Author: Andrew Grant
Compiled by: Chunzhen
Image source: EHT Collaboration
Sagittarius A*, hidden in the galactic gas haze, has proven to be a tricky imaging target for the Event Horizon Telescope team.
A bright ring of plasma orbits the supermassive black hole Sagittarius A* at the center of the Milky Way .
Three years after sharing a historical image of a black hole, the Event Horizon Telescope (EHT) collaboration has unveiled a second portrait of the subject: Sagittarius A*, the 4 million solar-mass black hole at the center of the Milky Way. There is a bright plasma ring at the radius of the photon orbit. Any inward-moving photons located in the area depicted in the shadow center of the image will eventually cross the point of no return, known as the event horizon. The findings were announced at several press conferences around the world and published in a series of papers in the Astrophysical Journal Letters.
If you think the blurry ring graphic looks familiar, that's a good thing. Although Sagittarius A* is nearly three orders of magnitude smaller than the central black hole of the banking system Messier 87 (M87) that the EHT imaged in 2019, Sagittarius A* is also three orders of magnitude less massive, so the angular sizes of the two black holes are very similar. According to general relativity , the size of the shadow is determined only by the radius of the event horizon, which in turn is proportional to the mass of the black hole. University of Arizona astrophysicist and EHT founding member Feryal Özel said at a press conference in Washington, D.C., "Spacetime warps around a black hole in exactly the same way, regardless of its mass or what's around it."
The images of the two black holes are the result of an analysis of data collected in April 2017 from eight millimeter-wave telescopes scattered around the world. By time-stamping the measurements using atomic clocks installed at each observation site, the researchers were able to combine the data using a technique called very long baseline interferometry to achieve resolution comparable to that of Earth-sized telescopes. After calibrating the data and improving the imaging algorithms, the EHT team was able to translate the 2017 observations of M87* into four images that resemble the final product by June 2018. Obtaining a Sagittarius A* portrait presents an extra challenge—and requires a bit of luck.
Image from: Keith Vanderlinde
Above, the 10-meter Antarctic Telescope at Amundsen-Scott South Pole Station in Antarctica is one of eight telescopes observing Sagittarius A*.
Dimitrios Psaltis, an astrophysicist at the University of Arizona and a founding member of the EHT collaboration, said that in many ways, M87* was the perfect first chosen subject. Although M87 galaxy is located 55 million light-years away, it is oriented almost perpendicular to the plane of our galaxy, which provides a relatively gas- and dust-free line of sight to the black hole target. By contrast, capturing the shadow of Sagittarius A* requires looking inward from our vantage point within the Milky Way's disk, directly into the crowded Galactic Center, some 27,000 light-years away. Fluctuations in the plasma free electron density along the line of sight introduce phase changes in the microwaves for EHT detection. The result is a blurry source image.
EHT The researchers knew they would have to deal with this blurring, which was the main reason they chose to observe at 1.3 millimeters rather than at a larger wavelength, where scattering is more intense. To further eliminate the effects of gas and dust, the team developed a model of blurring effects and conducted separate observations with the EHT Observatory to quantify various model parameters. In the end, Psaltis said, the ambiguity had less of an impact than the team feared.
The second complicating factor is the size of Sagittarius A*. If M87* were placed at the center of our solar system, its event horizon would be somewhere near the Voyager 8 probe in interstellar space; Sagittarius A* would not be able to enter the orbit of Mercury.This means that the plasma surrounding the black hole at the center of the Milky Way can complete an orbit in a matter of minutes, while the plasma surrounding M87* would take days or weeks.
EHT Part of the reason the telescope network is able to achieve such high resolution is that it can capture objects from different angles as the Earth rotates, much like a computed tomography machine scans from multiple locations. In both cases, the object is assumed to be static. "The algorithm assumes that what you're photographing is stationary," Psaltis said. "If it takes 10 hours to take a picture, M87* won't change much, but for Sagittarius A*, the plasma has already gone around 50 times." He and his colleagues worry that the black hole's environment is moving so fast that it will take long Time-exposure observations can never achieve sufficient resolution—and no model can counteract this effect. Fortunately for the EHT team, the blur is once again not that scary. "It turned out to be a gentler, more cooperative black hole than expected."
Image via: ESO/ S. Guisard
This 2009 photo from the Cerro Paranal mountain in Chile captures the Milky Way's busy, dusty center, including The supermassive black hole Sagittarius A*.
After a break following the release of M87* results in April 2019, a team of more than 300 members joined the work on Sagittarius A*. Face-to-face meetings critical to the imaging and modeling work on M87* were replaced by Zoom meetings among collaborators from around the world. "That's a big reason why it took three years instead of eight months," Psaltis said.
The new image adds to the knowledge scientists have gained in recent years about the Milky Way's supermassive black hole. For example, a team led by 2020 Nobel Prize winners Andrea Ghez and Reinhard Genzel tracked the motion of stars in dangerous orbits near Sagittarius A* and determined the mass of a black hole to an accuracy of about 1%. In today's image, the measured angular size of the shadow is 51.8 ± 2.3 μ, which is consistent with the predictions of general relativity for the black hole's mass and distance. The image also shows that Sagittarius A* faces roughly face-on as seen from Earth, rather than aligned with the plane of the Milky Way.
Article source: https://physicstoday.scitation.org/do/10.1063/PT.6.1.20220512a/full/
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