Shot at a Kilonova

For just the second time, physicists working on the Laser Interferometer Gravitational-Wave Observatory (LIGO) have caught the gravitational waves of two neutron stars colliding to likely form a black hole.The ripples in space-time traveled some 500 million light-years and reached the detectors at LIGO , as well as its Italian sister observatory, Virgo, at around 4 a.m. E.T. Thursday, April 25. Team members say there’s a more than 99 percent chance that the gravitational waves were created from a binary neutron star merger.In the moments after the event, a notice went out alerting astronomers around the world to turn their telescopes to the heavens in hopes of catching light from the explosion, called a kilonova. Kilonovae are 1,000 times brighter than normal novae, and they create huge amounts of heavy elements, like gold and platinum. That brightness makes it easy for astronomers to find these events in the night sky — provided they’ve been given a heads-up and location from LIGO first. LIGO’s twin L-shaped observatories — one in Washington state and one in Louisiana — work by shooting a laser beam down the long legs of their “L.” Their experimental setup is precise enough that even the minimal disturbance caused by a passing gravitational wave is enough to trigger a slight change in the laser’s appearance . It made the first ever detection of gravitational waves in 2016. Then it followed up by detecting merging neutron stars in 2017.Scientists use any slight delays between when signals reach the detectors to help them better triangulate where the waves originated in the sky. But one of LIGO’s twin detectors was offline Thursday when the gravitational wave reached Earth, making it hard for astronomers to triangulate exactly where the signal was coming from. That sent astronomers racing to image as many galaxies as they could across a region covering one-quarter of the sky.And instead of finding one potential binary neutron star merger, astronomers turned up at least two different candidates. Now the question is which, if any, are related to the gravitational wave that LIGO saw. Sorting that out will require more observations, which were already happening around the world as darkness fell.“I would assume that every observatory in the world is observing this now,” says astronomer Josh Simon of the Carnegie Observatories. “These two candidates (they’ve) found are relatively close to the equator, so they can be seen from both the Northern and Southern Hemisphere.”Simon also says that, as of Thursday afternoon in the United States, telescopes in Europe and elsewhere should be gathering spectra on these objects. His fellow astronomers at the Carnegie Observatories turned their telescopes at Chile’s Las Campanas Observatory to the event Thursday night.