How do binary black holes form?

Unravelling binary physics

We are making progress in understanding how two black holes can come together and merge.

During its first four months of taking data, Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) detected gravitational waves from two mergers of pairs of black holes, GW150914 and GW151226, along with the statistically less significant black hole merger candidate LVT151012. More recently, the discovery of another binary black hole merger, GW170104, has been announced.

These confirmed a major prediction of Albert Einstein's 1915 general theory of relativity and opened an unprecedented new window onto the cosmos. However, we still do not know how such pairs of merging black holes form.

In order for the black holes to merge within the age of the Universe by emitting gravitational waves, they must start out close together by astronomical standards, no more than about a fifth of the distance between the Earth and the Sun. However, massive stars, which are the progenitors of the black holes that LIGO has observed, expand to be much larger than this in the course of their evolution. The key challenge, then, is how to fit such large stars within a very small orbit. Several possible scenarios have been proposed to address this.

Figure 1 from Stevenson et al., Nature Communications 8, 14906 (2017) updated to include GW170104

In a paper published in Nature Communications, we have have shown that all three observed events can be formed via the same formation channel: isolated binary evolution via a common-envelope phase. In this channel, two massive progenitor stars start out at quite wide separations. The stars interact as they expand, engaging in several episodes of mass transfer. The latest of these is typically a common envelope - a very rapid, dynamically unstable mass transfer that envelops both stellar cores in a dense cloud of hydrogen gas. Ejecting this gas from the system takes energy away from the orbit. This brings the two stars sufficiently close together for gravitational-wave emission to be efficient, right at the time when they are small enough that such closeness will no longer put them into contact. The whole process takes a few million years to form two black holes, with a possible subsequent delay of billions of years before the black holes merge and form a single black hole.

The simulations have also helped the team to understand the typical properties of the stars that can go on to form such pairs of merging black holes and the environments where this can happen.

The simulations behind the paper are now available.



Summaries and data releases for our work

Hierarchical analysis of gravitational-wave measurements of binary black hole spin–orbit misalignment

Authors: Stevenson, S., Berry, C. P. L. & Mandel, I.
Journal: MNRAS
arXiv: 1703.06873 [astro-ph.HE]

LIGO recently detected gravitational-waves from 4 likely mergers of black holes, GW150914, LVT151012, GW151226 and GW170104. As LIGO detects more and more black hole mergers, we will be able to start learning about how binary black holes form. Here we study one possible fingerprint of binary black holes formation— the angles between the black holes' spins axes and the direction perpendicular to the binary's orbital plane.

Two possible families of ways to form a binary black hole exist:

  • 1. The evolution of a pair of stars away from anything else (isolated binary evolution). This predicts that the tilt angles are typically small; binaries are said to be aligned.
  • 2. Formation through many dynamical encounters in a dense stellar environment such as a globular cluster. In this case, black holes typically have their spins misaligned with the orbit.
We show that with as few as 5 gravitational-wave observations we could confidently distinguish between the extreme cases of all binary black holes forming through one of these two channels, assuming black holes spin reasonably rapidly. With around 100 detections we will start to be able to measure the fractions of binary black holes forming in each way (assuming that there is a mixture). The plot above shows in blue measurements of the fraction of binary black holes forming dynamically as a function of the number of gravitational-wave observations. The animation below shows how the measurements for a mixture of four different models (λ1, λ3 and λ4 are the fractions for models of isolated binary evolution with different assumptions about the amount of misalignment, and λ2 is the fraction for dynamically formed binaries).

Model-independent inference on compact-binary observations

Authors: Mandel, I., Farr, W. M., Colonna, A., Stevenson, S., Ti┼ło, P. & Veitch, J.
Journal: MNRAS
arXiv: 1608.08223 [astro-ph.HE]

The increasing number of gravitational-wave observations promises to lead to insights into stellar binary evolution. One approach is to compare specific evolution models against observations; however, given the uncertainty in the models, a model-independent approach to astrophysical inference is desirable. In an earlier paper, we advocated clustering in a multi-dimensional parameter space. Here, we propose a practical procedure to clustering observations which are subject to significant measurement errors. We apply our method to a mock data set of population-synthesis predictions for compact binaries incorporating realistic measurement uncertainties. We demonstrate that a few tens of observations are sufficient to accurately distinguish subpopulations of sources. The example shows what we could infer with varying numbers of observations. The plots shows inferred population density for different mass bins assuming a Gaussian process prior; you can make out clusters for binary neutron stars (in the bottom left corner), neutron star–black hole binaries (along the edges) and binary black holes (in the centre).

Distinguishing compact binary population synthesis models using gravitational wave observations of coalescing binary black holes

Authors: Stevenson S., Ohme, F. & Fairhurst, S.
Journal: ApJ
arXiv: 1504.07802 [astro-ph.HE]

Advanced LIGO began the hunt for gravitational waves from colliding black holes in 2015, with Advanced Virgo to join in 2017. Black holes are the remains of stars with masses tens of times the mass of the Sun. One way for two of these black holes to collide is to live and die as a binary. These stars constantly shed mass in stellar winds, violently strip mass from one another, explode as supernova, and even engulf one another in common-envelope events. Most of these processes have so far been studied using systems we can see, such as X-ray binaries, but remain poorly understood. However, they also leave an imprint on the masses of the black holes. Therefore, by comparing the distribution of masses of binary black holes that Advanced LIGO and Advanced Virgo find with a prediction from a model, we can learn about these extreme evolutionary processes and the lives of massive stars. The chequerboard type plot shown on the left compares simulated observations from each model in a set of models to all of the others in turn. Darker squares indicate that models are similar, and lighter squares indicate that we can tell models apart. The dark strip on the diagonal shows that models do look like themselves. Since there are lots of lighter squares, it is possible to distinguish between many models. We should be able to learn lots about binary evolution over the next couple of years!