Up until this May, 2019, Sagittarius A* (Sgr A*), the Milky Way’s central supermasive black hole appeared like a massive, dormant volcano, a sleeping monster, a slumbering region of spacetime where gravity is so strong that “what goes into them does not come out.”
On that beautiful May evening at Hawaii’s Keck Observatory set atop Mauna Kea, UCLA astrophysicist Tuan Do tweeted time lapse of images over 2.5 hours from May from @keckobservatory of the supermassive black hole Sgr A*. “The black hole is always variable,” Ko observes, “but this was the brightest we’ve seen in the infrared so far. It was probably even brighter before we started observing that night!”
“The black hole was so bright I at first mistook it for the star S0-2, because I had never seen Sgr A* that bright,” Do said in an interview with ScienceAlert “Over the next few frames, though, it was clear the source was variable and had to be the black hole. I knew almost right away there was probably something interesting going on with the black hole.”
It appears that something disrupted Sgr*A’s slumber. Conjectures for its recent flaring range from data errors to SO-2, one of two stars that approach very closely to Sgr. A* in an elliptical orbit. Every 16 years, it’s at its closest. In the middle of 2018 was its last closest approach, when it was only 17 light-hours away from the black hole. Another strong possibility is the massive gas cloud known as G2 that might be drawn into Sgr. A*’s accretion disk causing it to flare brightly as it was heated, triggering a chain of events that caused or contributed to the May 2019 flaring.
“We have wondered why the Milky Way’s black hole appears to be a slumbering giant,” observed Tatsuya Inui of Kyoto University in Japan. “But now we realize that the black hole was far more active in the past. Perhaps it’s just resting after a major outburst.” Tatsuya Inui is part of a team that used results from Japan’s Suzaku and ASCA X-ray satellites, NASA’s Chandra X-ray Observatory, and the European Space Agency’s XMM-Newton X-ray Observatory, to determine the history of our black hole.
It turns out that, approximately 300 years ago, Sagittarius A* let loose, expelling a massive energy flare. Data taken from 1994 to 2005 revealed that clouds of gas near the central black hole, known as Sagittarius B2, brightened and faded quickly in X-ray light. The X-rays were emanating from just outside the black hole, created by the buildup of matter piling up outside the black hole, which subsequently heats up and expels X-rays.
These pulses of X-ray take 300 years to traverse the distance between Sagittarius A* and Sagittarius B2, so that when we witness something happening in the cloud, it is responding to something that happened 300 years ago. Amazingly for us, in a rare occurrence of perfect cosmic timing, a region in Sagittarius B2, only 10 light-years across varied dramatically in brightness. “By observing how this cloud lit up and faded over 10 years, we could trace back the black hole’s activity 300 years ago,” says team member Katsuji Koyama of Kyoto University.
It appears that the cosmos is setting the stage is being set for the first ever image of SgrA* by the Event Horizon Telescope (EHT). When it’s completed, the image is sure to equal the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968. The obvious target for the Event Horizon Telescope, the team hopes to get imagery of our supermassive black hole soon, said Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of the Event Horizon Telescope that created the first-ever image of the gargantuan black hole 55 million light-years from Earth in neighboring galaxy M87.
Data from the EHT as opened a window on the inner workings of how material spirals towards black holes, finally disappearing across their event horizons, and growing into what physicist Avery Broderick of the Perimeter Institute calls “monsters lurking in the night.”
Taking images of the accretion disk around Sagittarius A*, which has an event horizon that is smaller than the orbit of Mercury, is a feat akin to trying to image a grapefruit on the moon. But the EHT array should be able to accomplish that. “There are now enough telescopes in the array, in principle, to make images in the next couple of years,” Broderick adds.
Those images will enable astrophysicists to transform our understanding of how black holes grow, how they interact with their surroundings, and even the nature of gravity. By studying the details of the cosmic “traffic jam” caused by gas as it rushes headlong towards the black hole, researchers will be able to check if Albert Einstein’s theory of general relativity, one of the pillars of modern physics, holds up in the extreme gravity conditions around black holes.
What we’ll see when the EHT actually sees Sagittarius A* is an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit.” Were you to float there, says astrophysicist Janna Levin, professor of physics and astronomy at Barnard College of Columbia University and author of Black Hole Blues, “you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face. Closer than that, all the light falls in.”
On May 5, 2019 The Galaxy reported that unknown objects were detected orbing Sgr*A: “They have clearly seen something moving. What it is, is not exactly clear.”” said Doeleman.
More than 50 years ago, scientists saw that there was something very bright at the center of our galaxy, says Paul McNamara, an astrophysicist at the European Space Agency. It has a gravitational pull strong enough to make stars orbit around it very quickly—as fast as 20 years, compared to our Solar System’s journey, which takes about 230 million years to circle the center of the Milky Way.
That “very bright something” was Sgr A*. Last October, 2018, before the release of the first image of the M87 black hole from the Event Horizon Telescope (EHT), astronomers announced that they found something orbiting the innermost possible orbit of the supermassive black hole. Their measurements suggest that these “hotspots” — perhaps made of blobs of plasma — are spinning not far from the innermost orbit allowed by the laws of physics.
The newly detected hotspots, reports Joshua Sokol in Quanta, “afford astronomers their closest look yet at the funhouse-mirrored space-time that surrounds a black hole. And in time, additional observations will indicate whether those known laws of physics truly describe what’s going on at the edge of where space-time breaks down.”
“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” marveled Oliver Pfuhl, a scientist at the Max Planck Institute for Extraterrestrial Physics
For astrophysicists, this glimpse at plasma is interesting in and of itself. “We have a totally new environment, which is totally unknown,” said Nico Hamaus, a cosmologist at Ludwig Maximilian University in Munich, who also developed the early hot spot theory.
While some matter in the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds — can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.
Relativistic speeds are those which are so great that the effects of Einstein’s Theory of Relativity become significant. In the case of the accretion disc around Sagittarius A*, the gas is moving at roughly 30% of the speed of light.
“We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*,” explained Pfuhl. “During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!”
Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE), who led the study, explained: “This always was one of our dream projects but we did not dare to hope that it would become possible so soon.” Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that “the result is a resounding confirmation of the massive black hole paradigm.”