Black holes are notoriouslybashful beasts. The supermassive monsters that dwell at the centers of galaxiesweigh millions to billions of times the mass of the sun and control the fatesof everything in their vicinity, including light. Despite such outsizeinfluence over their home galaxies, black holes never show their faces.
Until now.After more than a decade of work, results from the Event Horizon Telescope, orEHT, stunned the world this year with the first direct image of a black holesevent horizon, the region beyond which not even light can escape.
To make this remarkable image, scientists cobbled together a massive telescope by connecting seven observatories around the world to create a tool effectively the size of Earth (SN: 4/27/19, p. 7). The result: a picture of the round silhouette of a black hole against the ringlike backdrop of its brightly glowing accretion disk, the gas and other material drawn in by the black holes voracious gravitational appetite.
Almost immediately, that image shored up Einsteins general theory of relativity, weighed in on the best way to measure a black holes mass (SN Online: 4/22/19) and provided evidence that event horizons are real. Now the EHT team is digging into what else the telescopes vast amount of data can reveal, in the hopes of cracking more black hole mysteries.
This is justthe beginning of this kind of new era of observing event horizons, says KazuAkiyama, an EHT team member and astrophysicist at the MIT Haystack Observatoryin Westford, Mass.
The initial black hole snapshot, unveiled in April, focused on a distant galaxy, M87 (SN: 4/27/19, p. 6). At roughly 6.5 billion solar masses, M87s black hole is 1,000 times as massive as EHTs other target, the black hole in the center of the Milky Way. That black hole, Sagittarius A*, also known as Sgr A*, weighs about 4 million times the mass of the sun.
Being moremassive made M87s giant an easier subject. Gases swirling around that blackhole were more sluggish and changed brightness less often and less dramaticallythan those moving more nimbly around Sgr A*.
M87 was sittingstill for its portrait, says EHT team member Andrew Chael, an astrophysicistat Princeton University. Sgr A* is like a cheetah running across the frame.
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In datacollected during a week in April 2017, Sgr A* changed its appearance over thecourse of a few minutes. So while M87s black hole lent itself to a singlestill image, for Sgr A*, we may need to construct a movie, Akiyama says.
The simplestway to make a movie would be to break up one nights observations intosegments, make an image from each segment and string them together, says EHTteam member Katie Bouman, a data scientist at Caltech. But theres not enoughinformation in even the smallest segment to produce a reliable image. Youreconstruct nonsense, she says.
Instead, the team is working on techniques to fill in gaps and carry information about the black holes appearance forward in time. We assume that although the source is evolving, its not evolving randomly there is some continuity in how the gas is moving around the black hole, Bouman says. By stitching together a movie that plays smoothly, she and colleagues hope to understand the black holes structure.
Getting a good look at Sgr A*s event horizon will give physicists one of the best tests yet of general relativity, says physicist Feryal zel of the University of Arizona in Tucson. The century-old theory predicts how the mass of a black hole warps spacetime (SN: 10/17/15, p. 16). General relativity also makes precise predictions for the size of the bright ring and dark silhouette for black holes of a given mass.
M87s blackhole was too far away for astronomers to know precisely its mass beforecapturing the image. But Sgr A*s mass is well known, thanks to decades ofmeasurements of stars orbiting the Milky Ways black hole. Capturing Sgr A*simage would be a clean test of some of the things we want to look at, zelsays. The ring and the shadow, it either is the size you expect or its not.Thats an incredible opportunity for us.
A movie ofM87s black hole may be in the works, too. Our observations provided goodevidence that M87 is actually changing [within] the timescale of a week,Akiyama says. Studying how the black hole changes could reveal details of howit rotates, spinning magnetized plasma around it like a dancers skirt.
Among othertreasures waiting in already collected data is the polarization of lightemitted from the bright ring of M87s black hole. This measure of theorientation of light waves wiggling up and down, left and right, or at anangle lets scientists determine the arrangement of strong magnetic fieldsnear the black hole. Those magnetic fields are thought to control how the blackhole accretes matter.
Thearrangement tells you how the black hole eats, says astrophysicist and EHT teammember Michael Johnson of the Harvard-Smithsonian Center for Astrophysics inCambridge, Mass. Black holes are known for their hearty appetites, but actuallyits extremely difficult to fall into a black hole, he says. An orbiting bitof matter will just keep orbiting forever unless some friction or viscosity inthe environment drags it toward the black hole.
Physicists think magnetic fields are what make the environment around black holes viscous. In 2015, Johnson and colleagues published EHT observations of the polarization around Sgr A*, which showed tangled magnetic fields close to the black hole and more organized fields farther away. But those observations came from just four telescopes.
We have thisbeautiful theory of why black holes can eat, but weve never seen evidence forit, Johnson says. So if EHT can see these magnetic fields, we might have ourfirst glimpse into this accretion process.
Polarizationcould also help explain one mysterious feature of M87: It launches a bright,energetic jet that extends light-years into space. Magnetic fields that gettwisted around the black hole as it spins are important for launching the jet,physicists think, but the details are murky.
If we couldsee this polarization, we might be able to see these processes directly themagnetic fields and the jet and how theyre connected to the black hole,Johnson says.
EHT will fire up again in April 2020, this time with 11 observatories, including Kitt Peak in Arizona and NOEMA in the French Alps. Further in the future, EHT scientists are considering sending a telescope to space. Extending EHT into Earths orbit would alleviate worries about weather on the ground ruining observations and would help make even sharper images of even more black holes.
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