In June 2016, the first telescope to begin observations of GRB 160625B was the MASTER-IRC telescope at the Teide Observatory in the Canary Islands, which observed it within a minute of the satellite notification. It made optical light observations while the initial phase was still active, gathering data on the amount of polarized optical light relative to the total light produced.
Another telescope, the RATIR camera (Reionization and Transients InfraRed camera), a 1.5-meter (60-inch) optical and infrared telescope in Baja California, had to wait eight and a half hours to make observations until the right area of the sky came into view.
“This means the gamma-ray burst itself had ended, and we were observing what's called the afterglow,” Butler said. “This is the fading explosion as the radiation shocks up the interstellar medium around the star that exploded.”
RATIR continued its observations over the weeks that followed the June 2016 event. They revealed that the outburst jetted out in a beam that was roughly two degrees wide, which is about four times the size of the moon. The fact that Earth found itself within the beam was entirely random.
Based on all the observations from both ground-based and space telescopes, and the long-lasting afterglow, the researchers concluded this GRB most likely originated from collimated jets of particles spewing from a young black hole.
Researchers in this field have typically theorized that a black hole's energy emission jets are dominated either by its magnetic field or by matter. The new data suggests that both factors are key. The black hole's magnetic field dominates the jets at the outset, and matter becomes dominant as the magnetic field dissipates.
“We find evidence for both models, suggesting that gamma-ray burst jets have a dual, hybrid nature,” Troja said. “The jets start off magnetic, but as the jets grow, the magnetic field degrades and loses dominance. Matter takes over and dominates the jets, although sometimes a weaker vestige of the magnetic field might survive.”
The black hole's spin that results following the supernova explosion may produce the beaming effects, Butler said, with material jetting out along the black holes poles. Using MASTER to make observations within minutes enabled the scientists to measure polarization, which had never been done before. The team detected a large amount of polarization that indicated that the GRB was being focused by powerful magnetic fields, suggesting that the polarization of the radiation is determined by the strength of the magnetic fields.
“We think the gamma-ray emission is due to highly energetic electrons, propelled outward like a fireball,” Butler said. “Measuring the strength of magnetic fields by their polarization effects can tell us about the mechanisms that accelerate particles such as electrons up to very high energies and cause them to radiate at gamma-ray energies.”