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Washington, November 11 (ANI): Astronomers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have found that an X-ray source in galaxy NGC 5408 represents one of the best cases for a middleweight black hole to date.

Several nearby galaxies contain brilliant objects known as ultraluminous X-ray sources (ULXs).

They appear to emit more energy than any known process powered by stars but less energy than the centers of active galaxies, which are known to contain million-solar-mass black holes.

“ULXs are good candidates for intermediate-mass black holes, and the one in galaxy NGC 5408 is especially interesting,” said Richard Mushotzky, an astrophysicist at the University of Maryland, College Park.

The galaxy lies 15.8 million light-years away in the constellation Centaurus.

Using the European Space Agency’s orbiting XMM-Newton observatory, Tod Strohmayer, an astrophysicist at Goddard, and Mushotzky studied the source - known as NGC 5408 X-1 - in 2006 and 2008.

XMM-Newton detected what the astronomers call “quasi-periodic oscillations,” a nearly regular “flickering” caused by the pile-up of hot gas deep within the accretion disk that forms around a massive object.

The rate of this flickering was about 100 times slower than that seen from stellar-mass black holes.

Yet, in X-rays, NGC 5408 X-1 outshines these systems by about the same factor.

Based on the timing of the oscillations and other characteristics of the emission, Strohmayer and Mushotzky conclude that NGC 5408 X-1 contains between 1,000 and 9,000 solar masses.

“For this mass range, a black hole’s event horizon - the part beyond which we cannot see - is between 3,800 and 34,000 miles across, or less than half of Earth’s diameter to about four times its size,” said Strohmayer.

If NGC 5408 X-1 is indeed actively gobbling gas to fuel its prodigious X-ray emission, the material likely flows to the black hole from an orbiting star.

This is typical for stellar-mass black holes in our galaxy.

Strohmayer next enlisted the help of NASA’s Swift satellite to search for subtle variations of X-rays that would signal the orbit of NGC 5408 X-1’s donor star.

Swift detects a slight rise and fall of X-rays every 115.5 days.

“If this is indeed the orbital period of a stellar companion, then it’s likely a giant or supergiant star between three and five times the Sun’s mass,” Strohmayer said.

“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. (ANI)