(credit: NASA)
When the LIGO collaboration announced the first, unambiguous detection of the gravitational waves produced by a black hole merger, several of the researchers hinted that there would be further news emerging from the mass of data obtained during the first run of Advanced LIGO. That news has now arrived in the form of GW151226, a merger of two black holes roughly seven and 14 times the mass of our Sun.
Because of their small size, the black holes spent more time producing gravitational waves prior to their collision. In some ways, this gives us more information, but the lower intensity of the waves mean that there are much larger errors attached to most of its properties.
For physicists, GW151226 was a slightly delayed Christmas gift: it arrived at 3:40 in the morning UTC on December 26, 2015. LIGO has automated software systems that scan the data to look for events quickly enough to notify astronomers, who can turn conventional instruments in the direction of the detection. These systems realized there was something interesting going on 70 seconds after the gravitational waves hit Earth. The preliminary estimate was that random noise should produce an event like this only once every 1,000 years, so the astronomers (or, as the paper puts it, "electromagnetic partners") got sent an alert.
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