On the 17th August 2017, LIGO detected gravitational waves that had travelled over 140 million light years from a lenticular galaxy NGC 4993.
For the first time in human history, telescopes were able to then direct themselves and watch the event in visible and other electromagnetic frequencies.
Several days later the object GW170817 brightened to blue, then over 4 days faded out to a red colour.
At the very same time the Chandra X-Ray observatory was watching, and it saw nothing, which was a surprise, as the size of the gravitational waves suggested two neutron stars colliding, and the expected theory was an immediate jet outburst bright in X-Rays.
The x-rays did eventually show up, but slowly over a year and a bit began to fade, astronomers believe that there probably was a jet outburst, but that it wasn’t heading in our direction, and instead, what was detected was that outburst of material slamming into the interstellar material, heating it up and causing x-rays to be emitted. The fading is caused as the jet fans out and slows down as it moves away.
However, GW170817 had a further surprise, the fading abruptly stopped and held constant for a further few years.
There are two main possible explanations to what was going on here, either the neutron stars merged and the kilonova was caused by the ejected material of the event, or the two neutron stars had enough mass to collapse further into a black hole.
The problem is, both events would be expected to fade, the black hole probably faster as it quickly consumed the accretion disk.
The truth is, Astronomers have never witnessed either event, particularly not with gravitational waves and EM observations straight after, so it may be that something additional is going on.
The event will continue to be watched in the hope it will provide further evidence to the processes of neutron star merger.
