LIGO Finds Lightest Black-Hole Binary

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Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

A Routine Detection

GW170608

The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

GW170608 component masses

Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

Why Size Matters

With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

Compact object masses

GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

Citation

LIGO collaboration, submitted to ApJL. https://arxiv.org/abs/1711.05578