When the LIGO collaboration announced the first, unambiguous detection of the gravitational waves produced by a black hole merger, several of the researchers hinted that there would be further news emerging from the mass of data obtained during the first run of Advanced LIGO. That news has now arrived in the form of GW151226, a merger of two black holes roughly seven and 14 times the mass of our Sun.

Because of their small size, the black holes spent more time producing gravitational waves prior to their collision. In some ways, this gives us more information, but the lower intensity of the waves mean that there are much larger errors attached to most of its properties.

For physicists, GW151226 was a slightly delayed Christmas gift: it arrived at 3:40 in the morning UTC on December 26, 2015. LIGO has automated software systems that scan the data to look for events quickly enough to notify astronomers, who can turn conventional instruments in the direction of the detection. These systems realized there was something interesting going on 70 seconds after the gravitational waves hit Earth. The preliminary estimate was that random noise should produce an event like this only once every 1,000 years, so the astronomers (or, as the paper puts it, "electromagnetic partners") got sent an alert.

The data from this time period were later analyzed in more detail. LIGO includes various sensors that can pick up potential sources of background noise. When GW151226 was detected, these sensors were pretty quiet; at best, they could account for less than six percent of the apparent signal. That signal was then compared to the total data from Advanced LIGO's full first run. That analysis put the significance of its detection at a significance of 4.5σ—barely below the 5σ cutoff. But when the other potential detections (there are three total) were removed from the background data, the significance did shoot up above 5σ.

Gravitational waves are generated as dense objects (neutron stars or black holes) are in the final stages of spiraling inwards towards collision. The heavier they are, the less time they spend close enough to generate detectable waves. That's because the frequency of these waves is proportional to the distance between the two bodies, and LIGO is only sensitive to some frequencies.

In the first detection, the black holes involved were very massive, so they only spent a fraction of a second within the region where LIGO could detect the gravitational waves they produced. The compensation? The large mass made those gravitational waves very intense.

Here, the black holes are smaller, and the signal looks more like a series of fluctuations in the regular background of LIGO. They spent much longer orbiting each other in the range that LIGO is sensitive to, producing about 55 oscillations that lasted for roughly a second. Their frequency increased as they got closer.

But the lack of intensity in the signal means that there's large uncertainties about the mass of the black holes. One of them could be anywhere between 10.5 and 22.5 times the mass of the Sun (most likely value 14.2 times), while the second is likely 7.5 times the mass of the Sun with an error of 2.3 times on either side. The minimum possible mass is above that of a neutron star, so the authors argue that these are in fact black holes merging. The resulting black hole probably weighs about 21 times the mass of the Sun, meaning the equivalent of our Sun was converted into energy in the form of gravitational waves.

Distance information was equally uncertain. The most likely value is 440 Megaparsecs (about 1.4 billion light years), but the 90 percent confidence interval is 200 Megaparsecs on either side of that value.

It's also possible to infer things about the orbit of the black holes involved by measuring the polarization of the gravitational waves. (Small aside: we can measure the polarization of gravitational waves!??!?) Here, the low intensity doesn't get in the way; instead, it's an unhappy accident of geometry. The orientation of the two LIGO detectors just happens to be arranged so that a source signal in this direction is hard to measure.

And just to make sure, the LIGO team confirmed that this event is consistent with relativity. Einstein can rest in peace.

In a lot of ways, the data we have is pretty limited. But that doesn't mean this detection is insignificant. The first detection by LIGO involved black holes that are heavier than those that could form directly from the collapse of stars' cores, implying either that they had fed heavily or undergone previous mergers. The two bodies that merged to produce GW151226, in contrast, are typical of those formed by supernovae. Thus, this sort of merger should be typical of what we'd expect to find—and LIGO has now confirmed that we can spot them.

“It is very significant that these black holes were much less massive than those observed in the first detection,” Gabriela Gonzalez, LIGO's spokesperson, said in a statement. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our Universe.”

There's only one other event in Advanced LIGO's first run, which ran from September last year to January of this one. LVT151012 looks like another black hole merger, but it fell short of statistical tests for detection. So we'll apparently have to wait for LIGO's next data-taking run to get a better perspective on our gravitational wave environment.

Physical Review Letters, 2016. DOI: Not yet available.