that of an orange seen from the distance of the Moon.
To unmask this “tiny” shadow, the EHT team combines data from radio observatories around the world to create a virtual, planet-size telescope capable of resolving the silhouette. The first image came from a 2017 observing campaign. Subsequent releases have included the first image of our own galaxy’s central black hole, Sagittarius A*, and details about the magnetic fields around M87* — all using the 2017 observations.
Now, the collaboration has tackled data from its second run, that of 2018, to reveal how the black hole looked one year later.
The 2018 campaign included nine radio telescopes at seven sites, spread from the Arctic to the South Pole and from Spain to Hawai‘i. (The South Pole Telescope can’t observe M87; it helped observe other sources.) This campaign introduced important upgrades, including the addition of the newly commissioned 12-meter Greenland Telescope, which improved the array’s sensitivity.
Over several days in April, astronomers simultaneously turned the telescopes toward their targets, gathering reams of data that they then flew on physical hard drives to two processing centers, one in Germany and the other in Massachusetts. There, researchers carefully combined the observations, aligning the time stamps to within trillionths of a second and weeding out noise.
Due to a range of weather, technical, and security issues (astronomers driving to the Large Millimeter Telescope in Mexico were briefly detained by armed men), only two of the four days spent observing M87* produced enough data.
Once the data were ready, imaging teams set to work. Using eight different computational methods, researchers reconstructed the underlying image revealed in piecemeal fashion by the virtual telescope’s data. Each method produced a different image, but they all showed the same ring and dark central pit, the team reports January 18th in Astronomy & Astrophysics.
The ring’s size matches that of the 2017 shadow, confirming the earlier landmark results.
But the 2017 and 2018 images do differ. The ring’s brightest section has moved about 30° around the circumference, and it has a slightly different structure. While the ring’s size depends on the black hole’s mass and gravitational physics, these other traits arise from what’s happening with the hot gas.
Astronomers expect M87* to change its appearance with time. The gas swirling around the black hole is turbulent and woven through with magnetic fields, which abruptly rearrange themselves and release energy. Given the black hole’s size, we should see fluctuations on the scale of days.
The 2017 observations indeed showed small changes over the course of a week, and a 2020 analysis of data from 2009 to 2017 hinted at year-to-year changes. But the latter study had to assume the ring was there instead of finding it blindly, because the earlier data were so sparse.
The 2018 image thus is the first clear confirmation that M87* shadow changes from year to year.
The variations may have multiple sources, explains team member Dominic Chang (Harvard). Turbulence in the gas itself is likely to blame. But patchy coverage also left holes in the 2017 data, which the algorithms used to reconstruct the glowing ring may have incorrectly filled in as brighter or fainter.
A more concrete answer will require further analysis, including of data from more recent observing runs. Chang is hopeful that what we learn will help us test our assumptions about what happens in the mess of gas trying to fall into the black hole.
The EHT has since added more sites, and the team is working on the next-generation EHT (ngEHT). This endeavor will deploy four new radio telescopes and equip existing ones with new hardware to make them more sensitive. Researchers hope to complete those updates by 2030.
Scientists are also exploring the possibility of expanding the EHT to space. A telescope in Earth orbit would increase the array’s resolution substantially, looking past the infalling gas to see the thin rings of light traced by photons zipping just outside the event horizon. These photon rings carry unique information about how the black hole warps the fabric of spacetime.
Detecting them would give us an unprecedented look at this extreme environment.
The Event Horizon Telescope Collaboration. “The Persistent Shadow of the Supermassive Black Hole of M87.” Astronomy & Astrophysics. January 2024.