LIGO may have spotted a black hole-neutron star merger


Image of two long arms of the LIGO detector in the desert of eastern Washington.

Enlarge / Our gravitational wave detectors really are a series of tubes. (credit: Caltech/MIT/LIGO Lab)

On April 1, the teams behind the three gravitational wave detectors started them up for a new observational run, the first with all three operating in parallel for the full run. With the benefit of three detectors and some upgrades that were done during the downtime, we’re seeing a flood of new data. In just one month, LIGO/VIRGO has seen five gravitational wave events. Three of those are from merging black holes, one was the second neutron star merger, and another may have been the first instance of a neutron star-black hole merger.

A new season

The two LIGO detectors have been a work in progress for years, starting with an early version that everyone acknowledged was unlikely to pick up gravitational waves. But each iteration has allowed scientists to understand the sources of error in their detectors, and they’ve been taken down for regular upgrades. The international collaboration also benefits from the fact that two additional detectors, Europe’s VIRGO and Japan’s KAGRA, have similar designs, and all the teams share what they’re learning about the hardware.

VIRGO joined LIGO for roughly a month in 2017 before its second observational run came to a close. According to Caltech’s Jess McIver, a LIGO team member, work in the intervening time went into “pushing down the quantum noise limits in the detectors.” As a result of the lowered noise, McIver said that the detectors can pick up gravitational wave events farther out into space than was ever possible before. Having three detectors helps provide better spatial information as to where the event actually originated, necessary for doing follow-up observations with traditional observatories. And, as one of the events described today makes clear, three detectors let us continue to take data even if one detector is down temporarily.

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