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| Asrtist's impression of colliding Neutron Stars, Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet |
On Aug. 17, the Laser
Interferometry Gravitational-Wave Observatory (LIGO) detected the fifth
fingerprint of a massive disturbance in spacetime since LIGO began operations
in September 2015. Unlike the first four sets of ripples, which reflected
collisions between two black holes, the shape of these spacetime distortions
suggested a collision between two neutron stars.
While black hole collisions
produce almost no signature other than gravitational waves, the collision of
neutron stars can be—and was—observed up and down the electromagnetic spectrum.
"When neutron stars collide, all hell breaks loose," said Frans
Pretorius, a Princeton physics professor. "They start producing a
tremendous amount of visible light, and also gamma rays, X-rays, radio
waves…."
Princeton researchers have
been studying neutron stars and their astronomical signatures for decades. The
gravitational waves were the first evidence of the neutron star merger to
arrive at Earth, followed by a gamma-ray burst that arrived 1.7 seconds later.
The connection between
neutron stars and gamma-ray bursts was first identified by Princeton
astrophysicists in 1986, said James Stone, the Lyman Spitzer Jr., Professor of
Theoretical Astrophysics and chair of the Department of Astrophysical Sciences.
"Many of the discoveries announced [Oct. 16] confirm the basic predictions
made 30 years ago here in Princeton."
He was referring to a set of back-to-back papers
by Bohdan Paczynski, the late Lyman Spitzer Jr. Professor of Theoretical
Astrophysics, and Jeremy Goodman, a 1983 Ph.D. graduate who studied under
Paczynski and is now a professor in the department.
In their articles, Paczynski
and Goodman argued that colliding neutron stars could be the sources of
gamma-ray bursts, a mysterious, short-lived energy source first identified by
satellites in the late 1960s.
"We both referred to
that possibility. Who first floated that idea? I don't know, because we were in
constant conversation," Goodman said. "We knew that [neutron stars]
must occasionally collide—we knew that because of [Princeton physicist and
Nobel laureate] Joe Taylor's work."
In addition, Paczynski had
realized that most gamma-ray bursts were coming from distances far enough that
the expansion of the universe was affecting their apparent distribution.
"Bohdan Paczynski was
absolutely right," said Goodman. However, his ideas were not immediately
embraced by the field. "I remember going to a conference in Taos, New
Mexico. … Bohdan gave a short talk on his idea that gamma-ray bursts are coming
from cosmological distances. I remember these other astrophysicists … they were
respectfully quiet when he spoke, but regarded him as a bit of a lunatic."
He added, "Bohdan Paczynski was a very bold thinker."
The possibility of colliding neutron stars that
had prompted Paczynski and Goodman's discussion first surfaced in a 1981 paper
by Joseph Taylor, now the James S. McDonnell Distinguished University Professor
of Physics, Emeritus.
His 1974 discovery of binary
neutron stars with his then-graduate student Russell Hulse, who later worked at
the Princeton Plasma Physics Laboratory, was awarded the 1993 Nobel Prize in
Physics. They showed that the two neutron stars they had spotted were separated
by about half a million miles and orbiting each other every 7.75 hours.
In 1981, shortly after
coming to Princeton, Taylor and then-Assistant Professor Joel Weisberg
announced that with precise measurements taken over several years, they had
confirmed that the distance and period are changing with time, with an orbital
decay that matches Albert Einstein's prediction for energy loss due to
gravitational wave emission. The orbit is slowing so infinitesimally that it
will take roughly 300 million years for the neutron stars in the Hulse-Taylor
binary to collide and merge.
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| This snapshot of the first milliseconds in the violent merger of two neutron stars, calculated and rendered by associate research scholar David Radice and collaborators, reveals the stars’ gravitational tidal effects on each other. In the next 10 milliseconds, they will merge into one rapidly rotating massive neutron star, then collapse into a black hole surrounded by a transient disk of material. Credit: Princeton University |
"Once the Hulse-Taylor
neutron star binary was understood, with subsequent timing experiments showing
consistency with general relativity, it was clear that collisions would
happen," said Steven Gubser, a professor of physics. "So as we
celebrate the first gravitational wave detection of colliding neutron stars,
let's also credit Joe Taylor and Russell Hulse for their original discovery of
binary pulsars, and for the demonstration that they are in fact neutron stars
orbiting each other, just waiting to collide."
Picture a quarter spinning
on a tabletop. As friction bleeds energy from the system, the quarter starts to
wobble around its outer edge, making a "whop…whop…whop…whop" sound
that speeds up (whop-whop-whop-whop) and speeds up (whopwhopwhopwhop) until
it's just a blur of sound that rises in pitch into a final "whoooop"
as the quarter flattens on the table.
That's the demonstration
that Gubser and Pretorius provided as they described how black holes (or
neutron stars) collide—an astronomical marvel that LIGO has now detected five
times. At a recent talk for their book, "The Little Book of Black
Holes," published by Princeton University Press, Gubser and Pretorius used
a disk about three inches across instead of a quarter, so their audience could
more easily see and hear the disk's slow but steady increase of speed.
"You'd ordinarily think
of losing energy as corresponding to slowing down, not speeding up, but you saw
with the disk that in fact it can go the other way," said Gubser
afterward. "As the disk loses energy to friction, its point of contact
moves faster and faster around, and produces that characteristic rising
frequency."
Whether the colliding objects
are neutron stars or black holes—or one of each—the whirling motion and its
sound follows the same pattern. As the gravitational wave energy bleeds away,
the two objects will orbit each other faster and faster, heading to their
inevitable demise.
In the case of the collision
that LIGO detected on Aug. 17, the two stars—each the size of Manhattan and
with almost twice the mass of the sun—were ultimately whirling around each
other hundreds of times per second, moving at a significant fraction of the
speed of light before they collided.
"Taylor and Weisberg's
timing experiment showed the beginnings of this pattern, arising from a slow
in-spiral," said Gubser. "The frequency increases very slowly, and
that's why it was such an impressive measurement."
By contrast, he said,
"in the final phase of the in-spiral, the frequency increases rapidly, and
you get the kind of 'whoop' or 'chirp' waveform that LIGO saw."
When stars smash into each
other at an appreciable fraction of the speed of light, the collision fuses
atoms together and creates the elements that fill the bottom rows of the
periodic table.
"These
elements—platinum, gold, many other less valuable ones that are high up on the
periodic table—they have more neutrons than protons in their nuclei,"
Goodman said. "You can't get to those nuclei in the same way that we
understand elements up to iron being produced, by effectively adding one
neutron at a time. The problem is that you have to add a lot of neutrons very
quickly." This rapid process is known to physicists as the r-process.
For a long time, scientists
thought that r-process elements were created in supernovae, but the numbers
didn't add up, Goodman said. "But neutron stars are mostly neutrons, and
if you smash two of them together, it's reasonable to expect that some of the
neutrons will splash out."
"The products of this
merger could be gold, uranium, europium—some of the heaviest elements in
nature," said Adam Burrows, a professor of astrophysical sciences and the
director of the Program in Planets and Life.
Burrows and David Radice, an
associate research scholar, recently won funding from the U.S. Department of
Energy to investigate merging neutron stars and supernovae, which Burrows
collectively describes as "some of the most explosive phenomena, some of
the most violent, that occur on a regular basis in the universe."
Spectroscopic observations
from the European Southern Observatory's Very Large Telescope (VLT) in the wake
of the LIGO detection confirmed that heavy metals like platinum, lead and gold
were created in the collision of the two neutron stars.
The VLT data used to
identify these elements, the visible and near-visible wavelengths of light,
were gathered in the hours and days following LIGO's detection of the
gravitational waves. Once word had begun to spread of LIGO's discovery, the
worldwide astronomical community trained their telescopes and other instruments
on the patch of sky that the gravitational waves had come from, in what former
Princeton postdoctoral researcher Brian Metzger called the "most ambitious
and emotionally charged electromagnetic campaign in history, probably, for any
transient [short-lived event]."
Metzger, an assistant
physics professor at Columbia University, was one of the almost 4,000
co-authors on the paper describing the follow-up observations of X-rays, gamma
rays, visible light waves, radio waves and more. "This was a really
amazing panchromatic discovery of gravitational waves, at basically every
single wavelength," he said.
The impact on the
astronomical community compares to only one other event in his lifetime, said
Goodman: the 1987 supernova. Observations of that stellar explosion had
provided concrete resolution to countless astronomical questions and theories.
"People had been building up this model for supernovae, [a] towering
theoretical edifice, and the observational foundations were a little
shaky," Goodman said. "Nobody could think of a better model for these
things, but then to see it … I don't know how to describe it, it's like getting
a telegram from God, saying exactly what these events were."
The reams of data gathered
from the "electromagnetic fireworks" produced by the neutron star
merger have had a similar effect, Goodman said. "We had all sorts of
speculation … but now we have these gravitational waves. It's exactly as we
expected for two compact masses!"
"This is the future of
gravitational wave detection, which is a new astronomy that has been
opened," said Burrows. "It's a new window on the universe that has
been anticipated for decades, and it's an amazing coming-to-fruition of the
ambitions of thousands of scientists, technologists, that actually accomplished
what many people thought they could not."
Via Phys.org