What are gravitational waves and their source

Four new sources of gravitational waves

The LIGO and Virgo observatories also publish their first catalog

Scientists have scoured previously recorded data from the LIGO and Virgo gravitational wave detectors and identified four new signals in the process. They all come from the merging of pairs of black holes. This time, too, the researchers at the Max Planck Institute for Gravitational Physics in Potsdam and Hanover made decisive contributions in key areas to the observations and their interpretation.

During the first two observation runs, the LIGO detector in the USA registered six mergers of pairs of two black holes and one crash of two neutron stars. The four events now extracted from the data all come from the O2 observation phase, which lasted from November 30, 2016 to August 25, 2017. The new signals are designated as GW170729, GW170809, GW170818 and GW170823 - corresponding to the year, month and day on which they reached the earth detectors.

The masses of the black holes involved in the four mergers span a broad spectrum, ranging from 7.6 to 50.6 solar masses. The GW170729 event is the most massive and most distant source of gravitational waves that has been observed so far. In this merger, which took place about five billion years ago, nearly five solar masses were converted into gravitational waves. And: On closer inspection of all the data, it appears very likely that at least one of two of the observed black hole mergers will rotate.

The two LIGO detectors in the USA and the Italian Virgo Observatory jointly detected the event on August 18, 2017. Therefore, the position of the source in the northern sky could be localized comparatively well, precisely to 39 square degrees (corresponding to 195 full moon discs) - more precisely than ever before.

Simultaneously with the announcement of the new results at a workshop at the University of Maryland (USA), the scientists presented a catalog of all known gravitational waves and candidate events from the first two observation runs. It describes the properties of the population of merging black holes in the universe. Above all, the researchers note that almost all black holes formed from stars have less than 45 times the mass of our sun.

“State-of-the-art waveform models, advanced data processing and better calibration of the instruments have enabled us to determine the astrophysical properties of previously announced events even more precisely,” says Alessandra Buonanno. The director at the Max Planck Institute for Gravitational Physics in Potsdam is looking forward to the next observation run in spring 2019 and expects that an average of two signals of merging black holes will be found per month.

In the third observation run (O3), LIGO and Virgo start with once again improved sensitivity. In addition, the Japanese gravitational wave detector KAGRA may be added to the network towards the end of O3. And there is another innovation: At O3, the scientists want to announce their measurement results immediately so that all astronomers worldwide - amateurs and professionals alike - can start with follow-up observations.

In addition, a technical innovation will be used in the third measurement period at LIGO and Virgo, which will start in a few months: “So far, many technologies that we have developed on our GEO600 system have already been used on the large detectors. Now, with the squeezed light, we have added another process for which we have done pioneering work, ”says Karsten Danzmann, Director at the Max Planck Institute for Gravitational Physics in Hanover.