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LIGO spots a second gravitational wave

17 June 2016
The Laser Interferometer Gravitational-Wave Observatory collaboration has now fully scoured its first data run in its attempt to find evidence for black hole mergers.

Gravitational waves are distortions of spacetime that propagate away from accelerating masses at light speed. Like electromagnetic waves, they carry energy; indeed, the first gravitational waves were inferred from the steady loss of energy in pulsars. In February of this year, LIGO reported a direct observation of the spacetime-perturbing effects of a gravitational wave. Nearly simultaneously, in Michelson interferometers located in Livingston, Louisiana, and Hanford, Washington, laser light shot down the 4-km-long arms of the interferometers ceased to interfere nearly destructively, as it normally does. The changing interference pattern showed that the relative lengths of the interferometer arms were changing by a factor of 10−21 as a gravitational wave rapidly passed through the devices. The best astrophysical explanation for the terrestrial observations is a merger of two black holes, one of 36 solar masses (M) and one of 29 M; gravitational radiation carries away 3 M of energy and leaves a final black hole with a mass of 62 M.

This artist's illustration depicts the merging black hole binary systems for GW150914 (lower left) and GW151226 (upper right). The black hole pairs are shown together here, but were actually detected at different times and on different parts of the sky. The images have been scaled to show the difference in black hole masses. In the GW150914 event, the black holes were 29 and 36 times that of our Sun, and in GW151226, the two black holes weighed in at 14 and 8 solar masses. Image credit: LIGO/A. Simonnet.

This artist's illustration depicts the merging black hole binary systems for GW150914 (lower left) and GW151226 (upper right). The black hole pairs are shown together here, but were actually detected at different times and on different parts of the sky. The images have been scaled to show the difference in black hole masses. In the GW150914 event, the black holes were 29 and 36 times that of our Sun, and in GW151226, the two black holes weighed in at 14 and 8 solar masses. Image credit: LIGO/A. Simonnet.

In San Diego, California, at the American Astronomical Society meeting this week, Gabriela González, speaking for the LIGO team, reported that LIGO has confirmed a second gravitational wave in the data of their initial run (12 September 2015–19 January 2016) and identified a third candidate wave but has found no other evidence for black hole mergers with total masses below 100 M. The new confirmed wave was spotted on 26 December 2015 Universal Time—Christmas Day in the US, where intrepid LIGO workers forewent their holiday celebrations to maintain the interferometer. The event, which lasted for about 1 second, is best explained as arising from a merger of black holes with 14 and 7.5 M, with 1 M of energy liberated in gravitational waves. Although determining black hole spin is particularly challenging, the LIGO researchers established that at least one of the two merging black holes was spinning.

The black holes leading to the Christmas observation are lighter than those responsible for the February wave, and the changes in the interferometer arm lengths were correspondingly shorter. To see that they had a signal at all, the LIGO researchers systematically looked for correlations in the noisy interference pattern and each of the elements in a thousands-strong library of interference patterns that would correspond to mergers of black holes with various masses and spins.

From their observation of two, maybe three, black hole mergers, the LIGO team estimates that the cosmic frequency of roughly solar-mass black hole mergers is 9–240 Gpc−3 s−1. (Typically galaxies are separated by a few megaparsecs.) To convert that number into absolute density of black hole binaries, one would need a model of how black hole binaries form. Many models exist, but they fall into two classes: Black hole binaries form in situ from stellar binaries, or they form from dynamical interactions in dense stellar systems. The two events reported to date by LIGO are not enough to prefer any particular formation mechanism. But as Frederic Rasio (Northwestern University) pointed out in a press conference following this week’s LIGO announcement, spin measurements can be particularly informative in deciding between the two main classes. Black hole binaries formed in situ, he explained, should have aligned spins for the two black holes. Dynamical processes lead to randomized spins.

LIGO’s second run is scheduled to begin late this year. The observatory will make use of its improved sensitivity and a longer run time of six months. Moreover, the VIRGO observatory in Europe should be up and running; that third eye on the sky will greatly improve the ability to localize black hole mergers. Of course, black hole mergers are not the only mechanisms for forming gravitational waves, and they’re not the only mergers of astrophysical import. Neutron star mergers are of great interest, but their signal is simply too weak to have a good chance of being spotted at the moment. Observing the gravitational waves from a neutron star merger, says González, will probably have to wait for LIGO’s third data run, though she adds, “We can always be lucky.” Beginning in late 2017, that experiment will operate for nine months and will incorporate a new round of sensitivity upgrades.

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