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UAH physicist: Monster black hole ‘practically in our back yard’

HUNTSVILLE – The discovery of a so-called monster black hole that has about 12 times the mass of the sun is detailed in a new Astrophysical Journal research submission, authored by Dr. Sukanya Chakrabarti, a physics professor at the University of Alabama in Huntsville.

Dr. Sukanya Chakrabarti (UAH Photo/Michael Mercier)

“It is closer to the sun than any other known black hole, at a distance of 1,550 light years,” said Chakrabarti, the Pei-Ling Chan Endowed Chair in the Department of Physics at
UAH. “So, it’s practically in our back yard.”

Black holes are seen as exotic because, although their gravitational force is clearly felt by stars and other objects in their vicinity, no light can escape a black hole so they can’t be seen in the same way as visible stars.

“In some cases, like for supermassive black holes at the centers of galaxies, they can drive galaxy formation and evolution,” Chakrabarti said. “It is not yet clear how these noninteracting black holes affect galactic dynamics in the Milky Way.

“If they are numerous, they may well affect the formation of our galaxy and its internal dynamics.”

To find the black hole, Chakrabarti and a national team of scientists analyzed data of nearly 200,000 binary stars released over the summer from the European Space Agency’s Gaia satellite mission.

“We searched for objects that were reported to have large companion masses but whose
brightness could be attributed to a single visible star,” she said. “Thus, you have a good
reason to think that the companion is dark.

“Simple estimates suggest that there are about a million visible stars that have massive
black hole companions in our galaxy. But there are a hundred billion stars in our galaxy, so it is like looking for a needle in a haystack. The Gaia mission, with its incredibly precise measurements, made it easier by narrowing down our search.”

Sources were followed up with measurements from telescopes including the Automated Planet Finder in California, Chile’s Giant Magellan Telescope and the W.M. Keck Observatory in Hawaii.

“The pull of the black hole on the visible sun-like star can be determined from these
spectroscopic measurements, which give us a line-of-sight velocity due to a Doppler shift,”
said Chakrabarti. A Doppler shift is the change in frequency of a wave in relation to an
observer, like how the pitch of a siren’s sound changes as an emergency vehicle passes.

“By analyzing the line-of-sight velocities of the visible star – and this visible star is akin to
our own sun – we can infer how massive the black hole companion is, as well as the period
of rotation, and how eccentric the orbit is,” she said. “These spectroscopic measurements
independently confirmed the Gaia solution that also indicated that this binary system is
composed of a visible star that is orbiting a very massive object.”

The black hole has to be inferred from analyzing the motions of the visible star because it is not interacting with the luminous star.

Noninteracting black holes don’t typically have a doughnut-shaped ring of accretion dust and material that accompanies black holes that are interacting with another object. Accretion makes the interacting type relatively easier to observe optically, which is why far more of that type have been found.

“The majority of black holes in binary systems are in X-ray binaries – in other words, they
are bright in X-rays due to some interaction with the black hole, often due to the black hole
devouring the other star,” says Dr. Chakrabarti. “As the stuff from the other star falls down
this deep gravitational potential well, we can see X-rays.”

These interacting systems tend to be on short-period orbits, she said.

“In this case we’re looking at a monster black hole but it’s on a long-period orbit of 185 days, or about half a year,” Chakrabarti saidys. “It’s pretty far from the visible star and not
making any advances toward it.”

The techniques the scientists employed should apply to finding other noninteracting
systems, as well.

“This is a new population that we’re just starting to learn about and will tell us about the
formation channel of black holes, so it’s been very exciting to work on this,” said Peter
Craig, a doctoral candidate at the Rochester Institute of Technology who is advised on his
thesis by Chakrabarti.

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