APEX offers up-close view of black hole's event horizon

Astronomers are trying to take a picture of the shadow of a black hole, and they're getting closer thanks to the Atacama Pathfinder Experiment radio telescope, also known as APEX.

Five years ago, astronomers outfitted APEX with the equipment needed to be integrated into a global network of antennas known as Event Horizon Telescope. The addition of APEX allowed EHT to collect the most detailed observations yet of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy.

More specifically, the collective of telescopes was able to zoom in on the black hole's event horizon, the point of no return -- where the gravitational pull of the black hole becomes stronger than the speed of light and beyond, which no matter or particle can escape.

The Event Horizon Telescope uses a technique called Very Long Baseline Interferometry to integrate antennas around the world.

"The participation of the APEX telescope almost doubles the length of the longest baselines in comparison to earlier observations and leads to a spectacular resolution of 3 Schwarzschild radii only," Ru-Sen Lu, a researcher at the Max Planck Institute for Radio Astronomy in Bonn, Germany, said in a news release.

A Schwarzschild radius is the radius of the event horizon of a supermassive black hole as predicted by Einstein's general theory of relativity.

The resolution captured by the EHT network is the closest astronomers have come to an event horizon.

"It reveals details in the central radio source which are smaller than the expected size of the accretion disk," said astronomer Thomas Krichbaum.

The unprecedented observations have allowed astronomers to begin to make out the structures surrounding the event horizon, along the inner edge of the accretion disk.

"We started to figure out what the horizon-scale structure may look like, rather than just draw generic conclusions from the visibilities that we sampled," Ru-Sen Lu said. "It is very encouraging to see that the fitting of a ring-like structure agrees very well with the data, though we cannot exclude other models, e.g., a composition of bright spots."

Since the latest observations -- which were detailed in the Astrophysical Journal -- were gathered, the EHT network has brought on another powerful partner. The ALMA telescope was integrated in 2017. Astronomers are hopeful the next round of observations will offer even more detailed views of the event horizon of Sagittarius A*.

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