The universe-shaking collision of a black hole and a neutron star just led astronomers to a strange type of orbital interaction never seen before, forcing them to rethink their theories.
Before the two extremely dense objects crashed and combined, they first swirled around each other in an eccentric, oval shape that resembled the vortices of a Spirograph, researchers reported March 11 in The Astrophysical Journal Letters.
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“The fact that this system remains eccentric at the very end of its life is essentially a smoking gun signal that at least some binary neutron star black holes must form differently (than theory predicts),” study co-author Patricia Schmidtan associate professor of physics and astronomy at the University of Birmingham in the United Kingdom, told LiveScience in an email. This observation “forces us to reconsider where, and under what conditions, these systems arise.”
Einstein’s ripples
In January 2020, scientists discovered the first convincing evidence of a black hole swallowing a neutron star — the ultra-dense, collapsed core of a once-massive star — resulting in the formation of a new black hole with about 13 times the mass of Earth’s sun.
Although the event occurred about a billion light-years from Earth, the researchers measured the properties of the two objects using a pair of gravitational waves. These ripples in space-time are triggered by extreme cosmic collisions and were first predicted by Einstein’s theory of relativity. Scientists detected the two waves, which came 10 days apart, using the 1,900-mile (3,000-kilometer) Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States. The first wave, labeled GW200105, is the focus of the new study.
Using a new model developed by the University of Birmingham’s Institute of Gravitational Wave Astronomy, as well as complementary data from the Virgo interferometer gravitational wave detector in Italy, the team refined their measurements of the space-time ripple and found that some initial assumptions were wrong. For example, the previous studies of GW200105 underestimated the black hole’s mass while overestimating the neutron star’s mass. These values have now been corrected.
More importantly, previous studies also assumed a perfectly circular orbit for the black-hole-neutron-star system that led to the collision, which is often the case in pairs like these. The new research rules out this possibility with 99% certainty – and also calls into question the system’s origins.
The circle is broken
Both black holes and neutron stars form when once-powerful stars run out of fuel and collapse into dense remnants. Under certain circumstances, two remnants can fall into a shared, binary orbit that slowly pulls the objects toward a catastrophic collision.
“Canonically, neutron star-black hole binaries are thought to form from pairs of isolated massive stars that evolve together until one becomes a black hole and the other a neutron star,” Schmidt told LiveScience. “However, this formation path predicts that when the objects are close enough for LIGO and Virgo to detect them, their orbits should be almost perfectly circular. An eccentric orbit at such small separations is therefore very difficult to reconcile with this standard scenario.”
To paint a clearer picture of the doomed system’s orbit, the new analysis looked at two understudied properties: eccentricity (how oval the system’s orbit was, which elliptical orbit of the moon around the Earth) and precession (how the axis of rotation of an object changes or fluctuates over time). This was the first time scientists analyzed both properties simultaneously in a black hole and neutron star merger, according to the researchers.
The team found that the system’s orbit was highly eccentric (oval-shaped), but there was no convincing evidence of precession. According to the team, this means that the system’s oddly egg-shaped orbit had nothing to do with changes in the axis of rotation. Rather, it was most likely imprinted on the system long before it died – probably due to the gravitational pull of other objects in the environment.
“The orbit gives the game away,” study co-author Geraint Prattena Royal Society University Fellow at the University of Birmingham, said in a statement. “The elliptical shape just before the merger shows that this system did not evolve quietly in isolation, but was almost certainly shaped by gravitational interactions with other stars, or perhaps a third companion.”
A “new window” into the universe
This evidence of an oval-shaped orbit is a first among black hole-neutron star systems.
While the exact mechanism behind it remains a mystery, its existence proves that there is no single explanation for how these systems form, and points to a newly opened gap in our understanding of these extreme objects.
Narrowing this gap will require new models based on more unusual gravitational wave signals from across the universe. Finding the weak signals may require new technology, such as the upcoming one space-based Laser Interferometer Space Antenna (LISA) detectorunder construction.
“Future gravitational wave detectors, both on the ground and in space, will open a whole new window on the universe,” concluded Schmidt. “They will be far more sensitive than current instruments, allowing us to detect fainter and more distant sources, and even entirely new types of gravitational wave signals that are beyond our reach today.”
