Thursday, 8 January 2015

Neutron Star Sparks Brightest Flare Recorded from Milky Way's Supermassive Black Hole


When you’re waiting for a bus, it always seems as if nothing comes along for ages, then three arrive all at once. Something similar happened last year to x-ray astronomers waiting for cosmic fireworks when a cloud of gas and dust passed close to the supermassive black hole at the center of the Milky Way. The fireworks never erupted but the researchers, using NASA’s Chandra x-ray observatory, serendipitously caught three other rare events: two bright x-ray flares and emissions from a rare, highly magnetized star.

The gas cloud, known as G2, was calculated to come close to the central black hole—known as Sagittarius A*, or Sgr A**--in spring 2014. Astronomers were expecting a huge increase in emissions from the region at many different wavelengths as G2 ploughed through the disk of material orbiting the black hole, or perhaps was even swallowed by it. But for reasons that astronomers are still arguing vociferously over, nothing happened.

“We really expected to see a bow shock [when G2 hit], but we really didn’t see it,” says astronomer Daryl Haggard of Amherst College in Massachusetts.

But Haggard and her team, who like many others had booked copious amounts of observing time to witness the event, didn’t come away empty handed. First, somewhat to their annoyance, a magnetar—a type of neutron star with a very strong magnetic field--exploded into view very close to Sgr A* with a burst of x-rays. For a time it was brighter than Sgr A* and the researchers were concerned it might spoil their view of G2’s arrival. But in the end, says Haggard, it was an interesting event in itself: there are only around 30 known magnetars and this was the first anywhere near Sgr A*. If it remains bright, she says, they may be able to see it move around the black hole and use it “to probe this very busy neighbourhood.”

Then, in early 2014, a bright x-ray flare erupted from close to Sgr A*. Such flares can be common but this one was the brightest ever recorded. “It was really exciting,” Haggard says. It couldn’t have been caused by G2, she adds, because its short duration was characteristic of something much closer in to Sgr A* than G2.

But what caused it? Using models of the area around Sgr A*, the researchers were able to suggest two possibilities: that it could have been caused by an asteroid that strayed too close to the black hole and was torn apart, its remains being heated to enormous temperature before slipping below Sgr A*’s event horizon; or it could be caused by field lines of the black hole’s powerful magnetic field snapping apart and reconnecting, a process—common on the Sun—which releases strong bursts of energy. “It’s an unsolved mystery,” Haggard says.

That flare was followed by another one in October 2014, this time half as bright and also unexplained. "Such rare and extreme events give us a unique chance to use a mere trickle of infalling matter to understand the physics of one of the most bizarre objects in our galaxy," Gabriele Ponti of the Max Planck Institute for Astrophysics in Garching, Germany, a co-author on the study, said in a statement.

Astrophysicist Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics in Cambridge, says such observations are valuable because supermassive black holes are common throughout the universe. “Our theories about what happens at the center of a quasar are all about things falling into a supermassive black hole,” he says. “This is the first time we can see such things close up, although on a smaller scale. It’s a fair bet that this is happening at a larger scale elsewhere in the universe.”

The image at the top of the page shows orbits of stars that fly by at millions of miles per hour around the Milky Way's black hole (courtesy Stanford Astronomy Program).

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