This image shows two massive black holes in the OJ 287 galaxy. The
smaller black hole orbits the larger one, which is also surrounded by a
disk of gas. When the smaller black hole crashes through the disk, it
produces a flare brighter than 1 trillion stars. Credit:
NASA/JPL-Caltech. › Larger view
Black
holes aren't stationary in space; in fact, they can be quite active in their
movements. But because they are completely dark and can't be observed directly,
they're not easy to study. Scientists have finally figured out the precise timing
of a complicated dance between two enormous black holes, revealing hidden details
about the physical characteristics of these mysterious cosmic objects.
The OJ
287 galaxy hosts one of the largest black holes ever found, with over 18
billion times the mass of our Sun. Orbiting this behemoth is another black hole
with about 150 million times the Sun's mass. Twice every 12 years, the smaller
black hole crashes through the enormous disk of gas surrounding its larger
companion, creating a flash of light brighter than a trillion stars - brighter,
even, than the entire Milky Way galaxy. The light takes 3.5 billion years to
reach Earth.
The OJ 287 galaxy hosts one of the largest black holes ever found, with
over 18 billion times the mass of our Sun. Orbiting this behemoth is
another massive black hole. Twice every 12 years, the smaller black hole
crashes through the enormous disk of gas surrounding its larger
companion, creating a flash of light brighter than a trillion stars.
But the smaller black hole's orbit is oblong, not circular, and it's irregular: It shifts position with each loop around the bigger black hole and is tilted relative to the disk of gas. When the smaller black hole crashes through the disk, it creates two expanding bubbles of hot gas that move away from the disk in opposite directions, and in less than 48 hours the system appears to quadruple in brightness.
Because
of the irregular orbit, the black hole collides with the disk at different
times during each 12-year orbit. Sometimes the flares appear as little as one year
apart; other times, as much as 10 years apart. Attempts to model the orbit and
predict when the flares would occur took decades, but in 2010, scientists created
a model that could predict their occurrence to within about one to three weeks.
They demonstrated that their model was correct by predicting the appearance of
a flare in December 2015 to within three weeks.
Then,
in 2018, a group of scientists led by Lankeswar Dey, a graduate student at the
Tata Institute of Fundamental Research in Mumbai, India, published a paper with
an even more detailed model they claimed would be able to predict the timing of
future flares to within four hours. In a new study
published in the Astrophysical Journal Letters, those scientists report that
their accurate prediction of a flare that occurred on July 31, 2019, confirms
the model is correct.
The
observation of that flare almost didn't happen. Because OJ 287 was on the
opposite side of the Sun from Earth, out of view of all telescopes on the
ground and in Earth orbit, the black hole wouldn't come back into view of those
telescopes until early September, long after the flare had faded. But the
system was within view of NASA's
Spitzer Space Telescope, which the agency retired in January 2020.
After
16 years of operations, the spacecraft's orbit had placed it 158 million miles
(254 million kilometers) from Earth, or more than 600 times the distance
between Earth and the Moon. From this vantage point, Spitzer could observe the
system from July 31 (the same day the flare was expected to appear) to early
September, when OJ 287 would become observable to telescopes on Earth.
"When
I first checked the visibility of OJ 287, I was shocked to find that it became
visible to Spitzer right on the day when the next flare was predicted to
occur," said Seppo Laine, an associate staff scientist at Caltech/IPAC in
Pasadena, California, who oversaw Spitzer's observations of the system.
"It was extremely fortunate that we would be able to capture the peak of
this flare with Spitzer, because no other human-made instruments were capable
of achieving this feat at that specific point in time."
Ripples in Space
Scientists
regularly model the orbits of small objects in our solar system, like a comet
looping around the Sun, taking into account the factors that will most
significantly influence their motion. For that comet, the Sun's gravity is
usually the dominant force, but the gravitational pull of nearby planets can change
its path, too.
Determining
the motion of two enormous black holes is much more complex. Scientists must account
for factors that might not noticeably impact smaller objects; chief among them are
something called gravitational waves. Einstein's theory of general relativity
describes gravity as the warping of space by an object's mass. When an object
moves through space, the distortions turn into waves. Einstein predicted the
existence of gravitational waves in 1916, but they weren't observed directly
until 2015 by the Laser
Interferometer Gravitational Wave Observatory (LIGO).
The
larger an object's mass, the larger and more energetic the gravitational waves
it creates. In the OJ 287 system, scientists expect the gravitational waves to
be so large that they can carry enough energy away from the system to measurably
alter the smaller black hole's orbit - and therefore timing of the flares.
While
previous studies of OJ 287 have accounted for gravitational waves, the 2018
model is the most detailed yet. By incorporating information gathered from LIGO's
detections of gravitational waves, it refines the window in which a flare is
expected to occur to just 1 1/2 days.
To
further refine the prediction of the flares to just four hours, the scientists folded
in details about the larger black hole's physical characteristics. Specifically,
the new model incorporates something called the "no-hair" theorem of
black holes.
Published
in the 1960s by a group of physicists that included Stephen Hawking, the
theorem makes a prediction about the nature of black hole "surfaces."
While black holes don't have true surfaces, scientists know there is a boundary
around them beyond which nothing - not even light - can escape. Some ideas
posit that the outer edge, called the event horizon, could be bumpy or
irregular, but the no-hair theorem posits that the "surface" has no
such features, not even hair (the theorem's name was a joke).
In
other words, if one were to cut the black hole down the middle along its
rotational axis, the surface would be symmetric. (The Earth's rotational axis
is almost perfectly aligned with its North and South Poles. If you cut the
planet in half along that axis and compared the two halves, you would find that
our planet is mostly symmetric, though features like oceans and mountains
create some small variations between the halves.)
Finding Symmetry
In the
1970s, Caltech professor emeritus Kip Thorne described how this scenario - a
satellite orbiting a massive black hole - could potentially reveal whether the
black hole's surface was smooth or bumpy. By correctly anticipating the smaller
black hole's orbit with such precision, the new model supports the no-hair
theorem, meaning our basic understanding of these incredibly strange cosmic
objects is correct. The OJ 287 system, in other words, supports the idea that
black hole surfaces are symmetric along their rotational axes.
So how
does the smoothness of the massive black hole's surface impact the timing of the
smaller black hole's orbit? That orbit is determined mostly by the mass of the
larger black hole. If it grew more massive or shed some of its heft, that would
change the size of smaller black hole's orbit. But the distribution of mass
matters as well. A massive bulge on one side of the larger black hole would
distort the space around it differently than if the black hole were symmetric.
That would then alter the smaller black hole's path as it orbits its companion and
measurably change the timing of the black hole's collision with the disk on
that particular orbit.
"It
is important to black hole scientists that we prove or disprove the no-hair
theorem. Without it, we cannot trust that black holes as envisaged by Hawking
and others exist at all," said Mauri Valtonen, an astrophysicist at
University of Turku in Finland and a coauthor on the paper.
Spitzer
science data continues to be analyzed by the science community via the Spitzer
data archive located at the Infrared Science Archive housed at IPAC at Caltech
in Pasadena. JPL managed Spitzer mission operations for NASA's Science Mission
Directorate in Washington. Science operations were conducted at the Spitzer
Science Center at IPAC at Caltech. Spacecraft operations were based at Lockheed
Martin Space in Littleton, Colorado. Caltech manages JPL for NASA.
For more information about Spitzer, visit: https://www.nasa.gov/spitzer - http://www.spitzer.caltech.edu/
News Media Contact
Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov
Source: JPL-Caltech/News
