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EI2GYB > ASTRO 15.10.25 11:03z 46 Lines 4992 Bytes #29 (0) @ WW
BID : 46574_EI2GYB
Subj: When Black Holes Don't Play by the Rules
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Sent: 251015/0936Z 46574@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
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When Black Holes Don't Play by the Rules
Black holes are the remains of dead supermassive stars. When a star reaches the end of its life, one of two things will happen, either the thermonuclear pressure from fusion will cause the star's outer layers to expand or gravity wins and the star collapses. In this latter case, what gets left behind is often a black hole, an object whose conditions are so extreme that even light cannot escape.
When black holes orbit each other in isolation over long periods, gravitational radiation gradually circularises their orbits. By the time they reach the final stages before merger, when gravitational wave detectors like LIGO and Virgo can observe them, any eccentricity should have been smoothed away entirely. Finding a binary black hole system that retains a noticeably eccentric orbit at this late stage suggests something unusual happened in its past.
An international team led by Dr Isobel Romero-Shaw from Cardiff University's Gravity Exploration Institute used detailed simulations to explore three possible formation scenarios for this peculiar binary. Their analysis points toward formation in either a three body star system or within a densely packed environment like a star cluster. Both scenarios can drive orbital eccentricity through gravitational interactions with nearby objects.
Eccentricity serves as smoking gun evidence that a binary black hole system didn't form in complete isolation. If two black holes evolved together as a bound pair from the beginning, with no external influences, current instruments would detect them with circular orbits. The presence of detectable eccentricity means external forces must have shaped their orbital evolution, either through interactions with a third massive object or through dynamic encounters in crowded stellar environments.
Multiple independent research groups identified GW200208_222617 as eccentric using different analysis techniques during LIGO and Virgo's third observational run. By examining the event's other characteristics and comparing them with theoretical predictions for various formation theories, the team narrowed down how this binary most likely came together. While they cannot yet pinpoint the exact formation process with complete certainty, the eccentric orbit rules out the simplest scenario where two stars evolved together in isolation, eventually collapsing into black holes that remained gravitationally bound throughout.
Understanding how binary black holes form and evolve has become increasingly important as gravitational wave detections accumulate. The LIGO-Virgo-KAGRA collaboration has now detected hundreds of black hole mergers, and each one carries clues about the environments and processes that brought those systems together. Some binaries likely formed through isolated binary stellar evolution, where two massive stars were born together and remained paired throughout their lives. Others probably assembled through more chaotic processes in dense stellar environments where chance encounters and gravitational interactions dominate.
The research team hopes their study will motivate theorists to develop more sophisticated gravitational models that can detect and characterise eccentricity with greater confidence and accuracy. As detector sensitivity continues improving and more eccentric mergers emerge from the data, astronomers will gain clearer insights into the diverse ways through which binary black holes form throughout the universe.
Source : Likely origins of black hole collision with 'squashed' orbital path revealed
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