Hot on the heels of their revolutionary, first ever direct observation of gravitational waves announced in February this year, the LIGO and Virgo collaborations have identified a second gravitational-wave event in the data from the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) in the US. The observation was made on 26 December 2015, just three months after the first gravitational-wave event was detected on 14 September 2015.

Gravitational waves are ripples in the fabric of space–time, and like the first detection, this latest signal was generated by the collision of two black holes. This time the black holes weighed in at 14 and 8 solar masses and merged to form a single, spinning 21-solar-mass black hole, some 1.4 billion light-years (440 Mpc) away. The collision and subsequent merger released approximately one solar mass of energy, which was radiated away as gravitational waves.

The so-called “Boxing Day event” is officially dubbed GW151226. In October 2015, LIGO recorded another possible event, dubbed LVT151012, which was below the threshold for an official detection. LIGO has therefore detected three events in the four months of observation since it was upgraded to aLIGO. This kicks off the era of gravitational-wave astronomy, and researchers can now begin to constrain predictions about the population of black holes in the universe.

At a press conference held today at the AAS conference in San Diego, LIGO spokesperson Gabriela González said “We are very proud of this being the beginning of gravitational-wave-astronomy. It takes a lot of people, physics and engineering to build these exquisite instruments…and they detected these gravitational waves very clearly.”

Black holes aplenty

Indeed, LIGO changed our view of the universe from its first observation in September. LIGO scientists had expected that binary neutron-star mergers would be one of the first systems that they would detect gravitational waves from. To date, these mergers have not been seen, while black-hole mergers – which were thought to be rare – have instead been detected.

Furthermore, the observed mergers have all involved stellar-mass binary black holes. This is surprising because theories previously suggested that such stellar-mass binaries would either not form at all or, if they did, would be too far apart to merge within the age of the universe. LIGO’s detections have now shown that the opposite is true, and that the rate of binary-black-hole mergers is higher than expected – between six and 400 per cubic gigaparsec per year.

Since the first event in September, theorists have been looking into how such binaries may merge – possibilities include massive binary-stars that both evolve into black holes that eventually merge or black holes in dense stellar environments like globular clusters, where the black holes would “sink” towards the cluster’s centre and merge with others.

Boxing Day binary

B S Sathyaprakash – a physicist at Cardiff University in the UK and a member of the LIGO collaboration – told physicsworld.com that the masses of the two black holes observed in GW151226 are more typical of what they expected for those that form via the evolution of massive stars. “With this system, we widen the debate about formation channels of binary black holes.” Commenting on the great distance to the merger, he says, “The distance to this system is about 70 to 80% of the maximum distance up to which we can confidently detect such systems,” adding that it was “not too surprising [that] our second detection comes from close to the horizon distance of our sensitivity; we survey a greater volume of space at that distance and so [there is] a greater chance of seeing an event.”

The masses of the black holes were smaller than those detected in the first event, which means that the signal was much weaker and was not as immediately visible to the human eye in the data as the September event was. On the other hand, the smaller masses meant that this signal lasted for longer in the detector, and was quickly flagged by LIGO’s online analysis systems and algorithms as a candidate gravitational wave. This occurs because lower-mass systems merge at higher frequencies than systems of larger masses. Heavier systems end up merging more quickly than lighter ones as their orbits quickly reduce, as they emit more radiation.

“This makes some intuitive sense when considering that the size of the black hole is directly proportional to the mass; the larger black holes will merge before they can get close enough to have high frequencies,” explains LIGO scientist Amber Stuver. Although the signal from the December event was not as strong as the first one (which involved masses that were about three times bigger) the statistical significance of the signal was comparable at 5.3σ, because it lasted for longer in the detector – nearly 1 s.

The latest signal was picked up within 70 s of it hitting LIGO’s detectors in December, and was instantly recognized as a good candidate. “I happened to be awake when the trigger came and a bunch of us got together on a teleconference and talked about it for a couple of hours. It was a fantastic Boxing Day present,” says Sathyaprakash.

The signal itself was composed of 55 gravitational-wave cycles that were produced as the two black holes spiralled towards one another. Because the orbital frequency of a system is half the frequency of the wave, the LIGO scientists know they observed about the last 27 orbits before merger. (To read more about the characteristic waveform of gravitational-wave events, including the inspiral, chirp and ringdown features, see “Ligo detects first ever gravitational waves – from two merging black holes”.)

Although only one solar mass of energy was radiated away in the GW151226 event – compared with the three solar masses in the first event – Stuver says that the fraction of mass radiated away in all three detections was nearly the same, up to about 5% of the total mass of the system. This suggests that the merger process for black holes is similar and any differences arise mainly due to differences in mass.

Another contributing factor that could affect the merger process is if either of the original black holes in the binary are spinning – if the spins are aligned, then more energy will be released and vice-versa. In the latest discovery, the LIGO researchers were able to tell that one of the companions had a spin parameter of at least 0.2 – this corresponds to a black-hole rotational speed of a tenth of the speed of light. All of the final black holes formed in the merger were spinning (Kerr) black holes – the spin of the final GW150914 black hole is around 0.7.

Sense and sensitivity

aLIGO’s detectors are currently being upgraded following the first run, further increasing its sensitivity and stability, before a planned engineering run that will begin next month. “Any new or persisting issues will be addressed and tested again before the next observing run, currently scheduled for the fourth quarter this year,” explains Stuver. “We are also very much looking forward to the Advanced Virgo detector [in Italy] joining the search sometime during the next LIGO observing run. This will greatly increase our ability to extract information about the gravitational-wave source from the signal, especially the location in the sky,” she adds.

Indeed, with three or more geographically separate detectors, researchers should be able to nail down where the signal originates from more clearly. Currently, aLIGO can only assay that a signal came from a general area of the sky, mainly by measuring how long it takes a gravitational wave to travel between the two detectors.

With these detections under its hat and an expected rate of one to two detections per month in the next run, it is fair to say that gravitational-wave astronomy is really taking off. “So many years of work by hundreds of scientists has gone into this search, and we are seeing clear evidence that not only do our detectors work, but there may be more things out there to see than we previously thought,” says Stuver.

Sathyaprakash agrees, adding that future detectors and observations will help us to “understand the formation and growth of black holes throughout cosmic history, which could shed some light on a long-standing riddle: when and how did monstrous black holes we now find at centres of most galactic nuclei form and grow?”

The research is published in Physical Review Letters.