University of Leicester scientists chase colliding black holes

Within hours of this discovery on 26th December, scientists from the University of Leicester and an international team used the Swift satellite to search for associated X-ray and UV emission.

"Despite it being the middle of the holidays, we were able to respond very quickly," recalled Dr Phil Evans of the University of Leicester, who coordinates the Swift response to Gravitational Wave events. "And we had modified our approach, so we could search nearly twice as much sky with Swift following the Boxing Day event than we had for the previous LIGO detection in September."

“This second gravitational wave detection shows that we have now entered the era of gravitational wave astronomy, and that the first detection was not just a one-off event. Scientists are very excited by the prospects opened up by this new way of seeing the distant Universe,” said Prof Julian Osborne, who leads the Swift project in Leicester.

The team found no sign of the merging stars with Swift, nor with the VISTA telescope in Chile, which performed observations 7-13 days after the trigger. This is in itself a result. "There is a lot of debate at the moment about whether we should expect to see electromagnetic radiation from merging black holes," explained University of Leicester Professor, Nial Tanvir, "so finding nothing can be an important result."

However, the large uncertainty in where the gravitational wave event occurred makes it hard to be sure whether the astronomers looked in the right place. "It really is like searching for a needle in a haystack," Professor Julian Osborne admitted, but the Swift team has ways of narrowing down where to look. "Black holes occur in galaxies, and LIGO can only detect cosmically nearby events. So we only observe where we know there are nearby galaxies; this can shrink the search area by as much as 5 or 6 times."

Later this year a second gravitational wave observing run will start, with the European advanced VIRGO facility expected to join advanced LIGO. Scientists hope for the first gravitational discovery of merging neutron stars with the resulting improved sensitivity and precision. "We may still be debating whether merging black holes produce light, but we are pretty certain that merging neutron stars do,'' commented Professor Paul O'Brien, head of the department of Physics & Astronomy at the University of Leicester. "Detecting both light and gravitational waves from the same event would be of huge scientific value."

But will the Swift team be able to find such light, given the size of the haystack they have to search? Dr Evans is confident that they will: "We've recently enabled a new observing mode, which will let us cover something like ten times as much sky for future events than we've been able to do so far,'' he said. "In the future we can simultaneously shrink the haystack, identify which bits are most likely to contain the needle, and search through it more rapidly."