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Illustration of the unprecedented “elliptical” orbit of a black hole-neutron star system. The strange orbits point to gaps in our understanding of how these systems form. . |Image credit: Geraint Pratten, Royal Society University Fellow at the University of Birmingham
A collision that shook the universe black hole A neutron star has just led astronomers to discover a strange orbital interaction they’ve never seen before, forcing them to rethink their theories.
Before the two extremely dense objects collided and combined, they first orbited each other in an eerie oval shape similar to the vortices of a spirograph, scientists reported March 11 in the journal Science . Astrophysical Journal Communications.
The study authors say the new discovery challenges common assumptions about how black hole and neutron star systems form and whether they must fall into perfectly circular orbits before dying.
“The fact that this system remained eccentric towards the end of its life is essentially a conclusive sign that at least some neutron star-black hole binaries must have formed differently. [than theory predicts],” study co-author Patricia SchmidtAn associate professor of physics and astronomy at the University of Birmingham in the UK told LiveScience in an email. This observation “forces us to rethink where and under what conditions these systems emerge.”
Einstein’s Ripples
In January 2020, scientists discovered the first convincing phenomenon Evidence that a black hole swallows a neutron star — the ultra-dense, collapsed core of a once massive star — resulting in the creation of a new black hole with a mass roughly 13 times the mass of Earth’s sun.
Although the event occurred about a billion light-years away from Earth, researchers measured the properties of both objects using a pair of gravitational waves. These space-time ripples were released by extreme cosmic collisions and are the first Einstein’s theory of relativity predicts. Researchers using the United States’ 1,900-mile (3,000-kilometer) Laser Interferometer Gravitational-Wave Observatory (LIGO) detected two waves that arrived 10 days apart. The first wave, labeled GW200105, is the focus of the new study.
The two LIGO gravitational wave observatories in Washington state and Louisiana are about 1,880 miles (3,030 kilometers) apart. This allows scientists to measure millisecond differences in gravitational wave signals. |Image source: Virgo cooperation/CCO 1.0
Using new models developed by the University of Birmingham’s Institute of Gravitational Wave Astronomy, and complementary data from Italy’s Virgo Interferometer gravitational wave detector, the team improved their measurements of the ripples in space-time and found that some of the original assumptions were wrong. For example, early studies of GW200105 underestimated the mass of the black hole and overestimated the mass of the neutron star. These values have now been corrected.
What’s more, previous research also assumed that the black hole-neutron star system that caused the collision had a perfectly circular orbit, as is the case in pairs like this. New research rules out this possibility with 99% certainty, while also casting doubt on the origins of the system.
The circle is broken
Black holes and neutron stars both form when once-powerful stars run out of fuel and collapse into dense remnants. In some cases, two remnants may fall into a shared binary orbit, slowly pulling the objects toward a catastrophic collision.
“Normally, neutron star-black hole binaries are thought to form from an isolated pair of massive stars that evolve together until one becomes a black hole and the other a neutron star,” Schmidt told Live Science. “However, this formation path predicts that when these objects are close enough for LIGO and Virgo to detect them, their orbits should be almost perfectly circular. Therefore, eccentric orbits so closely spaced are difficult to reconcile with this standard scenario.”
To paint a clearer picture of the doomed system’s orbit, the new analysis looks at two underexplored properties: eccentricity (the ellipticity of the system’s orbit, as Moon’s elliptical orbit revolves around the Earth) and precession (how an object’s axis of rotation changes or oscillates over time). Researchers say this is the first time scientists have simultaneously analyzed the two characteristics of a black hole and a neutron star merger.
The team found that the system’s orbit was highly eccentric (elliptical) but had no convincing evidence of precession. The team says this means the system’s strange egg-shaped orbit has nothing to do with changes in its axis of rotation. Instead, it was likely imprinted on the system long before its death—perhaps due to the gravitational pull of other objects in its environment.
Study co-author ‘The tracks give it all away’ Geraint PlattenRoyal Society University Fellows at the University of Birmingham said in a report statement. “Its elliptical shape before the merger suggests that the system did not quietly evolve in isolation, but almost certainly formed through gravitational interactions with other stars or possibly a third companion star.”
A “new window” to the universe
This evidence of an elliptical orbit is the first of its kind in a black hole-neutron star system.
Related stories
—Gravitational waves reveal first merger between neutron star and mysterious object
– ‘Impossible’ black hole collision pushes relativity theory to breaking point – scientists finally understand how it happens
—Stephen Hawking’s long-controversial black hole theory finally confirmed — scientists “hear” two event horizons merge into one
While the exact mechanism behind it remains a mystery, its very existence demonstrates that there is no one-size-fits-all explanation for how these systems form, and points to an emerging gap in our understanding of these extreme objects.
Closing the gap requires new models based on more unusual gravitational wave signals from across the universe. Finding these weak signals may require new technologies, such as the upcoming Space-based Laser Interferometer Space Antenna (LISA) detectorcurrently under construction.
“Future gravitational wave detectors, both on the ground and in space, will open a whole new window on the universe,” Schmidt concluded. “They will be much more sensitive than existing instruments, allowing us to detect fainter, more distant sources and even entirely new types of gravitational wave signals that we cannot detect today.”
Editor’s note: This article was updated at 10:15 a.m. March 11 to link to published research.
