New Study Finds Middle Child of Black Hole Family
Swift X-ray observations of galaxy NGC 5408 indicate its ultraluminous X-ray source undergoes periodic changes every 115.5 days. This cycle, astronomers suspect, is linked to the orbit of star circling a middleweight black hole, donating mass to it.
Credit: NASA

Black holes usually come in either the little or big variety, but astronomers have found compelling new evidence that supports the existence of a long-sought middleweight class of the deep space objects.

The "just right"-sized candidate is an X-ray source in the NGC 5408 galaxy, 15.8 million light-years away from Earth in the constellation Centaurus.

"Intermediate-mass black holes contain between 100 and 10,000 times the sun's mass," said Tod Strohmayer, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. "We observe the heavyweight black holes in the centers of galaxies and the lightweight ones orbiting stars in our own galaxy. But finding the 'tweeners' remains a challenge."

The new study used the European Space Agency's orbiting XMM-Newton observatory to observe the candidate, called NGC 5408 X-1, in 2006 and then again in 2008. It found further proof that the signal is a medium-sized black hole.

Black holes are infinitely dense objects that trap matter and even light with their gravity. Though astronomers can't see them directly, they detect a black hole's presence by its gravitational pull on other objects, and by the light that's emitted when massive things fall into it.

NGC 5408 X-1 emits more X-rays than a typical star, but less than supermassive black holes, which can weigh as much as a million suns.

Strohmayer and colleague Richard Mushotzky, an astrophysicist at the University of Maryland, College Park, found that NGC 5408 X-1 flickers, getting periodically brighter and dimmer over time. Scientists think this pattern reflects hot gas piling on to the accretion disk of matter swirling around a black hole.

The NGC 5408 X-1 signal is unique because the flickering is about 100 times slower than that seen from stellar-mass black holes, yet this source outshines these systems by about the same factor. That tells the researchers it's probably a middleweight.

"Astronomers have been studying NGC 5408 X-1 for a long time because it is one of the best candidates for an intermediate-mass black hole," said Philip Kaaret at the University of Iowa, who has studied the object at radio wavelengths but is unaffiliated with the new study. "These new results probe what is happening close to the black hole and add strong evidence that it is unusually massive."

The case still isn't closed on the controversial idea of medium-sized black holes, though. More evidence will be needed to convince skeptics that black holes aren't all huge or small.

The research is slated to be published in a future issue of The Astrophysical Journal.

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