January 5, 2015

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An image of a simulation of the gas cloud’s encounter with Sgr A*. The blue lines
mark the orbits of the so-called “S” stars that are in close orbits around
the supermassive black hole. (Image Credit: Image by ESO/MPE/Marc Schartmann.)

Last September, after years of watching, a team of scientists led by Amherst College astronomy professor Daryl Haggard observed and recorded the largest-ever flare in X-rays from a supermassive black hole at the center of the Milky Way. The astronomical event, which was detected by NASA’s Chandra X-ray Observatory, puts the scientific community one step closer to understanding the nature and behavior of supermassive black holes.

Haggard and her colleagues discussed the flare today at a press conference during this year’s meeting of the American Astronomical Society in Seattle.

Supermassive black holes are the largest of black holes, and all large galaxies have one. The one at the center of the Milky Way is called Sagittarius A* (Sgr A*for short), and scientists say it contains about four and a half million times the mass of our Sun.

Scientists working with Chandra have observed Sgr A* repeatedly since the telescope was launched into space in 1999. Haggard and fellow astronomers were originally using Chandra to see if Sgr A* would consume parts of a cloud of gas, known as G2.

“Unfortunately, the G2 gas cloud didn’t produce the fireworks we were hoping for when it got close to Sgr A*,” she said. “However, nature often surprises us and we saw something else that was really exciting.”

Haggard and her team detected an X-ray outburst last September that was 400 times brighter than the usual X-ray output from Sgr A*. This “megaflare” was nearly three times brighter than the previous record holder that was seen in early 2012. A second enormous X-ray flare, 200 times brighter than Sgr A* in its quiet state, was observed with Chandra on Oct. 20, 2014.

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Astronomy Professor Daryl Haggard

Haggard and her team have two main ideas about what could be causing Sgr A* to erupt in this extreme way. One hypothesis is that the gravity of the supermassive black hole has torn apart a couple of asteroids that wandered too close. The debris from such a “tidal disruption” would become very hot and produce X-rays before disappearing forever across the black hole’s point of no return (called the “event horizon”).

“If an asteroid were torn apart, it would go around the black hole for a couple of hours – like water circling an open drain – before falling in,” said Haggard’s colleague and co-principal investigator Fred Baganoff of the Massachusetts Institute of Technology in Cambridge, MA. “That’s just how long we saw the brightest X-ray flare last, so that is an intriguing clue for us to consider.”

If that theory holds up, it means astronomers have found evidence for the largest asteroid ever to be torn apart by the Milky Way’s black hole.

A second theory is that, within the material flowing towards Sgr A*, the magnetic field lines are packed unusually tightly. If this were the case, these field lines would occasionally interconnect and reconfigure themselves. When this happens, their magnetic energy is converted into the energy of motion, heat and the acceleration of particles—which could produce a bright X-ray flare. Such magnetic flares are seen on the Sun; the Sgr A* flares have a similar pattern of brightness to those solar events.

“At the moment, we can’t distinguish between these two very different ideas,” said Haggard. “It’s exciting to identify tensions between models and to have a chance to resolve them with present and future observations.”

In addition to the giant flares, Haggard and her team collected data on a magnetar—a neutron star with a strong magnetic field—located close to Sgr A*. This magnetar is undergoing a long X-ray outburst, and the Chandra data are allowing astronomers to better understand this unusual object.

As for the G2: Astronomers estimate that the gas cloud made its closest approach—still about 15 billion miles away from the edge of the black hole—in spring 2014. The researchers estimate the record breaking X-ray flares were produced about 100 times closer to the black hole, making it very unlikely that the Chandra flares were associated with G2.

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