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Black Hole Collision ¹

An historic event that heralded a new phase of astronomy was the detection of gravitational waves. According to Eldridge it is now possible to tell the story of the black hole system that generated the wave based on a numerical model of the Universe.

The first gravitational wave source was detected on 14 September 2015. The fact that the signal came from the merger of 2 black holes, each of which was about 30 times the mass of the Sun, was a surprise to some. Now it has been shown by Belczynski et al, that this issue, that such a system can arise naturally for the present understanding of how the stars in binary systems interact, and also unlock the history of the black holes from their origins as 2 massive stars.

There are other groups (Eldridge & Stanway, 2016; Hartwig et al., 2016; Lipunov, 2016) who sought to characterise the source of the gravitational waves, GW 150914, but Belczynski et al. have developed a numerical model of the Universe that allows the following of every phase of the evolution of a binary system from the formation of the Universe to the present. This model allowed them to search lists of black hole binaries to find those matching the parameters of the source of the gravitational wave. They estimated the relative probability that the source could have caused the event, and thereby which was the most likely, by tracking back the evolution of each candidate.

Belczynski et al. concluded that it is likely the black holes began as 2 stars with masses of 40-100 times that of the Sun and were formed about 2 billion years after the Big Bang. About 5 million years later these stars collapsed to form black holes, then merged 10.3 billion years later when they emitted the gravitational wave signal that was detected 1.2 billion years later. There are other possible scenarios though they are considered to be less likely.

Eldridge suggests the black holes were monsters, and it is shown that their progenitor stars would have been among the brightest and massive in the Universe. Eldridge suggests that the stars might have contributed to the reionisation of the Universe, one of the key events in the evolution of the Universe, if the proposed age for the formation of the stars is correct. It is also likely that the composition of the stars was relatively pure, consisting mostly of hydrogen and helium, and contained less than 10 % of the heavy elements, such as carbon, oxygen and iron, that pollute the Sun. It is indicated by this that the stars would have been in a small dwarf galaxy, instead of a large spiral galaxy, such as the Milky Way.

Eldridge suggests this study is important for 2 reasons. GW 150914 provides an exciting test for the theory of stellar evolution. Previously, core collapse supernovae represented the latest stage of the life of a star that could be used to constrain the nature of the progenitor stars (Smartt, 2015). Belczynski et al. have gone beyond that to the final event occurring within a stellar binary that has already survived 2 supernovae. Therefore, their work places firm constraints on stellar evolution and how stars die in supernovae. Secondly, a new way to measure the accuracy of models of star formation and cosmic evolution throughout the history of the Universe is provided by it.

Adding uncertainty to the model by Belczynski et al. are caveats and assumptions. One of the uncertainties is how massive the black hole formed by a star can be; the explosiveness of the supernovae that formed the black hole determines this. The explosive nature of massive stars is a common topic of research, and there is some evidence that black holes can form directly from stars without requiring the supernova, which is the assumption used by Belczynski et al. Though stars might also form black holes and explode. For binary systems, the nature of the final black hole system would be affected by this, and the time required for the black holes to merge.

An intermediate phase of the evolution of binary stars involves another uncertainty. The radii of the stars in a binary star system increase as the component stars evolve, the stars sometimes expanding to the size of their orbit with the result that they get in each other’s way is called the common-envelope phase. It has been found that it is typical for the star that is the first to form loses its outer gas envelope to leave a small, hot core that eventually forms the black hole. During this process the size the orbit of the binary decreases. The closer the 2 objects were when they formed, the sooner they will merge during the merger of 2 black holes. It is not known how much the orbit can shrink during the common envelope phase, in spite of decades of work being spent trying to find this answer (Ivanova et al., 2013).

Astrophysicists may be helped constrain both the uncertainties by future gravitational-wave signals, but for now, Belczynski et al. generate an ‘optimistic’ and a ‘pessimistic’ model Universe in order to assess the highest and lowest possible rates of black hole mergers. They have demonstrated that in both models, systems that would form binary black holes of the sort that generated GW 150914, and the rate of mergers of black holes in the Universe matches that inferred from the gravitational waves that were detected. Belczynski et al. also suggest that the rotation of stars about their own axes is not a requirement for an explanation of most sources of gravitational waves, though it has been suggested that the number of mergers of black holes could be increased by such rotation (de Mink & Mandel, 2016). There is still more work that needs to be carried out and more physics to be included in these models.

According to Eldridge the study by Belczynski et al. is tremendously exciting as it examines the effects of a new constraint on how the stars and the Universe evolve, identified by GW 150914. There are rumours that more gravitational wave signals will be announced soon, it may not be long to wait for the next lesson.

Black Hole Collision Released No Neutrinos ³

The first search for neutrinos produced by the merger of 2 black holes, which in 2015 produced the gravitational waves that were the first to be detected directly, has not found any neutrinos. The data from 2 neutrino detectors: ANTARES, which is located beneath the Mediterranean Sea, and IceCube at the South Pole, were analysed by Imre Bartos et al. In this study no neutrinos were found at ANTARES in the 500 seconds before or after the black holes collided and only 3 were detected at IceCube, all 3 coming from a direction other than that of the merging black holes. An upper limit is placed on the amount of energy it could have radiated through near-massless particles by the scarcity of neutrinos, say the Bartos et al. The relatively high spatial resolution of neutrino telescopes could be used to pinpoint the location of any future signals from merging black holes.

Sources & Further reading

1. Eldridge, J. J. (2016). "Astrophysics: Recipe for a black-hole merger." Nature 534(7608): 478-479.

2. Belczynski, K., D. E. Holz, T. Bulik and R. O’Shaughnessy (2016). "The first gravitational-wave source from the isolated evolution of two stars in the 40–100 solar mass range." Nature 534(7608): 512-515.

3. (2016). "Astrophysics: No neutrinos from black hole smash." Nature 535(7610): 10-10.