[ad_1]
Just in time for Halloween, astronomers have delivered the best-yet view of a real-life cosmic monster-Sagittarius A *, a supermassive black hole lurking at the center of the Milky Way. Or, rather, a view of hot clumps of gas that orbit it, teetering on the edge of oblivion. The results reveal new, previously unknown properties of our galaxy's largest black hole and point towards a deeper understanding of gravity.
Black holes like terrifying monsters, can scarcely be understood, let alone seen. Even Einstein doubted they existed, despite his theory of general relativity predicting that they must. They are knots of gravitation bound so tightly that they are spacetime dissolves; spectral shadows so voracious they devour light itself. Yet they can be glimpsed indirectly, like appearances at the corner of your eye. Most spectacularly, when they get stars or other black holes they can give off gravitational waves, ripples in the fabric of reality that scientists first detected in 2016. Scientists can also measure a black hole for example, Sagittarius A * has somehow swallowed the equivalent of four million suns. (Those in other galaxies can be far larger, tipping the scales at billions of solar masses.) And, ironically, black holes do not shine, and the gases that pile up in spinning accretion disks around their maws can be heated to trillions. of degrees, becoming more than a few times and even ejecting even brighter sprays of radiation. When they come from supermassive black holes, these outbursts can shape and possibly even sterilize a galactic host-and such black holes seem to squat at the center of every large galaxy. For more than 40 years astronomers have warily studied such circumstantial evidence as bones and ashes scattered at the threshold of a dragon's air.
Now, an international team of scientists has studied the Milky Way's monster using an instrument called GRAVITY to combine the infrared light of four-meter telescopes at the European Southern Observatory's Very Large Telescope in Chile. Combining light from multiple telescopes is a technique called interferometry, and can dramatically boost the sensitivity and precision of astronomical observations. The results appeared October 31 in Astronomy & Astrophysics. "This is a major breakthrough," says Reinhard Genzel, an astrophysicist at Max Planck Institute for Extraterrestrial Physics and leader of the group. "We have observed the galactic center by using telescopes as a gigantic single telescope with an effective 130-meter diameter to make interferometric images of a few times before having done so before." This is not the first breakthrough from GRAVITY: In May of this year the team successfully measured the relativistic distortion of light from a star, S2, during their closest approach to Sagittarius A * in its 16-year-orbit around the monstrous black hole.
This latest discovery unfolds after the same observations of S2, when GRAVITY team members Oliver Pfuhl and Jason Dexter, both study co-authors at Max Planck, noticed three flares, or "hot spots," that emanated from Sagittarius A * ' s accession disk between mid-May and late July. To appreciate the GRAVITY team's feat, imagine looking up at the moon from the Earth and discerning on the lunar surface at a corner (Sagittarius A * -or at least its shadow) sitting on a beach ball with Christmas lights (Sagittarius A * ' s accession disk and accompanying flares).
These hot spots are thought to be "magnetic thunderstorms" that occur when intense magnetic fields form a snap and reconnect, releasing copious energy to heat a black hole. Each hot spot is akin to a short-lived, 10-million-kilometer-wide lightbulb-after perhaps an hour, it's cool in the whirling, turbulent maelstrom. That would make them exceedingly challenging-particularly if their emissions were varied and varied by various extreme relativistic effects predicted to arise in the vicinity of a black hole. Those same effects, in turn, could be investigated by Einstein's theory of gravity and stringent testing, potentially leading to new physics.
Such outbursts have been detected before, but for the first time, and they have been detected at 30 percent light-speed in a roughly 45-minute orbit around some central location. object weighing four million suns. GRAVITY's data is also measured by polarization, which shifted in accordance with each other's motion through the disk's powerful magnetic fields, further reinforcing the orbital interpretation. "When we saw the first one, we had to ask ourselves, 'Is this real or not?' But then we found two more," Pfuhl says. "They all showed the same rotation, the same orientation and the same scale, which reassured us."
When first presented with the data, Genzel initially reacted with shock. "I could not believe my eyes," he says. "We're doing it, we've been doing this, but this is a beautiful orbital motion." Besides being fortunate enough to catch multiple flares in the act, the GRAVITY team also seems Sagittarius A * 's accretion disk is coincidentally oriented almost face-on rather than edge-on to Earth, allowing astronomers to study its swirling hot spots Much like meteorologists use satellite views to track thunderstorms in a hurricane. "This is like winning the lottery, because a priori probability that you would see something like this face-on is very low," Genzel says. "It almost seems that somebody has this place for us; I guess the galactic center is a place for lucky people. "
Avo Loeb, an astrophysicist at Harvard University who was not directly involved with GRAVITY's studies. More than a decade ago, while working with then-postdoc Avery Broderick (now at the Perimeter Institute for Theoretical Physics in Canada), Loeb developed models of hot spots around Sagittarius A * and suggested methods for observing their orbital motion. "Seeing is believing," he says. "This is fully consistent with what we expected …. Most everyone was so naive, but amazingly enough.
The most important overlap between these predictions and GRAVITY's observations is that Sagittarius * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Beyond this boundary will be more important than ever before. The end of the horizon is the end of the line. Because Sagittarius A *. "By themselves, black holes are simple objects-mass, spin [electric] "It's all about getting", says Andrea Ghez, an astronomer at the University of California, Los Angeles, who has a team that has used the twin Keck telescopes in Hawaii to compete with Genzel's group for more than a decade. "The innermost stable orbit is tied to the black hole's mass and spin-and we already know [Sagittarius A*’s] mass-so if you believe the hot spot is there, you could pin this black hole's spin and measure this fundamental property. That fundamental property is linked to how these things grow, which tells you how they form and evolve over time. Black holes are basic constituents of our universe, so when you study them, you are asking about the building blocks of the cosmos. "
Unfortunately, for more empirical validation of GRAVITY's results. In 2012 NASA pulled the plug on an initiative to give the Keck telescopes an interferometric capability similar to that used by GRAVITY on the Very Large Telescope; without it, independent observations-and confirmation-from Ghez's team will be able to wait until sometime in the 2020s when two as-yet-unbuilt 30-meter-class US observatories, the Giant Magellan Telescope and the Thirty Meter Telescope, are slated to beginning.
In the meantime, GRAVITY will observe the galactic center for more flares beginning next spring, and additional corroboration may come from the realm of radio astronomy. The Event Horizon Telescope, which seeks to image the enigmatic shadow of Sagittarius A * by interferometrically linking radio observatories from around the world, should be published soon after its first full-scale observing run last year. That work probes much closer to the black hole, where gravity traps light in a revolving ring just outside the horizon event. But according to the director's project, Sheperd Doeleman, those observations might also reveal radio blips produced by the circulation of hot spots farther out.
"Whether you're looking at it in infrared or radio or gravitational waves, black holes really are the crux, one of the world's deepest and most profound mysteries," Doeleman says. "How can there be a doorway out of our universe? What does that mean? Right now we are still just seeing bones outside the dragon's air-we have not seen the dragon. "
Source link
