LIGO spots a third black hole merger, tightens mass limits on gravitonsEnlarge (credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet))
Today, the team behind the Laser Interferometer Gravitational-Wave Observatory (LIGO) is announcing the detection of a third black hole merger, the first from its second operational run.

The merger shares some features with the previous ones: the black holes were bigger than expected, and their merger released a staggering amount of energy.

But the LIGO team was also able to extract some information about the details of the collision and propagation of gravitational waves.

These details tell us someting about the limits of general relativity and the history of the black holes themselves.
As with the earlier detections, the new event (GW170104) showed up as a series of curves that came to briefly dominate the noise in LIGO’s two detectors for a few seconds before fading back into the background. Normally, an event like this would trigger an alert to the astronomy community, which could then attempt observations in the area of the sky where the event took place.

But, in this case, a recent period of maintenance had left one of the two detectors set in a calibration mode.

So, instead, the event was picked up by software triggers that scan the incoming data in near-real time.

The operators then sent out an alert manually; about 30 groups of astronomers searched the appropriate areas of the sky, but there’s no word yet on whether anything came out of those searches.
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