Yes, we've found another one

After the excitement of February when we (the LIGO Scientific Collaboration & Virgo Collaboration, or LVC for short) announced the first direct detection of a gravitational wave signal a lot of people having understandably been asking "Well, did you see any more?". The analysis performed for the announcement of the first detection (of the source called GW150914) used just over a month of data from the start of a longer observing run (that we called O1), which ran from the 12^th September 2015 until the 19^th January 2016. So we did have more data "in the can". And, as it happened that additional data did indeed provide us with another highly significant detection^*. This new signal was observed in what was the early hours of Boxing Day in the UK, although it was still Christmas Day in the US when it hit the two LIGO detectors - so we can call it a late Christmas present. As we work in Coordinated Universal Time (UTC), which follows Greenwich Mean Time (GMT), the signal has been given the title GW151226 (i.e. it arrived on 26^th December 2015), but internally has generally been called "The Boxing Day Event".

So, is this signal different from the first one? Well, like GW150914 it appears to be the result of two black holes inspiralling in to each other and merging, although the two merging black holes are smaller at roughly 14 and 8 times the mass of the Sun. A nice illustration of where these black holes sit, in terms of mass and radius, compared to other known black holes is shown here. However, unlike GW150914 we can pretty definitively say that at one of the merging black holes is spinning. Another thing to note is that if you look at Figure 1 from our detection paper for GW151226 you can't really see the signal in the data time series (whereas GW150914 stuck out like a sore thumb!) and it pretty much looks like noise.  As the system was less massive (but at a similar distance to) GW150914 the amplitude of the signal was intrinsically smaller. The saviour to this was that it also lasts longer in the detector's sensitive frequency bands (see Figure 1 in this paper that discusses all the detection's together) which means that you can integrate (basically sum together) over the longer signal and still "see" it in the noise.

Given that we'd already announced the detection you may be wondering what's important about this new one. The main thing is that we are now starting to reveal a population of objects rather than a single one. From looking at the population you can start to understand the distribution of source properties and investigate how they form. Admittedly with just two (and a bit) sources you really can't say much - it would be hard to work out the distribution of everyone's height by measuring just two people, but you at least get a rough idea of the likely range. It also allows us to be sure that the first signal wasn't a fluke, and suggests that we'll see many more of these objects in our upcoming observing runs (the next one, O2, should start this autumn with slightly better sensitivity than O1, and hopefully include the Virgo detector).

We often say that these gravitational waves are opening a new astronomical window on the Universe. And they really are! Imagine that the sky had always been covered in cloud, so you'd never been able to see the Moon, planets or stars (although in this scenario assume that you had a pretty good theory that the diffuse light coming through the clouds was being emitted by very distant objects called "stars".). Then, imagine that one night there's a slight chink in the clouds and through that you see a black sky with a single shining point of light in it. Wow! Your theory about "stars" was right! As the nights go on the clouds clear even more and reveal even more stars and other astronomical objects and the wonders of the Universe (and the exciting physics they reveal(!)) open up to you. It's a slightly tortured analogy, but you can kind of see that we're just seeing the first few points of light as the clouds are just starting to clear.

Some further information/reading:

• The science summary of these results can be found here.
• The GW151226 detection paper is here and a paper detailing all the binary black hole events detected in O1 is here.
• The data, and tutorials on using it (with ipython/jupyter notebooks), for GW151226 can be found here.
• A (probably non-exhaustive) list of blog posts by other LVC members about this detection (in no particular order):
• The Wave that Stole Christmas by Daniel Williams
• Merry Christmas, LIGO: Another Gravitational Wave! by Amber Stuver
• The Cosmic Classroom on Boxing Day by Shane Larson
• The Boxing Day Event by Christopher Berry
• Gravitational waves found again: here’s how they could whisper the universe's secret by Graham Woan
• Black holes rule! by Mark Hannam

^*The eagle-eyed of you may have know that amongst all the papers produced about our detection announcement there was also mention of a candidate gravitational event that was within the originally analysed dataset. We've estimated that this candidate, dubbed LVT151012 (for LIGO-Virgo Trigger), has a roughly 90% chance of being a real astrophysical signal, but we like to be far more certain than that to claim it as a definite signal.