Frontiers Physics World  July 2017

LIGO spots third black-hole merger

Spinning around An artist’s impression of a black-hole binary. (LIGO / Caltech / MIT / Sonoma State / Aurore Simonnet)

A third gravitational wave has been detected at the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US. The wave was produced by two black holes that merged about 3 billion light-years from Earth. By studying the characteristic “chirp and ringdown” signal, LIGO physicists worked out that the two initial black holes weighed in at 31 and 19 solar masses, while the merged object was 49 solar masses – the difference having been radiated away in the form of gravitational waves.

The new event, dubbed GW170104, was observed on 4 January 2017. While the previous two detection events were also produced by black-hole mergers, researchers think this is the first in which the spin of one of the merging black holes could have been pointing in the opposite direction to the orbital rotation of the system.

When the black holes coalesce, the total rotational velocity of the merged black hole cannot exceed a certain upper limit. So if the spins of the two merging black holes point in the same direction as the orbital spin, some of the orbital angular momentum must be discarded to meet this criterion before the merger can occur. This is done by emitting additional gravitational waves before the merger.

In LIGO’s first event (GW150914), recorded on 14 September 2015, researchers believe the spins of both black holes were aligned with the orbital angular momentum. Meanwhile, for the second detection (GW151226), recorded on 26 December 2015, there is some evidence that the spin of one of the black holes was at an angle to the orbital angular momentum but still had a component in that direction. For this third event, however, it is thought that the spin of at least one of the black holes was at an angle and had a component in the opposite direction to the orbital angular momentum.

According to Bangalore Sathyaprakash of Cardiff University in the UK, the relative orientations of the spin and orbital angular momenta of a binary black hole provide important information about how the system formed. If they are aligned, it is likely that the system developed in isolation as two large stars that then collapse to create a binary black hole. Misalignment suggests that the black holes formed separately and then came together to create a binary system (Phys. Rev. Lett. 118 221101).