Gravitational wave observatories detect rare mergers of black holes with neutron stars for the first time

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Image of a numerical relativity simulation of the MAYA collaboration of an NSBH binary fusion, showing the perturbation of the neutron star. Credit: Deborah Ferguson (UT Austin), Bhavesh Khamesra (Georgia Tech) and Karan Jani (Vanderbilt)

Salvatore Vitale describes how gravitational wave signals suggest that black holes have completely devoured their companion neutron stars.

Recently, an international team of scientists, including researchers at MIT, announced the detection of a new type of astrophysical system: a collision between a black hole and a neutron star, two of the densest and most exotic objects in the universe.

Scientists have detected collision signals from black holes and neutron stars, but so far have not confirmed the fusion of a black hole with a neutron star. In a study published today in Letters from the astrophysical journal, scientists report observing not one, but two of these rare events, each of which emitted gravitational waves that reverberated through much of the universe before reaching Earth in January 2020, just 10 days from ‘interval.

The gravitational waves of the two collisions were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) of the National Science Foundation in the United States and by Virgo in Italy. The events are named GW200105 and GW200115, for the date on which each gravitational wave was observed. The two signals represent the last moments as a black hole and a neutron star coiled into a spiral and merged. For GW200105, the black hole is estimated to be around 9 times the mass of the sun, with a companion neutron star of around 1.9 solar mass. It is estimated that the two objects merged around 900 million years ago. GW200115 is the product of a black hole of 6 solar masses, which collided with a neutron star about 1.5 times the mass of our sun, about 1 billion years ago. In both events, the black holes were large enough to likely devour their neutron stars completely, leaving very little or no light behind them.

LIGO team member Salvatore Vitale, assistant professor of physics at MIT and member of the Kavli Institute of Astrophysics and Space Research, spoke with MIT News on the rarity of the two detections and on what mergers of black holes and neutron stars can reveal about the evolution of stars in the universe.

Salvatore Vitale

Salvatore Vitale, assistant professor of physics at MIT and member of the LIGO scientific collaboration. Credit: Courtesy of MIT Kavli Institute for Astrophysics and Space Research

Question: Tell us about these extreme and elusive systems. In general, what was known about collisions involving black holes and neutron stars before these detections?

A: Neutron stars and black holes are left behind by massive stars once they run out of nuclear fuel. Since a large portion of the stars in the universe are in binary systems, one would expect all possible pairwise combinations to exist: two neutron stars, two black holes, or one neutron star and one black hole. .

Neutron star binaries have been known for decades, discovered using electromagnetic radiation. Black hole binaries were first observed in 2015, with the detection of gravitational waves GW150914. After that, gravitational wave detectors such as LIGO and Virgo discovered dozens of binary black holes and two binary neutron stars. However, binaries with a neutron star and a black hole (NSBH) had never been found using electromagnetic radiation, nor with gravitational waves, at least until now.

Question: What can you say from the signal about the possible scenarios that could have brought these objects together in the first place?

Unfortunately, not much at this point! The most likely scenario is that the two objects of each binary have been together all their lives, as giant stars. While running out of fuel, they suffered powerful explosions called supernovae, leaving behind a neutron star and a black hole. The two objects of the binary then got closer and closer, because they lose energy by emitting gravitational waves, until they collide. LIGO and Virgo saw the final seconds lead to a collision.

Theoretically, these mergers could produce light, which is extremely exciting! However, for this to happen, some material must be left around the system after the collision. Unfortunately, if the black hole is too massive, or if it doesn’t spin around its axis fast enough, it will swallow the neutron star entirely before it has a chance to tear apart. When this happens, no matter is left behind, and therefore no light. This is what could have happened with these two detections of gravitational waves.

However, it is also possible that light was in fact emitted but was not detected by the telescopes that tracked these systems. This is because their position in the sky – based on gravitational wave data – was rather uncertain, implying that telescopes might not have had a chance to find the electromagnetic counterpart before it vanished. .

Question: What is the overall significance of this new detection? And what avenues does this open up in our understanding of the universe?

A: These two systems are important because they are the first clear discovery of neutron star black hole binaries, a type of source that has never been observed, with electromagnetic or gravitational waves. He tells us that these systems exist but are rarer than binary neutron stars. With only two sources, the numbers are still very uncertain, but roughly speaking: for 10 neutron star binaries, there is an NSBH merger.

The fusion rate we calculated using these two signals, and the properties of compact objects, will be of great help to astronomers and modelers trying to understand the formation and evolution of NSBHs.

In fact, as none had been observed before, there was no good way to refine the theoretical and numerical models. These models are complicated and depend on many physical parameters of the binary system, as well as its history. For example: how violent is the explosion of the supernova which leaves behind neutron stars and black holes? Is it so powerful that it can completely destroy the binary system?

Finally, having access to NSBH mergers will allow us to refine these models, and therefore our understanding of the formation and evolution of compact objects.

To learn more about this research:

Reference: “Observation of gravitational waves from two coalescences of neutron stars and black holes” by R. Abbott, TD Abbott, S. Abraham, F. Acernese, K. Ackley, A. Adams, C. Adams, RX Adhikari, VB Adya, C. Affeldt […] AB Zimmerman, Y. Zlohower, ME Zucker, J. Zweizig and the scientific collaboration LIGO, the collaboration Virgo and the collaboration KAGRA, June 29, 2021, Letters from astrophysical journals.
DOI: 10.3847 / 2041-8213 / ac082e


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