For the first time ever, astronomers using NASA’s Hubble telescope has reported the observation of a free-floating black hole wholly independent of a stellar mass companion.
Heretofore, black holes have only been identified as supermassive objects at the centers of massive galaxies like our own milky way, or gravitationally bound to a stellar companion. Although such free-floating black holes have long been predicted to exist, this marks the first time such an object has been detected.
Two separate observational teams —- one led by the Space Telescope Science Institute in Maryland, and another led by the University of California at Berkeley —- announced the findings today and detail their observations in papers accepted in The Astrophysical Journal and The Astrophysical Journal Letters.
Following six years of meticulous observations, NASA’s Hubble Space Telescope has detected wandering black hole lies about 5,000 light-years away, in the Carina-Sagittarius spiral arm of our galaxy, NASA reported.
The teams used Hubble to catch the object warping spacetime via gravitational microlensing. With microlensing, a foreground object can act as a gravitational lens to bend and amplify the 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 reason the teams suspected this lensing foreground object was actually a black hole was due to the duration of the lensing event which was amplified for 270 days.
Because two separate surveys caught the same object, this putative black hole has two names, known in part by the lensing events in which they were found: MOA-2011-BLG-191 and OGLE-2011-BLG-0462.
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This illustration reveals how light from a background star is deflected and brightened by the black … [+] NASA, ESA, STSCI, JOSEPH OLMSTED
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 resulting remnant stellar core is crushed by gravity into a black hole, says NASA. Because the self-detonation is not perfectly symmetrical, the black hole may get a kick, and go careening through our galaxy like a blasted cannonball, NASA notes.
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What is this black hole’s actual size?
Theoretical models suggest that the mass of the progenitor of such a black hole is 20 to 25 solar masses, which means the progenitor must be an O-type star, Kailash Sahu, an astronomer at the Space Telescope Science Institute and the lead author of The Astrophysical Journal paper, told me.
O-type stars, which are still burning hydrogen on what is known as the main sequence range up to 90 times the mass of our Sun and can be a million times as luminous as our own star.
Sahu says that if this solar mass black hole is 7.1 times the mass of our Sun, its event horizon (or its outer boundary) would be about 26 miles in diameter.
How fast is it traveling through the galaxy?
Sahu’s team, says NASA, estimates that the isolated black hole is traveling across 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-off detection, Sahu says it still might be possible to do follow up observations of this free-floating black hole.
“We estimate that the black hole lies in a region of high density of interstellar matter,” said Sahu. In that case, the black hole would be accreting material from the interstellar matter which would produce x-rays and radio waves, he says.
So, deep x-ray and radio observations may detect the black hole, which can be used to better characterize the object, says Sahu.
As for how many of these free floaters might lie within our galaxy?
Estimates range from 100 to 200 million, with the nearest expected to be at least 80 light years from Earth.
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