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Only one month after the start of a new series of observations, gravitational wave observatories have announced five new signals, one of which could turn out to be a black hole swallowing a neutron star.
On April 1st, the teams behind the three gravitational wave detectors started them for a new series of observation, the first one with the three detectors running in parallel for the duration. With three detectors and some upgrades during downtime, we are witnessing a flood of new data. In just one month, LIGO / VIRGO witnessed five gravitational wave events. Three of these come from the fusion of black holes, one was the second fusion of neutron stars, and another could be the first case of melting neutron stars and holes black.
A new season
The two LIGO detectors are a work in progress for years, starting with a first version that, everyone recognized, was unlikely to capture gravitational waves. But each iteration has allowed scientists to understand the sources of error in their detectors, which have been removed for regular upgrades. International collaboration also benefits from the fact that two additional detectors, VIRGO in Europe and KAGRA in Japan, have similar designs and that all teams share what they learn on the hardware.
VIRGO joined LIGO for about a month in 2017 before the end of his second tour of observation. According to Jess McIver of Caltech, a member of the LIGO team, the work done in the meantime has been to "lower the quantum noise limits in the detectors". As a result of the reduced noise, McIver said the detectors could capture gravitational waves further. in the space only what was ever possible before. Having three detectors helps provide better spatial information about the location of the event, necessary to make follow-up observations with traditional observatories. And, as one of the events described today indicates, three detectors allow us to continue recording data, even if a detector is temporarily out of order.
McIver added that the KAGRA detector should come online later in this operation, offering even better resolution.
So what have we seen since the beginning of the race on April 1st? Three events appear to be fusions of two black holes, similar to the fusion that provided the first detection of gravitational waves. The identity is determined by matching the waves captured by the detectors to the models of different masses in collision; Neutron stars and black holes exist in different mass ranges, which allows detectors to deduce what the collision is doing. However, Salvatore Vitale of MIT explained that there is a difference in mass between about two solar masses (upper limit of neutron stars) and the smaller known black holes, which are about five solar masses. Vitale said that events that appear to have an object in this mass gap are a top priority for follow-up observations so that we can better understand what is going on there.
The other two events are more intriguing. One of them, occurring on April 25, appears to be a collision of neutron stars that occurred 500 million light years away. Unfortunately, LIGO-Hanford in Washington being offline that day, it was only recorded in two detectors, which provided a very poor spatial resolution. It is not yet known whether optical telescopes collected information about the event, but they should have searched for about a quarter of the sky to do so.
A day later, the three detectors were online when an extremely distant event occurred about 1.2 billion light years away. This was identified experimentally as the first fusion we saw between a black hole and a neutron star. It is unclear whether we should expect an optical counterpart for this type of merger, but its position has been reduced to only three percent of the total sky, suggesting that we should have good data on this, if there is something to see.
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