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Gravitational wave hunters have picked up signals from the lightest black holes yet

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The fifth black hole merger that’s been detected

An artist’s illustration of two black holes merging.
Image: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

The collaboration of scientists called LIGO — famously known for making the first ever detection of gravitational waves — has once again found these ripples in space-time, stemming from a pair of black holes violently merging many light-years away.

On June 8th, LIGO’s two facilities in Louisiana and Washington picked up the wave signals from the two black holes, which spun around each other rapidly many times a second before colliding to form an even more massive black hole. This marks the fifth black hole merger that LIGO’s wave-detecting facilities have found so far. And it’s possible a sixth one has been seen, too, though that hasn’t been confirmed yet.

This latest merger, detailed in a study in The Astrophysical Journal Letters, is still unique, since it’s the lightest one that has been seen so far: the black holes were just seven and 12 times the mass of our Sun. That means they’re similar in mass to black holes that astronomers have already detected through other indirect methods. Because black holes emit no light, they can’t be observed directly with light-collecting telescopes. But astronomers can sometimes measure the X-rays coming from the super-heated material that churns around a black hole. Now, scientists can compare the properties of these black holes seen with gravitational waves to the ones observed indirectly with X-rays.

A chart showing the masses of all the black holes LIGO and Virgo have detected so far, including a sixth potential merger.
Image: LIGO / Caltech / Sonoma State (Aurore Simonnet)

As gravitational wave detection grows more routine, we’re now entering a new era of research known as “multi-messenger astronomy.” It allows scientists to observe celestial events, such as mergers, in multiple types of spectrums, by looking at the different types of light they produce, as well as their gravitational waves. And that gives scientists even more information to better understand the distant cataclysmic events happening in our Universe.

Black hole mergers aren’t really the best candidates to do multi-messenger astronomy, since they just produce another black hole that doesn’t give off any light. A much better candidate came to the astronomy community in August, when LIGO detected the merger of two neutron stars colliding 130 million light-years away. Afterward, the collaboration alerted hundreds of astronomers operating ground- and space-based telescopes to find the result of the merger. Up to 70 observatories were able to pinpoint the aftermath, measuring its light for weeks and months, and learning how the explosion evolved over time.

How LIGO detects gravitational waves.

Multi-messenger astronomy is also bolstered by the fact that we now have three facilities detecting gravitational waves, up from two. A European collaboration known as Virgo has its own wave observatory in Italy, and it has already detected two gravitational wave signals. The addition of Virgo to the mix significantly helps astronomers better locate where these mergers are happening in the sky, similar to how three GPS satellites are used to find a location here on Earth.

However, Virgo didn’t detect the black hole merger announced today as it wasn’t taking data at the time. In fact, it will be a while before LIGO and Virgo start observing the sky again. Their most recent observational run came to an end on August 25th, and the facilities won’t start back up until fall of 2018. During the downtime, the scientists will work to improve the sensitivity of their observatories to make them even better at detecting gravitational waves. So once the three observatories are up and running, we may see a new explosion of gravitational wave detections.