NASA Hubble finds first free-floating black hole

For the first time ever, astronomers using NASA’s Hubble telescope have reported the sighting of a free-floating black hole completely independent of a stellar companion.

Until now, black holes have only been identified as supermassive objects in the centers of massive galaxies like our own galaxy, or gravitationally bound to a stellar companion. Although the existence of such free-floating black holes has long been predicted, this is the first time such an object has been detected.

Two separate observation teams — one led by the Space Telescope Science Institute in Maryland and another led by the University of California at Berkeley — released the findings today, detailing their observations in documents accepted in The astrophysics magazine and The astrophysical journal letters

After six years of close observations, NASA’s Hubble Space Telescope has detected a wandering black hole about 5,000 light-years away, in our galaxy’s Carina-Sagittarius spiral arm, NASA reported.

The teams used Hubble to capture the object distorting spacetime via gravitational microlensing. Microlensing allows a foreground object to act as a gravitational lens to bend and amplify light from a distant background star. In this case, a black hole lensed a star located an estimated 19,000 light-years away in the galactic bulge of our Milky Way galaxy. One of the reasons the teams suspected that this lensing foreground object was actually a black hole was due to the length of the lensing event amplified for 270 days.

Because two separate investigations captured the same object, this alleged black hole has two names, known in part from the lensing events in which they were found: MOA-2011-BLG-191 and OGLE-2011-BLG-0462.

How do such black holes end up as solo objects?

The most likely scenario is that giant stars, at least 20 times the mass of the sun, explode as supernovae. The remaining stellar core is crushed by gravity into a black hole, NASA says. Because the self-detonation isn’t perfectly symmetrical, the black hole could be kicked and blasted through our galaxy like a blown cannonball, NASA notes.

What is the actual size of this black hole?

Theoretical models suggest that the mass of the precursor to such a black hole is 20 to 25 solar masses, meaning the precursor must be an O-type star, Kailash Sahu, an astronomer at the Space Telescope Science Institute and the lead author of theoretical models. Astrophysical Journal paper, told me.

O-type stars, still burning hydrogen in what’s known as the main sequence, range up to 90 times the mass of our sun and can be a million times more luminous than our own star.

Sahu says that if this sun-mass black hole is 7.1 times our sun’s mass, its event horizon (or its outer limit) would be about 26 miles in diameter.

How fast does it travel across the galaxy?

Sahu’s team, NASA says, estimates that the isolated black hole is traveling through the galaxy at 100,000 miles per hour (fast enough to travel from Earth to the moon in less than three hours).

Although microlensing is a one-time detection, Sahu says it’s still possible to make follow-up observations of this free-floating black hole.

“We estimate that the black hole is in a region with a high density of interstellar matter,” Sahu said. In that case, the black hole would attract material from the interstellar matter that would produce X-ray and radio waves, he says.

So deep X-ray and radio observations can detect the black hole, which could be used to better characterize the object, Sahu says.

As for how many of these free floaters could be in our galaxy?

Estimates range from 100 to 200 million, with the closest expected to be at least 80 light-years from Earth.

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