Milky Way’s ‘Lurking, Dormant Monster’ –“Our Central Black Hole Currently Fails to Outshine a Single Star”





Like a massive, dormant volcano, the Milky Way’s central black hole appears to be a sleeping monster. Black holes are regions of spacetime where gravity is so strong that “what goes into them does not come out,” says Avery Broderick, a faculty member at the Perimeter Institute. As the name implies, black holes are intrinsically dark, with no light or matter able to escape once they have passed the threshold of no return known as the event horizon. But as black holes feast on the surrounding gas and stars, their accretion disks can shine and produce extraordinary energy. They can even outshine their host galaxies.


Compared to some black holes, Sagittarius A* is much more anemic and fails to outshine a single bright star despite its comparatively enormous mass. But the data from the Event Horizon Telescope has opened a window on the inner workings of how material spirals towards black holes, finally disappearing across their event horizons, and growing into what Broderick calls “monsters lurking in the night.”

In December of 2015, the international Event Horizon Telescope research team measured for the first time the magnetic fields that contribute to black hole growth. The ETH, a linked array of millimeter-wavelength telescopes that spans the globe and is set to take the highest-resolution images in the history of astronomy. When trained on the black hole at the center of our galaxy, Sagittarius A*, it can see the structural details in the accretion flow that surrounds the black hole horizon.

For the first time, astronomers have detected evidence of black-hole-scale magnetic fields near the black hole at the center of our galaxy. Were these magnetic fields not there, “a lot of theoretical astrophysics would have to go back to the drawing board,” says Broderick, jointly appointed at the University of Waterloo. The discovery, published in the journal Science, moves the understanding of how black holes grow from the realm of theoretical speculation to the territory of empirical fact, Broderick says.

Broderick was part of a collaboration that discovered high levels of polarization in the radio emission from Sagittarius A*, the bright radio source believed to be the astronomical manifestation of the 4.5-million-solar-mass black hole.

The current observations are from only three of the sites in the global EHT array, comparable to having just a handful of pixels of the larger picture that will eventually be produced. Nevertheless, these few pixels are already writing the preface to the coming revolution in our understanding of black holes. Researchers are able to begin the process of putting our best current ideas of what is happening near the black hole to the test.

It will also shed light on the reverse process, whereby some black holes are capable of launching outflows of energy and material at nearly the speed of light, extending the black hole’s impact to intergalactic scales. Decades of theoretical work, including enormous computer simulations, painted a picture of how strong magnetic fields near the black hole horizon contribute to the processes that enable a black hole to grow. But now, with the data from the EHT, scientists can begin to see how these processes work in practice.

The radio emission in Sagittarius A* is generated by high-energy electrons zipping around magnetic field lines. This produces highly polarized emission on microscopic scales, tied to the local orientation of the magnetic field, so the polarization traces the structure of the magnetic fields. Detecting high polarization on the size of the black hole horizon at Sagittarius A* does two things. First, it verifies that magnetic fields are there and that they must be ordered. Second, it provides a measurement of the typical size of these magnetic structures.

There is much more to come. 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.




The Chandra Observatory image above provides a panoramic X-ray view extending 400 light years by 900 light years shows that, even at this distance from the center of the Galaxy, conditions are getting crowded, and the energy level is increasing dramatically. Supernova remnants (SNR 0.9-0.1, probably the X-ray Thread, and Sagittarius A East), bright binary X-ray sources containing a black hole or a neutron star (the 1E sources), and hundreds of unnamed point-like sources due to neutron stars or white dwarfs light up the region. The massive stars in the Arches and other star clusters (the DB sources) will soon explode to produce more supernovas, neutron stars, and black holes.

Infrared and radio telescopes have also revealed giant star-forming molecular clouds (Sagittarius A, B1, B2 and C, and the cold gas cloud near the Radio Arc), the edges of which are glowing with X-rays because of heating from nearby supernovas.

‘If, however, Sagittarius A* was more active in the past,’ Christopher van Eldik explains, ‘then it could indeed be responsible for the bulk of today’s galactic cosmic rays that are observed on earth.’ If true, this would dramatically influence the century-old debate on the origins of galactic cosmic rays, as the theory that their components are primarily accelerated to PeV energies by remnants of supernovae – shock waves that occur after the explosion of massive stars – would have to be revised to take this into account.

“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.

The Weekend Feature

The Daily Galaxy via Perimeter Institute for Theoretical Physics


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