We Still Don't Know Where Those Gravitational Waves Came From

Shown is the Pan-STARRS 1 telescope in Hawaii. University of Hawaii Institute for Astronomy / Rob Ratkowski

Last week it was revealed that gravitational waves had been spotted for the first time. There’s only one problem – we don’t know exactly where they came from.

What we do know is that this detection came from a pair of merging black holes 1.3 billion light-years away. And we know the size of the black holes – 36 and 29 times the mass of our Sun. But the observation of a signal was made with just two detectors; without a third, we can’t triangulate the signal.

So when the first indications of a detection came out of the Laser Interferometer Gravitational-wave Observatory (LIGO) on September 14, scientists at various telescopes immediately looked in the general known direction of the source of gravitational waves – the southern sky – to see if they could pick up any sort of flash of visible light from the merger.

One of those was the University of Hawaii’s Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The Pan-STARRS 1 Telescope maps the sky looking for changing celestial objects, such as supernovae or variable stars. It had been hoped the telescope might also be able to see the “sparks” from the merger, but sadly it was to no avail, despite spotting about 50 new supernovae, or massive stellar explosions.

It’s possible the merger may simply have been too far in the southern hemisphere, and thus not visible from the telescope, or it may have been too faint to observe in visible light. "That is science," said Pan-STARRS Director Ken Chambers in a statement. "Sometimes all you can report is what you don’t see, because that is important information too. LIGO has opened a brand new field of astronomy, and confirmation from facilities like Pan-STARRS will be very important to understanding them."

This is the approximate location of the gravitational waves detected on September 14, 2015, in the southern sky, ranging from yellow (most confident) to pink (least confident). LIGO

Another team that tried to find visible light from the merger was that behind the Dark Energy Camera (DECam) in Chile. Within a day of the discovery, they rapidly observed the southern sky to try and spot any sign of the merger. This involved scanning 700 square degrees of sky (2,800 times the size of the full Moon) for three weeks. Sadly, again, they could not detect the flash of light from the merger.

However, although both attempts were unsuccessful, they allowed astronomers to refine the techniques they will use next time a signal from gravitational waves is found. “This first attempt to detect visible light associated with gravitational waves was very challenging,” said Edo Berger, principle investigator of the DECam follow-up team, in a statement. “But it paves the way to a whole new field of astrophysics.”

So, for now, we can’t quite combine visible light observations with those of gravitational waves. This shows how useful gravitational waves can be, though, letting us glean information from parts of the universe we simply couldn't see before. But the next time a gravitational waves signal is spotted, scientists from these teams and others will be ready and waiting to see where it came from.

With Italy’s upcoming VIRGO instrument to complement the existing two LIGO detectors, there will be three instruments in operation, allowing a source to be triangulated, but seeing the visible light from it too will be of interest. Not least, it will help confirm that gravitational waves travel at the speed of light, if they are confirmed to arrive at the exact same moment as the flash of visible light.

For this particular pair of merging black holes, the event has passed, and it's unlikely we'll know their location. But the future is much more bright for subsequent detections.

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