I was travelling back from Cambridge on the train yesterday afternoon when I saw the announcement that the Advanced LIGO team had found a second gravitational wave source. Actually, I knew this one was coming – the event actually registered last Christmas – but I had forgotten that it was to be announced at the American Astronomical Society meeting that’s happening now in San Diego. There’s also a second possible discovery, but with much lower signal-to-noise.
The full discovery paper can be found here, from which I have taken this figure:
You can find the arXiv version here.
The signal shown above, code-named GW151226, like the previous one, appears to be from a black hole binary coalescence but it involves two black holes of rather lower masses (about 14 and 8 solar masses respectively). This means that the timescale is rather longer and so more orbits can be observed. It may not look visually as clear as the first source, GW150914, which involved black holes with masses in the region of 30 solar masses, but it’s a clear detection and it’s also interesting that the models suggest that at least one of the black holes has a significant spin. Interesting!
So, that’s two sources. Now we can do statistics! I was wondering last night how long it will take before every individual discovery like this is no longer reported. The same thing happened with the first few extra-solar planets but now that we have thousands, it’s only a subset – those that might plausibly be similar to Earth – that get press attention. At the current rate of discovery gravitational-wave sources may well become quite common over the next few years. In fact a reasonable prediction for when LIGO is switched on again at the end of the summer that there might be a detection every week or so. The era of gravitational wave astronomy is definitely upon us!
Actually from my point of view the really interesting challenge is to make full use of the low signal-to-noise detections that are probable sources but with some uncertainty. I hope to write a blog post soon about how Bayesian methods can help a great deal with that.
Anyway, that’s all I’ve got time for right now. After three days in Cambridge as External Examiner, I now have to chair our undergraduate finalist examination board here at Sussex. So I’ll just say congratulations again to the LIGO team. Great stuff.