Astrophysicists
are redrawing the textbook image of pulsars, the dense, whirling remains of
exploded stars, thanks to NASA’s
Neutron star Interior Composition Explorer (NICER), an X-ray
telescope aboard the International
Space Station. Using NICER data, scientists have obtained the first
precise and dependable measurements of both a pulsar’s size and its mass, as
well as the first-ever map of hot spots on its surface.
The pulsar in question, J0030+0451 (J0030
for short), lies in an isolated region of space 1,100 light-years away in the constellation Pisces.
While measuring the pulsar’s heft and proportions, NICER revealed that the shapes
and locations of million-degree “hot spots” on the pulsar’s surface are much
stranger than generally thought.
“From
its perch on the space station, NICER is revolutionizing our understanding of
pulsars,” said Paul Hertz, astrophysics division director at NASA Headquarters
in Washington. “Pulsars were discovered more than 50 years ago as beacons of
stars that have collapsed into dense cores, behaving unlike anything we see on
Earth. With NICER we can probe the nature of these dense remnants in ways that
seemed impossible until now.”
A series of papers analyzing
NICER’s observations of J0030 appears in a focus issue of The Astrophysical
Journal Letters and is now available
online.
When
a massive star dies, it runs out of fuel, collapses under its own weight and
explodes as a supernova. These stellar deaths can leave behind neutron stars,
which pack more mass than our Sun into a sphere roughly as wide as the island
of Manhattan is long. Pulsars, which are one class of neutron star, spin up to
hundreds of times each second and sweep beams of energy toward us with every
rotation. J0030 revolves 205 times per second.
For decades, scientists have
been trying to figure out exactly how pulsars work. In the simplest model, a
pulsar has a powerful magnetic field shaped much like a household bar magnet.
The field is so strong it rips particles from the pulsar’s surface and
accelerates them. Some particles follow the magnetic field and strike the
opposite side, heating the surface and creating hot spots at the magnetic
poles. The whole pulsar glows faintly in X-rays, but the hot spots are
brighter. As the object spins, these spots sweep in and out of view like the
beams of a lighthouse, producing extremely regular
variations in the object’s X-ray brightness. But the new NICER studies
of J0030 show pulsars aren’t so simple.
Using NICER observations from
July 2017 to December 2018, two groups of scientists mapped J0030’s hot spots
using independent methods and converged on similar results for its mass and
size. A team led by Thomas Riley, a doctoral student in computational
astrophysics, and his supervisor Anna Watts, a professor of
astrophysics at the University of Amsterdam, determined
the pulsar is around 1.3 times the Sun’s mass and 15.8 miles (25.4 kilometers)
across. Cole Miller, an astronomy professor at the University of Maryland (UMD)
who led the second team, found J0030 is about 1.4 times the Sun’s mass and
slightly larger, about 16.2 miles (26 kilometers) wide.
“When
we first started working on J0030, our understanding of how to simulate pulsars
was incomplete, and it still is,” Riley said. “But thanks to NICER’s detailed
data, open-source tools, high-performance computers and great teamwork, we now
have a framework for developing more realistic models of these objects.”
A
pulsar is so dense its gravity warps nearby space-time — the “fabric” of the
universe as described by Einstein’s general theory of relativity — in much the
same way as a bowling ball on a trampoline stretches the surface. Space-time is
so distorted that light from the side of the pulsar facing away from us is
“bent” and redirected into our view. This makes the star look bigger than it
is. The effect also means the hot spots may never completely disappear as they
rotate to the far side of the star. NICER measures the arrival of each X-ray
from a pulsar to better than a hundred nanoseconds, a precision about 20 times
greater than previously available, so scientists can take advantage of this
effect for the first time.
“NICER’s
unparalleled X-ray measurements allowed us to make the most precise and
reliable calculations of a pulsar’s size to date, with an uncertainty of less
than 10%,” Miller said. “The whole NICER team has made an important
contribution to fundamental physics that is impossible to probe in terrestrial
laboratories.”
Our
view from Earth looks onto J0030’s northern hemisphere. When the teams mapped
the shapes and locations of J0030’s spots, they expected to find one there based
on the textbook image of pulsars, but didn’t. Instead, the researchers
identified up to three hot “spots,” all in the southern hemisphere.
Riley and his colleagues ran
rounds of simulations using overlapping circles of different sizes and
temperatures to recreate the X-ray signals. Performing their analysis on the
Dutch national supercomputer Cartesius took
less than a month — but would have required around 10 years on a modern desktop
computer. Their solution identifies two hot spots, one small and circular and
the other long and crescent-shaped.
Miller’s group performed
similar simulations, but with ovals of different sizes and temperatures, on
UMD’s Deepthought2 supercomputer.
They found two possible and equally likely spot configurations. One has two
ovals that closely match the pattern found by Riley’s team. The second solution
adds a third, cooler spot slightly askew of the pulsar’s south rotational pole.
Previous
theoretical predictions suggested that hot spot locations and shapes could
vary, but the J0030 studies are the first to map these surface features.
Scientists are still trying to determine why J0030’s spots are arranged and
shaped as they are, but for now it’s clear that pulsar magnetic fields are more
complicated than the traditional two-pole model.
NICER’s
main science goal is to precisely determine the masses and sizes of several
pulsars. With this information scientists will finally be able to decipher the
state of matter in the cores of neutron stars, matter crushed by tremendous
pressures and densities that cannot be replicated on Earth.
“It’s
remarkable, and also very reassuring, that the two teams achieved such similar
sizes, masses and hot spot patterns for J0030 using different modeling
approaches,” said Zaven Arzoumanian, NICER science lead at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland. “It tells us NICER is on the right path
to help us answer an enduring question in astrophysics: What form does matter
take in the ultra-dense cores of neutron stars?”
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