We Just Observed Gravitational Waves Again!

The Black hole detections (and one unconfirmed one) from the gravitational observatories. LIGO/Virgo

2017 has been a tough year for many, but at least for gravitational wave observatories, it has been extremely successful. Thanks to a new announcement by the LIGO and Virgo observatories, four detections happened this year, reaching a grand total of six.

This sixth detection was produced by the merger of two relatively small black holes, which were seven and 12 times the mass of the Sun, respectively. The final object was about 18 times the mass of the Sun, which tells us that the energy equivalent to one solar mass was emitted in the form of gravitational waves.

The detection is known as GW170608, and as its number indicates, it was observed on June 8, 2017. The analysis of the other two detections, GW170814 and GW170817, took precedent as they were more complex and impactful. GW170814 was the first to be seen by all three LIGO and Virgo observatories, and GW170817 was the first detection of a neutron star merger ever.

While not as headline-catching as previous gravitational wave discoveries, this detection is still very interesting. It represents observations of the lightest black hole merger yet – more similar in size to those seen with X-rays than previous gravitational observations that were much bigger. Comparing the properties from these different observations could provide astronomers with some important insights into the workings of black holes.

The study, currently available on arXiv and submitted for publication in the Astrophysical Journal Letters, places the cosmic collision at about 1 billion light-years away, making this detection the closest black hole merger yet.

The press briefing also mentioned how luck played a role in the discovery. The detection was done by just two LIGO observatories, since Virgo only came online on August 1. Both of the LIGO observatories had undergone some maintenance during the previous month, and the detector at Hanford in Washington State was only back online a few hours before. When the other detector, located in Livingstone in Louisiana, “heard” the gravitational wave, the Hanford one was undergoing some tests. Although it detected the gravitational wave, it didn’t get the automatic alert from the network.

Luckily, the test didn’t affect much of the data and the team was able to confirm that GW170608 was indeed a detection.

The two LIGO detectors and the one Virgo detector will be offline for about a year while they undergo some minor tweaks to make them more “in-tune”. The teams will continue to analyze the current data and prepare for the third observation run.

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