Gravitational Waves from Second Neutron-star Collision Observed

Scientists with the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo collaborations have announced the detection of a second pair of neutron stars that went bump in the night. The announcement was made at the meeting of the American Astronomical Society in Honolulu and the results will appear in an upcoming issue of Astrophysical Journal Letters.

The merger is the first bona fide event of the LIGO detector’s third observing run (the second observing run to be conducted with the company of the Virgo detector in Italy).

Since the beginning of this (still ongoing) observing run in April 1, 2019, 43 unretracted alerts of gravitational-wave events have been sent out to the astronomy community, dwarfing the 10 mergers announced in the catalog of the first two observing runs. The observing run will continue until April 30, 2020, and a full accounting of the events seen in the first half of the year of observations are expected to come out around April.

Neutron Stars Collide

The new event, GW 190425, gives astronomers plenty of food for thought in the meantime. However, despite more than 100 attempts to catch a light-emitting counterpart to the gravitational waves — at wavelengths ranging from radio through visible to X-rays and gamma-rays — astronomers came up empty. This is unlike the first detection of a neutron star merger, GW 170817, where dozens of follow-up observations were able to pick up the explosion of material around the colliding neutron stars, known as a kilonova.

The new surge of gravitational waves was spotted on April 25, 2019, and marks the first event to be seen with only a single detector. LIGO’s Hanford detector was unfortunately offline at the time, and while Italy’s Virgo detector was online, its reach is substantially smaller, extending “only” to 130 million light-years. So LIGO’s Livingston detector was the only one to record the event between 290 million and 744 million light-years away — much farther than the first neutron star merger, which was 150 million light-years away.

A single detector isn’t able to triangulate sources as well as two or three detectors. Between the larger sky area to cover and the extreme distance to the source, it’s no surprise that follow-up was difficult. Nevertheless, astronomers gleaned insights from the gravitational waves alone.

A Pair of Neutron Stars . . . or a Neutron Star-Black Hole Collision?

Astronomers with LIGO used the gravitational-wave signal to “weigh” the neutron stars. One was 1.1 to 1.7 times the Sun’s mass; the other was heavier, between 1.6 and 1.9 solar masses. And, if the LIGO team doesn’t assume any prior knowledge about the second neutron star’s spin, their mathematical modeling suggests it could even have up to 2.5 solar masses. That would put it within black hole range, making this the first neutron star-black hole merger.

However, the LIGO team insists that the involvement of a black hole remains a more exotic possibility — due to the range of estimated masses, the more likely scenario remains the neutron star collision. While a black hole can technically have a mass a little more than double the Sun’s, no black hole has ever been observed with such a low mass.

Numerical Relativity Simulation: T. Dietrich (Nikhef), Wolfgang Tichy (Florida Atlantic University) and the CoRe-collaboration; Scientific Visualization: T. Dietrich (Nikhef), S. Ossokine, and A. Buonanno (Max Planck Institute for Gravitational Physics)

Still, even if the event marks the collision of two neutron stars, the pair are strange — and quite unlike the first neutron star collision LIGO and Virgo observed. That’s because the sum of the neutron stars’ masses before the merger makes for a much heavier system than any other neutron star pair ever observed in our galaxy.

“No matter what the source of this signal is,” says Katerina Chatziioannou (Flatiron Institute), “it challenges our understanding of how these systems form and merge.”

Only the Beginning

Although we are nearing the end of the third observing run, this is far from our last chance at detecting a neutron star collision.

In LIGO’s third observing run, which started April 1, 2019, and will continue until April 30, 2020, the detectors in Livingston, Louisiana, and in Hanford, Washington, could sweep up sources up to 420 million and 360 million light-years away, respectively. Virgo sees out to 180 million light-years. Once data collection stops in April, though, the detectors will undergo upgrades to their sensitivity.

Meanwhile, the KAGRA detector in Japan is coming online, and two other detectors in India and Germany will see first their first signals within a few years. By the end of the decade, the network of gravitational-wave detectors will be able to see events out to a billion light-years away.

Astronomers expect this network to detect up to hundreds of black hole mergers every month, and up to a dozen neutron star mergers a year. “Going from three detectors to five within the decade, we will go from some 50 events to more than 10,000,” says Aidan Brookes (LIGO).

Not only will this near-future network be extraordinarily sensitive to the gravitational-wave universe, it will also be able to pinpoint locations to a sky area 10 times smaller, which will make follow-up observations more feasible. The delay for Processing of the signals will also be sped up, so that alerts can be sent out mere seconds after an event occurs, compared to the several minutes required now.

“The take-away message,” says LIGO Executive Director David Reitze (Caltech), “is buckle your seat belts!”