Gravitational waves hunt now in overdrive

https://www.bbc.com/news/science-environment-48137011

Gravitational waves hunt now in overdrive

2 May 2019

Scientists working to detect gravitational waves switched on their instruments for a third time at the beginning of April and immediately began to register events that could be interpreted as cosmic collisions. All five trigger events still need confirmation. The BBC's Roland Pease examines how telescopes worldwide are helping.

The alert on Mansi Kasliwal's phone went off at two in the morning. Shrugging off the sleep, she squinted at the message. It was from LIGO, the Nobel Prize-winning scientific collaboration that operates gravitational wave detectors. A far-off violent event had sent ripples in space-time through the Universe, to be picked up by LIGO's sensor in Louisiana, and it looked from the data like there should be visible "fireworks", too.

Thanks to the smartphone revolution, she could react without leaving her bed. A few taps on the screen, and the Zwicky Transient Facility, a robotic telescope on Mount Palomar, was reprogrammed to start the hunt.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, and its European counterpart, VIRGO, have just completed upgrades that mean they should be spotting space-distorting events several times each week - collisions of black holes, of neutron stars, and even more exotic phenomena. And since they started running again at the start of April, expectations are holding up: two in the second week; three last week.

The first three, still to be formally confirmed, were probably collisions between black holes, like the first, Nobel-anointed event detected in 2015.

It was the next, on 25 April, that woke Mansi Kasliwal up. The gravitational ripples hinted at the involvement of neutron stars, which would be engulfed in a scorching nuclear flame as they devoured each other and became one.

The radio, optical, X-ray and gamma-ray colours would reveal new details about these normally secretive objects. But before the specialist telescopes could zoom in on the action, astronomers needed to know exactly where to look.

That is where the Zwicky Transient Facility (ZTF) comes in, a new instrument able to scan large swathes of the sky swiftly for anything new and unusual.

The 25 April "candidate" event was going to test that capacity. With LIGO's second detector temporarily out of action (the one in Hanford, Washington State), LIGO and VIRGO could only narrow the search to a quarter of the entire sky: somewhere in that vast darkness, a new spark could be brightly shining, but beginning to fade.
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