Astronomical Milestone: The Event Horizon Telescope (EHT) has observed cosmic objects at the short radio wavelength of 870 micrometers for the first time — achieving a new resolution record. This allows the radio telescope array to image black holes up to 50 percent sharper and with more detail in the future. It also makes observations of smaller black holes possible for the first time, as the team reports in the “Astrophysical Journal”. However, achieving this milestone was anything but easy.
In recent years, the coupled radio telescopes of the Event Horizon Telescope (EHT) have delivered some groundbreaking images, including the first photo of a black hole and the first portrait of the central black hole of the Milky Way, Sagittarius A*. Astronomers were also able to visualize the photon ring predicted by Albert Einstein, as well as the magnetic fields and jets around such gravitational giants using the worldwide telescope network.
However, these images were not particularly sharp until now: “The bright ring of light bent by gravity still looked blurry because we were operating at the absolute limit of the sharpness we could achieve,” explains lead author Alexander Raymond from the Harvard & Smithsonian Center for Astrophysics in the USA.
Shorter Means Sharper
But how can the resolution be increased? With a telescope network like the EHT, there are two possibilities: On the one hand, you can increase the distance between the telescopes, the so-called baseline. The further apart the outermost telescopes are, the larger the virtual “dish” formed by the network — and the higher the resolution. But the EHT’s radio telescopes are already at maximum distance from each other — they form a receiver the size of our entire planet.
The second possibility is to shorten the observation wavelength, because the shorter it is, the higher the possible resolution. Until now, the Event Horizon Telescope operated at a wavelength of 1,300 micrometers (230 gigahertz). But 870 micrometers would be better. The problem, however: Shorter radio waves are more strongly absorbed by water vapor in the atmosphere. This greatly attenuates the detectable astronomical signals and increases noise interference. The shorter wavelength signals are also more susceptible to weather-related turbulence.
To overcome these obstacles, the EHT collaboration had to optimize and further develop their ultra-cold cooled receivers, the transmission technology for their time-precise coupling, and data processing.
New Resolution Record
Now the breakthrough has been achieved: The Event Horizon network has observed cosmic objects at a wavelength of 870 micrometers for the first time — a new record for radio interferometry. “We are achieving the highest resolution ever achieved from the Earth’s surface,” Raymond and his colleagues report. Although only part of the complete telescope network was active for the test, the EHT achieved a resolution of 19 microarcseconds in this observation of distant galaxies.
While the few receivers used were not enough to create a sharp image from the data yet, the resolution of the radio data was already higher than ever before. And it can go even further: With its complete network, the Event Horizon Telescope could resolve structures as small as 13 microarcseconds at this new, shorter wavelength, the astronomers explain. For comparison: This corresponds to imaging a coin lying on the lunar surface from Earth.
“Like Switching From Black and White to Color Photos”
This milestone opens up completely new possibilities for exploring black holes and their surroundings: “To understand why this is such a breakthrough, compare it to the extra details that become visible when switching from black and white to color photos,” explains Raymond’s colleague Sheperd Doeleman. By expanding the frequency spectrum, black holes and their light ring can be imaged about 50 percent more detailed and sharper.
This allows astronomers to check Einstein’s predictions more accurately, for example. “The extension to 870 micrometer wavelength can, for instance, make the substructure of the photon ring around our black hole Sagittarius A* visible,” the astronomers report. “We can try to detect the orbit of light that completes a full rotation around the black hole.”
New Insights Into the “Feeding Behavior” of Black Holes
But the effect of gravity on the hot gases and magnetic fields at the black hole can now also be investigated more precisely. “By examining changes in the surrounding gas at different wavelengths, we can solve the puzzle of how black holes attract and absorb matter and how they can generate powerful jets that extend over galactic distances,” says Doeleman.
Moreover, the shortened wavelength now also allows imaging of smaller or more distant black holes. “These signal measurements with VLBI at 870 micrometers are groundbreaking because they open a new observation window for studying supermassive black holes,” explains co-author Thomas Krichbaum from the Max Planck Institute for Radio Astronomy in Bonn.
