SA telescopes observe major cosmic event
The discovery marks the birth of a new era in astrophysics
THE South African Astronomical Observatory (SAAO) and the Southern African Large Telescope (Salt) have provided some of the very first data in what is turning out to be one of the most-studied astrophysical events ever.
The Salt and SAAO telescopes are part of an unprecedented international collaboration to investigate the origin of the first detection of gravitational waves produced by two colliding neutron stars.
The discovery marks the birth of a new era in astrophysics, the first cosmic event observed in both gravitational waves and light.
SAAO and Salt are among the 70 ground- and space-based observatories that observed the explosion of two colliding neutron stars, immediately after their gravitational shock waves were detected by the US-based Laser Interferometer Gravitational-Wave Observatory (Ligo) and the European-based Virgo detector.
Neutron stars are the smallest, densest stars known. They are the remains of massive stars which exploded as supernovae. In this event, called GW170817, two neutron stars spiralled inwards then collided, emitting gravitational waves detectable for about 100 seconds.
The collision also resulted in a kilonova explosion of light, initially in the form of gamma rays which were detected by space-based telescopes. (Theorists have predicted that what follows the initial collision is a “kilonova” explosion, a phenomenon by which the material left over from the neutron star collision, which glows with light, is blown out of the immediate region far into space.)
The gamma rays were then followed by X-rays, ultraviolet, optical, infrared, and radio waves. GW170817 is the first time light and gravitational waves from the same event have been observed. This allowed astronomers to localise the event within hours and launch follow-up observations by Salt and numerous other telescopes in South Africa and around the world.
South African activities also included the first observations contributing to published scientific results by the Meer-KAT radio telescope under construction in the Karoo.
Gravitational waves from colliding black holes were first detected only two years ago, and have been detected three more times since then, leading to the 2017 Nobel Prize in physics being awarded to three US scientists. Black hole collisions, however, are not expected to emit light – the significance of the present event lies in the combination of the gravitational waves and light.
Salt Astronomy Operations head Petri Vaisanen said: “Imagine you have only one sense. All your life you have merely looked at the world. Two years ago you heard something, voices coming from somewhere around you. But then, suddenly, you actually see someone talking. How much more will you understand about how the world works when you put those together? Immensely more. That sums up the momentous discovery, and hints at the possibilities going forward.”
On August 18, the day and night following the Ligo detection and initial successful searches in Chile for the counterpart, was a busy day for observational astronomers.
“After a flurry of messages and e-mails that afternoon in Sutherland, I finally got the co-ordinates”, Vaisanen said. “There was a new object, which had caused the whole of space-time to ripple, sitting at the outskirts of the galaxy NGC 4993 some 130 million light-years away.
“I knew that everyone with a working telescope in the southern hemisphere was scrambling to get data on it. We decided to drop all other plans for that evening, and went for a spectral observation with Salt, since you need a large telescope for such observations breaking up the light into all its colours.
“It was a difficult observation since we had to do it in twilight, before it got properly dark. I’m very proud of the whole team: Salt was only the third observatory to provide a spectrum of the target, and the first spectrum that clearly started showing anomalous behaviour proving this was no run-of-themill transient event.”
The early Salt observations showed the explosion was relatively bright and blue. Only two or three days later, further observations by Salt, SAAO and other major international telescopes showed the light was rapidly fading and turning red, due to the dusty debris blocking the bluer light, as predicted by the theory of the evolution of a kilonova explosion.
Simultaneously, Master, a joint Russian-South African optical telescope located in Sutherland, and IRSF (Infra-Red Survey Facility), a joint Japanese-South African infrared telescope also in Sutherland, continued to monitor GW170817 for two weeks, showing it gradually faded in the visible light but brightened in the infrared, consistent with the final stages of the afterglow from the surrounding debris.
Piecing together the new science from the event requires combining observations spanning the first hours, days and weeks after the merger.
SAAO astronomy head Dr Stephen Potter said the ability of Salt and SAAO telescopes to respond rapidly to unexpected discoveries is a major reason for the success of these observations and will ensure similar successes in the future.
“We are proud to have played a major role in such a historical event thanks to the sterling efforts and expertise of SAAO and Salt staff who ensure our observatory is at the forefront of world-class scientific endeavours.”