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Telling apart two types of gravitational-wave signals

What is needed to distinguish between binary black hole and neutron star black hole mergers?

August 02, 2021

As the number of gravitational wave observations increase, many questions arise; one of growing importance is this: when a gravitational-wave from a low mass merger is detected without a concurrent electromagnetic signal how can one distinguish a light weight binary black hole merger from a neutron star black hole merger? Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute Hannover, AEI), at Leibniz University Hannover, and at Radboud University have used a simulated search for such gravitational-wave signals to answer this question. Their results were published in The Astrophysical Journal today. Third-generation detectors such as Cosmic Explorer and the Einstein Telescope will be able to make a clear-cut distinction under favorable circumstances. This shows that new instruments will be needed for precise gravitational-wave astrophysics.

Gravitational-wave signals are like fingerprints. They allow astrophysicists to find the culprit for a signal they detect. Moreover, the researchers can infer many properties of the source that emitted the signal. All signals observed so far by LIGO and Virgo came from collisions of compact objects, such as black holes and neutron stars. Their masses and spins and the nature of the merging objects can be extracted from the gravitational wave.

Gravitational-wave look-alikes

Simulation of a neutron-star–black-hole coalescence in which the neutron star is tidally disrupted during the merger.

Some types of signals can be harder to tell apart than others. “If you only consider the masses of the objects, a binary black hole merger where one black hole is very small can be difficult, if not impossible, to distinguish from a neutron star black hole merger. We don’t know with certainty the maximum mass of a neutron star or the minimum mass of a black hole,” says Stephanie Brown, a PhD student at the Max Planck Institute for Gravitational Physics and at Leibniz University Hannover, lead author of the study published in The Astrophysical Journal today.

If mass alone is not sufficient to unambiguously distinguish the two merger types, then what is? Neutron stars – unlike black holes – consist of matter. Therefore, there are two ways in which the presence of a neutron star in the merger can be determined: the presence of an electromagnetic counterpart or the imprint of matter effects on the gravitational wave itself.

If there is an electromagnetic signal such as a flash of gamma-rays associated to the event, there must be matter in the system, and one of the two merging compact objects must be a neutron star. In most cases, it is unlikely that electromagnetic radiation will be detected for such events: perhaps because the distance is large and the light is too faint or because it is not directed at Earth at all.

The fingerprints of deformed neutron stars

“Independent of their elusive light show, neutron stars leave behind other traces in the gravitational-wave signal. They will be deformed by the enormous gravitational pull from their black hole partners well before the merger,” says Collin Capano, a researcher at AEI Hannover, and co-author of the study. “These ‘tidal effects’ – similar to the tides induced by the Moon on Earth – leave characteristic, yet faint, fingerprints in the gravitational-wave signal. Black holes on the other hand have no tidal effects,” Capano adds.

Simulation of GW200115 – a Neutron-Star–Black-Hole merger

The movie represents a system compatible to GW200115, i.e., the black hole mass was chosen to be 6.1 solar masses and the neutron star mass was set to 1.4 solar masses. Both objects were non-spinning.

So far it has been impossible to unambiguously detect the tidal effects from neutron star deformation in any of the observed LIGO-Virgo signals. The international team led by Stephanie Brown estimated just how difficult this task is. They simulated gravitational-wave signals from neutron star black hole mergers at different distances to Earth for both current and future detector configurations. Building on their earlier work, they predicted the amount a companion black hole will tidally deform a neutron star by using nuclear physics models that describe the behavior of the matter inside the star.

They considered the present LIGO and Virgo detectors at their design sensitivities, their near term upgrades (LIGO A+ and LIGO Voyager), and also Cosmic Explorer, a third generation detector. They identified those cases in which the data analysis of their simulated search was able to provide decisive evidence for the presence of tidal effects.

Third-generation detectors are required

Artist's impression of the underground Einstein Telescope, a planned third-generation gravitational-wave detector.

Their results: Only when the black hole was relatively light-weight (about 4 times the mass of the neutron star, equivalent to five times the mass of our Sun) was there a chance of clearly observing tidal effects in the gravitational-wave signal. This is because the tidal effects are stronger if the black hole’s mass is more similar to the neutron star’s mass. Even at design sensitivity the current detector network, LIGO A+, and even LIGO Voyager will not be able to distinguish between the two types of merger signals based on the detection or non-detection of tidal effects.

According to the study, only the third-generation detector Cosmic Explorer would be able to tell apart the two types of signals, but even this much more sensitive detector will likely need an event to take place very close to Earth (130 million light-years, as close as GW170817, the first binary neutron star merger observed by LIGO and Virgo).

“We find that third-generation detectors like Cosmic Explorer can detect tidal effects and can use that to distinguish between binary black hole mergers and neutron star black hole mergers,” says Brown. “This underlines the necessity of third generation detectors for precise gravitational-wave astronomy and astrophysics.”

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