The key actors in kilonovae are neutron stars, the central cores of stars that
collapsed under gravity during supernova explosions. They each have a mass
similar to the Sun, but are only about 6 miles (10 kilometers) in diameter. And
when they collide, they send out debris moving near the speed of light. These
explosions are also thought to forge heavy elements, like gold, platinum, and
strontium (which gives actual fireworks their stunning reds). Kilonovae shoot
those elements across space, potentially allowing them to end up in rocks forming
the crust of terrestrial planets like Earth.
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NASA’s Roman Space Telescope will survey the same areas of the sky every
few days following its launch in May 2027. Researchers will mine these data to
identify kilonovae – explosions that happen when two neutron stars or a neutron
star and a black hole collide and merge. When these collisions happen, a
fraction of the resulting debris is ejected as jets, which move near the speed
of light. The remaining debris produces hot, glowing, neutron-rich clouds that
forge heavy elements, like gold and platinum. Roman’s extensive data will help
astronomers better identify how often these events occur, how much energy they
give off, and how near or far they are. Credits: NASA, ESA, J. Olmsted
(STScI)
The astronomical community captured one of these remarkable kilonova events in 2017. Scientists at the
National Science Foundation’s Laser Interferometer Gravitational-Wave
Observatory (LIGO) detected the collision of two neutron stars first with gravitational waves – ripples in
space-time. Almost simultaneously, NASA’s Fermi Gamma-ray Space Telescope
detected high-energy light. NASA quickly pivoted to observe the event with a
broader fleet of telescopes, and captured the fading glow of the blast’s
expanding debris in a series of images.
But the players in this example collided practically in our “backyard,” at
least in astronomical terms. They lie only 130 million light-years away. There
must be more kilonovae – and many that are farther flung – dotting our
ever-active universe.
“We don’t yet know the rate of these events,” said Daniel M. Scolnic, an
assistant professor of physics at Duke University in Durham, North Carolina.
Scolnic led a study that estimates the number of kilonovae that could be
discovered by past, present, and future observatories including Roman. “Is the
single kilonova we identified typical? How bright are these explosions? What
types of galaxies do they occur in?” Existing telescopes can’t cover wide
enough areas or observe deeply enough to find more distant examples, but that
will change with Roman.
Spotting More, and More Distant, Kilonovae
At this stage, LIGO leads the pack in identifying neutron star mergers. It can detect gravitational waves in all areas of the sky, but some of the most distant collisions may be too weak to be identified. Roman is set to join LIGO’s search, offering complementary qualities that help “fill out” the team. Roman is a survey telescope that will repeatedly scan the same areas of the sky. Plus, Roman's field of view is 200 times larger than the Hubble Space Telescope’s infrared view – not as vast as LIGO’s, but huge for a telescope that takes images. Its cadence will allow researchers to spot when objects on the sky brighten or dim, whether nearby or very far away.
How will NASA’s Roman Space Telescope detect kilonovae – brief flashes of
light sent out by the merger of two neutron stars or a neutron star and a black
hole? In part, due to the telescope’s wide field of view. Roman’s view is 200
times larger than the Hubble Space Telescope’s infrared view. Once Roman starts
observing the sky at a regular cadence following its launch, planned by 2027,
researchers expect to be able to identify more of these spectacular events,
both nearby and very far away. Although we do not yet know the rate of these
events, when Roman’s data pours in we will begin learn how frequent these
mergers are – and what results. Credits: NASA, Alyssa
Pagan (STScI)
Roman will provide researchers a powerful tool for observing extremely
distant kilonovae. This is due to the expansion of space. Light that left stars
billions of years ago is stretched into longer, redder wavelengths, known as
infrared light, over time. Since Roman specializes in capturing near-infrared
light, it will detect light from very distant objects. How distant? “Roman will
be able to see some kilonovae whose light has traveled about 7 billion years to
reach Earth,” explained Eve Chase, a postdoctoral researcher at Los Alamos
National Laboratory in Los Alamos, New Mexico. Chase led a more recent study
that simulated how differences in kilonovae ejecta can vary what we expect to
observe from observatories including Roman.
There’s a second benefit to near-infrared light: It provides more
time to observe these short-lived bursts. Shorter wavelengths of
light, like ultraviolet and visible, disappear from view in a day or two.
Near-infrared light can be gathered for a week or more. Researchers have been
simulating the data to see how this will work. “For a subset of simulated
kilonovae, Roman would be able to observe some more than two weeks after the
neutron star merger occurred,” Chase added. “It will be an excellent tool
for looking at kilonovae that are very far away.”
Soon, researchers will know far more about where kilonovae
occur, and how often these explosions occur in the history of the universe.
Were those that occurred earlier different in some way? “Roman will allow the
astronomy community to begin conducting population studies along with a slew of
new analyses on the physics of these explosions,” Scolnic said.
A survey telescope offers enormous possibility – and also a ton of data
that will require precise machine learning. Astronomers are meeting this
challenge by writing code to automate these searches. Ultimately, Roman’s
massive data sets will help researchers unravel perhaps the greatest mysteries
about kilonovae to date: What happens after two neutron stars collide? Does it
produce a single neutron star, a black hole, or something else entirely? With
Roman, we will gather the statistics researchers need to make substantial
breakthroughs.
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, with participation by NASA's Jet Propulsion
Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science
Institute in Baltimore, and a science team comprising scientists from various
research institutions. The primary industrial partners are Ball Aerospace and
Technologies Corporation in Boulder, Colorado; L3Harris Technologies in
Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks,
California.
By Claire Blome, Space Telescope Science Institute
Source: How NASA’s Roman Telescope Will Scan for Showstopping Explosions | NASA
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