GRO J1655-40: Evidence for a Spinning Black Hole

[Dana McPeek]

Explanation: In the center of a swirling whirlpool of hot gas is likely a beast that has never been seen directly: a black hole. Studies of the bright light emitted by the swirling gas frequently indicate not only that a black hole is present, but also likely attributes. The gas surrounding GRO J1655-40, for example, has been found to display an unusual flickering at a rate of 450 times a second. Given a previous mass estimate for the central object of seven times the mass of our Sun, the rate of the fast flickering can be explained by a black hole that is rotating very rapidly. What physical mechanisms actually cause the flickering -- and a slower quasi-periodic oscillation (QPO) -- in accretion disks surrounding black holes and neutron stars remains a topic of much research.

Interesting.
So is it thought the black hole is spinning at 450x per second, or does the star create multiple oscillation points, like the blades of a fan?

[Qev]

That's an interesting idea, actually, some sort of harmonic in the rotation of the object, causing an apparently shorter period. Then again, high rotation rates like this aren't exactly uncommon when dealing with massive, ultracompact objects like neutron stars and black holes. Just like figure-skaters pulling their arms in during a spin speed up, so do collapsing stars... a lot.

[Dana McPeek]

But 450 times per SECOND???

Assuming a 25,000 mile circumference (approximately earth size) the surface speed at the equator would (please excuse my math) be 11.2 million miles PER second. That's only 45 BILLION miles per hour. 6% c??? That is an appreciable spin...

[Qev]

Yup! That's fast for an ultracompact object, but not unbelievably fast. There're such things as millisecond pulsars, which means yes, they're spinning around 1000 times per second.

Also bear in mind that this object is much, much smaller in diameter than the Earth. A black hole of seven solar masses has a radius of approximately only 20km.