Bright flashes in the Milky Way's supermassive black hole on Aug 8-10 2019
bSeptember
12, 2019
Milky Way's supermassive black hole is feasting
CNN)
Earlier this year, astronomers
were surprised to spot "unprecedented" changes in the supermassive
black hole at the center of our galaxy. Now, they may have a better
understanding of what they witnessed: the black hole enjoying an interstellar
feast of gas and dust.
Astronomers were using the European Southern Observatory's
Very Large Telescope in Chile and the W.M. Keck Observatory in Hawaii to
observe the supermassive black hole at the center of the Milky Way in April and
May.
It's 26,000 light-years from Earth, but it's the closest
black hole we can see, even if it's largely obscured by dust.
This black hole, known as Sgr A*, has shown variability before.
It's been observed for years in multiple wavelengths of light. But on three
of the evenings they observed it [Aug 8-10 2019], it was unusually bright.
The unusually bright flux levels and variability showed peaks
that exceeded twice the historical measurements of the black hole. What the
astronomers were seeing was an eruption of the black hole unleashing bright
radiation.
In addition to these bright levels, on two nights in May
they also saw large drops in brightness occur over the course of minutes.
Some of the astronomers involved in the study, including Tuan
Do at the University of California, Los Angeles who initially drew
attention to the outburst by tweeting about it, have been observing the black
hole for years. They've analyzed 13,000 observations from 133 nights since
2003.
"The first image I saw that night, the black hole was
so bright I initially mistook it for the star S0-2, because I had never seen
Sagittarius A* that bright," said Tuan Do, lead author of
the latest research on the black hole. "But it quickly became clear the
source had to be the black hole, which was really exciting."
The study has been published in the journal Astrophysical Journal Letters.
"We have never seen anything like this in the 24 years
we have studied the supermassive black hole," said Andrea Ghez, UCLA
professor of physics and astronomy and co-senior study author.
"It's usually a pretty quiet, wimpy black hole on a diet. We don't know
what is driving this big feast."
So what's causing the brightness? The gas and dust being
devoured by the black hole are throwing off radiation. Now, the astronomers
want to know if this is a single event or something leading up to more
activity.
"The big question is whether the black hole is entering
a new phase -- for example if the spigot has been turned up and the rate of gas
falling down the black hole 'drain' has increased for an extended period -- or
whether we have just seen the fireworks from a few unusual blobs of gas falling
in," said Mark Morris, UCLA professor of physics and astronomy and
co-senior study author.
The astronomers have several thoughts about what led to the
increased activity. In 2018, the star S0-2
made a close approach to the black hole, releasing gas that reached it this
year. It could also be the death of large asteroids as they're swallowed by the
black hole. Or it could include G2, a strange object that could be a pair of
binary stars, which closely approached the black hole in 2014.
It's possible that the black hole stripped off the entire
outer layer, Ghez said.
And there's nothing to fear from the radiation, given the
distance and the amount. It would have to be 10 billion times brighter to
affect us, Do said.
For now, researchers are learning as much as they can from
this event.
"We want to know how black holes grow and affect the
evolution of galaxies and the universe," Ghez said. "We want to know
why the supermassive hole gets brighter and how it gets brighter."
Hungry black holes
Our galaxy isn't the only hotspot for supermassive black
holes acting like Pac-Man.
At the heart of a galaxy called GSN 069, which is 250
million light-years from Earth, astronomers have detected repeating X-ray
bursts that happen every nine hours. The supermassive black hole at the center
of this gallery is consuming enough material to comprise four of our moons at
least three times a day.
That's a million billion billion pounds per meal.
"This black hole is on a meal plan like we've never
seen before," said Giovanni Miniutti, study author of a new paper
in Nature at the European Space Agency's Center for Astrobiology in Spain.
"This behavior is so unprecedented that we had to coin a new expression to
describe it: "X-ray quasi-periodic eruptions."
Observations by NASA's Chandra X-ray Observatory and the
European Space Agency's XMM-Newton helped astronomers track the repeating
outbursts and see when the black hole was speeding up and slowing down.
During the events, the X-ray emissions would be 20 times brighter and the
temperature of the gas flowing in would spike from 1 million degrees Fahrenheit
to 2.5 million degrees.
Researchers want to gather more data on the repeating
outbursts to learn more about them. As for the material the black hole is
eating, it could be a star slowly being ripped apart.
For the first time, astronomers have detected tones like the
ring of a bell from a newly formed black hole.
Albert Einstein predicted that when black holes collide
and form a bigger one, they would send out ripples in space and time called
gravitational waves. And those waves would carry a specific tone much
like a bell that's been struck.
But the researchers unexpectedly detected two tones, rather
than one. Their finding was published this week in the journal Physical
Review Letters.
"Before, it was as if you were trying to match the
sound of a chord from a guitar using only a single string," said Matthew
Giesler, study author and graduate student at the California Institute of
Technology.
When black holes merge, it's a violent process. All of that
energy has to go somewhere, so it's shed through gravitational waves.
A reanalysis of data captured by the LIGO detector, which
helped detect gravitational waves for the first time in 2015, revealed the
overtones.
"This was a very surprising result. The conventional
wisdom was that by the time the remnant black hole had settled down so that any
tones could be detected, the overtones would have decayed away almost
completely," said Saul Teukolsky, study co-author and Robinson Professor
of Theoretical Astrophysics at Caltech. "Instead, it turns out that the
overtones are detectable before the main tone becomes visible."
REF/ https://edition.cnn.com/2019/09/12/world/black-hole-hungry-ring-scn-trnd/index.html
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