Making Massive Black Hole Binaries Merge

Many fascinating questions remain unanswered by last week’s detection of gravitational waves produced by a coalescing binary black hole system (GW150914) by LIGO. One of these is whether the fact that the similarity of the component masses (29 and 36 times the mass of the Sun respectively) is significant.

An interesting paper appeared on the arXiv last week by Marchant et al. that touches on this. Here is the abstract (you can click on it to make it larger):



Although there is some technical jargon, the point is relatively clear. It appears that very masssive, very low metallicity binary stars can evolve into black hole binary systems via supernova explosions without disrupting their orbit. The term ‘low metallicity’ characteristises stars that form from primordial material (i.e. basically hydrogen and helium) early in the cycle of stellar evolution. Such material has very different opacity properties from material with significant quantities of heavier elements in it, which alters the dynamical evolution considerably.

(Remember that to an astrophysicist, chemistry is extremely simple. Hydrogen and helium make up most of the atomic matter in the Universe; all the rest is called “metals” including carbon, nitrogen, and oxygen…. )

Anyway, this theoretical paper is relevant because the mass ratios produced by this mechanism are expected to be of order unity, as is the case of GW150914.  One observation doesn’t prove much, but it’s definitely Quite Interesting…

Incidentally, it has been reported that another gravitational wave source may have been detected by LIGO, in October last year. This isn’t as clean a signal as the first, so it will require further analysis before a definitive result is claimed, but it too seems to be a black hole binary system with a mass ratio of order unity…

You wait forty years for a gravitational wave signal from a binary black hole merger and then two come along in quick succession…




9 Responses to “Making Massive Black Hole Binaries Merge”

  1. chrisnorth Says:

    The details of LVT151012 are discussed in the paper at Although, as you say, the signal is weak, the best for parameters are that it is a merger of a 23{+18,-5} & 13{+4,-5} Msun binary black hole, also with no overall spin (so consistent with forming together, though that’s very speculative) at a distance of 1100{+/-500} Mpc. The usual 50% error bars are because there’s no constrain on the system inclination, due to only measuring one polarisation – something Mark Hannam discusses over at

  2. I think LVT151012 is the second most significant event mentioned in the press-released paper.
    It’s followed up further in one of the other papers.

  3. Peter: Does no one believe the evidence for BHB in OJ287
    (as argued for many years by Mauri Valtonen and collaborators)?
    They actually predicted when optical bursts from OJ287 will happen
    and these have been vindicated.

    • Phil Uttley Says:

      I think OJ287 is interesting but many including myself are withholding judgement The problem is that the early light curve is sparse, and the source has only undergone a couple of the putative cycles with good sampling. There is clearly also a lot of other flaring going on, as is also seen in other blazars, and it isn’t always clear which flare should be considered as the ‘prediction’, especially since the model has also been updated on a few occasions. Unfortunately the period is so long that it is hard to really test in an unambiguous way without only a priori reasoning, since the theory is developing on the same kind of time-scale.

      And don’t even get me started on…74G ….

      • Phil Uttley Says:

        Link broken above, so you could try this for the offending paper I tried to mention:

        Number of trials folks, number of trials…. sigh.

      • Phil Uttley Says:

        Oops, and I meant ‘with only a priori reasoning’… sorry!

      • Phil, thanks for the info on OJ287. I cannot find a reference offhand , but I thought for OJ287 also we have evidence that the BHB is coming closer together by emission of GWs similar to Hulse-Taylor binary pulsar?

      • Phil Uttley Says:

        There are predictions (e.g. Valtonen) but no clear evidence as far as I know, for the same reasons as above (plus the model has a number of other components, e.g. a geometry for the accretion flow needs to be assumed, since outbursts are hypothesised to occur when the companion BH passes through the accretion disk). But even neglecting the model uncertainties, if the period is not even certain or confirmed we certainly cannot test for GW effects on the orbit.

  4. Apologies if this appears twice, as I don’t see my comment.

    Peter: Does no one believe the evidence for BH binary from OJ287
    (as argued by Mauri Valtonen and collaborators?)
    They actually correctly predicted the time of occurence of optical
    bursts from OJ287

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