At 10:49pm Western Australian time on February 2 this year, cosmic gamma rays hit the NASA satellite, Swift, orbiting the Earth.
Within seconds of the detection, an alert was automatically sent to the University of WA’s Zadko Telescope. It swung into robotic action, taking images of the sky location in the constellation Ophiuchus.
What emerged from the blackness, where nothing was seen before, was a rapidly brightening “optical transient”, which is something visible in the sky for a brief period of time.
The event, named GRB170202, was a very energetic gamma ray burst (GRB). After less than a minute, the gamma rays switched off, and the GRB appeared as a brightening and then fading optical beacon.
The Zadko Telescope recorded the entire evolution of the optical outburst. During its biggest outburst, GRB170202 was equivalent in brightness to millions of stars shining together from the same location.
About 9 hours 42 mins after the GRB, the Very Large Telescope in Chile acquired the spectrum of the light from the optical afterglow.
This enabled a distance to the burst to be measured: about 12 billion light years. The Universe has expanded to four times the size it was then, 12 billion years ago, the time it took the light to reach Earth.
GRB170202 was so far away, even its host galaxy was not visible, just darkness. Because the GRB was a transient, never to be seen again, it is like turning on a light in a dark room (the host galaxy) and trying to record the detail in the room before the light goes out.
Mystery of gamma ray burst
The flash of gamma radiation and subsequent optical transient is the telltale signature of a black hole birth from the cataclysmic collapse of a star.
Such events are rare and require some special circumstances, including a very massive star up to tens of solar masses (the mass of our Sun) rotating rapidly with a strong magnetic field.
These ingredients are crucial to launch two jets that punch through the collapsing star to produce the gamma ray burst (see animation)