It is a clash of the titans. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has spotted the signs of yet another enormous collision in space, and this one seems to be between a black hole and a neutron star – the first time we’ve observed the two massive objects together.
Gravitational waves are ripples in space-time that occur when massive objects move about. As they pass through Earth, these ripples stretch and squeeze the fabric of space-time in a way that LIGO’s twin detectors and the Virgo detector in Italy can measure to extraordinary precision.
On 26 April, the three detectors measured a new signal. “This candidate is different from everything else that we’ve observed,” says LIGO team member Gabriela González at Louisiana State University. LIGO has felt the rumble from many pairs of black holes smashing together, and two sets of neutron stars, which are the dense remains of a dead star.
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This signal was unlike any of those. Instead, it may have been a black hole devouring a neutron star. Initial automated data analysis put the odds of this at a fairly low 13 per cent, but it may be more likely than that, González says, given the signal differences. The LIGO team is now conducting more detailed analysis to clear things up.
Meanwhile, other teams of astronomers are using observatories around the world to look for signs of the same event – signals in visible light, radio waves, X-rays and more.
These counterpart signals would come from the neutron star as it gets shredded and falls into the black hole. “Just before it merges or gets shredded, the neutron star is going around the black hole with something like the same speed as a kitchen blender,” says LIGO spokesperson Patrick Brady at the University of Wisconsin-Milwaukee. This rapidly-shifting mass produces the gravitational waves detected by LIGO.
“The neutron star could get shredded before it falls in, and the material falls in very rapidly and causes an outburst,” which would create the electromagnetic signal, he says. So far, though, astronomers haven’t spotted any obvious matches.
“The fact that we haven’t found a counterpart yet would mean that it’s farther away, which is more consistent with a neutron star-black hole system,” says González. “We wouldn’t see binary neutron stars that far.” Preliminary analysis has shown that the signal is coming from about 1.2 billion light years away.
Brady warns that there is still a chance the signal could be a statistical fluke, a result of noise on Earth rather than gravitational waves from afar. “There’s a great deal of ongoing analysis because this signal is quite weak,” he says. It may take months to be sure if it is real.
Signals like this could help us probe both neutron star structure and black hole physics. Even if this signal turns out to be a fluke, hopes are high that LIGO’s current observing run, which is expected to last a year, will spot others like it. “We know that this run is going to be a lot more exciting, and we still have 11 months to go,” says González.
