‘Milky-Way-Sized’ Supermassive Black Hole Observed Flaring Up




In May of 2010  a team of astronomers at the Harvard-Smithsonian Center for Astrophysics spotted what appeared to be a flare from a previously inactive, Milky-Way-sized supermassive black hole in a galaxy about two billion light-years away. The CfA team led an aggressive follow-up campaign of observations to see what was going on.

The team began observing the flare about 40 days after it went off and about 40 days before it peaked, providing excellent data over most of the event. Detailed modeling of the light led the team to conclude that the black hole is less massive than previously thought, only about two million solar masses, and that the object it devoured was probably an evolved star (about 5 billion years old) whose mass was about 0.2 solar masses. 

Black holes can come in a wide range of masses. Some, with only about one solar mass, result from the supernova death of a massive star, while those at the center of galaxies (called supermassive black holes) have millions or even billions of solar masses. 

Supermassive black holes are relatively famous because they are responsible for the powerful jets and other dramatic phenomena seen in some galaxies. The center of our Milky Way galaxy contains a modest-sized supermassive black hole, with about four million solar masses, and (fortunately for us) it is inactive – it lacks the extreme phenomena seen elsewhere.

Black holes are so dense that nothing, not even light, can escape from their gravitational clutches. Still, black holes can be detected because matter that falls into them heats up, and emits bright radiation. A short-lived flare, for example, can result when a body (perhaps a cloud of gas or a star) wanders too close to a black hole and is eaten. 

Astronomers are particularly interested in measuring the way the brightness of the flare increases, versus its decline, because the shape of the rising emission holds clues to the actual infall process. Observing such events is difficult, though, because the flaring activity may only last for a few months — by the time it is spotted in the sky the most diagnostic phases of flare activity may have passed.

Moreover, flares from smaller supermassive black holes (like the one in the center of the Milly Way) may be correspondingly weaker.

Pan-STARRS (Panoramic Survey Telescope & Rapid Response System) is a telescope with a small mirror (1.8 meters) but a very large field of view, and large digital cameras (1.4 billion pixels) developed especially to look for transient events. It can observe the entire available sky several times a month. 
These new results provide a particularly impressive, detailed view of what goes on in these exotic cosmic flares, and offer support for the overall model of these flaring events.

The optical-IR image below shows the galaxy that suddenly brightened when the supermassive black hole at its center shredded and absorbed a star that wandered too close. 

The rapid X-ray flare shown in image at the top of the page was observed from the direction of the supermassive black hole that resides at the center of our galaxy. This violent flare, captured by NASA's Chandra X-ray Observatory, has given astronomers an unprecedented view of the energetic processes surrounding this supermassive black hole.

A team of scientists led by Frederick K. Baganoff of MIT detected a sudden X-ray flare while observing Sagittarius A*, a source of radio emission believed to be associated with the black hole at the center of our galaxy.

"This is extremely exciting because it's the first time we have seen our own neighborhood supermassive black hole devour a chunk of material," said Frederick K. Baganoff of MIT . "This signal comes from closer to the event horizon of our galaxy's supermassive black hole than any that we have ever received before. It's as if the material there sent us a postcard before it fell in."

"It almost certainly came from the suspected supermassive black hole there, known as Sagittarius A*," said Fulvio Melia of the University of Arizona. Melia, who is in Washington for the "Two Years of Science with Chandra" symposium, was contacted by e-mail.

"What is so exciting about this is that its properties require an active region no bigger than about 20 times the size of the event horizon predicted by general relativity," Melia said.

The findings validate research that Melia and his colleagues have done over the past decade, particularly on the size and radiative characteristics of Sagittarius A*.



The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics and Nature.com

Image Credit: NASA; Gezari, Rest, and Chornock

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