Today, an international scientific collaboration released the largest catalog ever of collisions involving black holes and neutron stars, raising the total to 90 events. The results suggest that intermediate-mass black holes are more common than scientists previously thought. The catalog also includes the second discovery of an intriguing object that seems too small to be a black hole, yet too large to be a neutron star.
Scientists with the LIGO-Virgo-KAGRA Collaboration picked up gravitational waves, ripples in space time, from these collisions using an international gravitational-wave observatory network. The collaboration includes Deirdre Shoemaker, Pablo Laguna and Aaron Zimmerman, faculty members in the Department of Physics at The University of Texas at Austin.
"Finding these intermediate-mass black holes points to the fact that we don't understand everything about how black holes form and grow," said Shoemaker, whose research group uses the equations of General Relativity to predict the waveforms generated when these objects collide.
In a paper published Nov. 7th on the preprint server ArXiv, the team has detected a further 35 gravitational wave events since the last catalog release in October 2020, bringing to 90 the total number of observed events since gravitational-wave observations began.
Of the 35 events detected, 32 were most likely to be black hole mergers – two black holes spiraling around each other and finally joining together, an event which emits a burst of gravitational waves and produces a larger black hole.
Several of the resulting black holes that formed from these mergers exceed 100 times the mass of our Sun and are classed as intermediate-mass black holes. This type of black hole has long been theorized by astrophysicists. These most recent LIGO-Virgo-KAGRA observations confirm that this new class of black holes is more common in the universe than previously thought.
The UT Austin team also includes postdoctoral researchers Jacob Lange, Deborah Ferguson and Miguel Gracia; graduate student Richard George; and others. Former postdoctoral researcher Cody Messick is also an author on the paper describing the latest observations.
An international effort
The catalog updates the list of all gravitational-wave events observed to date with events observed between November 2019 and March 2020, using three international detectors:
●The two Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Louisiana and Washington state in the US.
●The Advanced Virgo detector in Italy.
Data from these three detectors have been carefully analyzed by an international team of scientists from the LIGO Scientific Collaboration, the Virgo Collaboration and the Japan-based KAGRA Collaboration.
What else did LIGO-Virgo-KAGRA see?
Three of the 35 events added to the catalog were potentially not mergers of two black holes.
One event came from two objects merging where one was almost certainly a black hole (with a mass around 24 times the mass of our Sun) but the other was either a very light black hole or a very heavy neutron star of around 2.8 times the mass of our Sun. Scientists have deduced it is most likely to be a black hole but cannot be entirely sure. A similar ambiguous event was discovered by LIGO and Virgo in August 2019, and was included in a previous version of the catalog published in 2020. The mass of the lighter object is puzzling, as scientists expect that the most massive a neutron star can be before collapsing to form a black hole is around 2.5 times the mass of our Sun. However, no black holes had been discovered with electromagnetic observations with masses below about 5 solar masses. This led scientists to theorize that stars do not collapse to make black holes in this range. The new gravitational wave observations indicate that these theories may need to be revised.
Aaron Zimmerman said if these are small black holes, they likely formed when stars exploded in so-called supernovae. These events are very difficult to model and understand.
"If it is common to get black holes right at the edge of [the mass] where neutron stars collapse under their own weight, that will tell us something new about what is going on inside stars when they explode," he said. "I think we need to see a few more of these objects to know more, but having a second example of these mysterious systems gives me confidence that there are many more out there to be discovered."
Finally, two of the 35 events were likely to be neutron stars and black holes merging – a much rarer event, and a type that was only discovered in the most recent observing run of LIGO and Virgo (previously announced in June 2021).
Of these rare neutron star and black hole mergers, one event seems to show a massive black hole (about 33 times the mass of our Sun) with a very low-mass neutron star (about 1.17 times the mass of our Sun). This is one of the lowest-mass neutron stars ever detected, either using gravitational waves or electromagnetic observations.
The masses of black holes and neutron stars are key clues to how massive stars live their lives and die in supernova explosions.
The future of the field
The LIGO and Virgo observatories are currently undergoing improvements before the upcoming fourth observing run, expected to begin in the latter half of next year.
The KAGRA observatory in Japan will also join the next full observing run. Located deep under a mountain, KAGRA completed a successful first observing run in 2020, but has yet to join LIGO and Virgo in making joint observations. With more detectors, potential events can be located more accurately.
As more detections are confidently added to the gravitational wave catalog, researchers are learning more and more about these astronomical phenomena.
Before the next observing run, scientists will be busy further analyzing the existing information, learning more about neutron stars and black holes, and searching for new types of signals hidden in the data.
Even farther in the future, the European Space Agency is planning a space-based gravitational wave observatory called the Laser Interferometer Space Antenna (LISA), which Shoemaker, Laguna and Zimmerman plan to participate in. This week the National Academy of Sciences released a once-a-decade report called Pathways to Discovery in Astronomy and Astrophysics for the 2020s, or "Astro2020," which recommends support for a suite of gravitational wave experiments including LISA.
This article is based on a press release from the LIGO Scientific Collaboration.
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