Astronomy

A Dying Star's Explosive ‘Fireball' Blast Aimed a Beam of Gamma Rays at Earth

One of the brightest gamma-ray bursts ever observed briefly released as much energy as the sun will emit over its lifetime, directing a narrow beam of gamma rays toward Earth by chance.

On June 25, 2016, something went boom in deep space, about nine billion light-years away. Thanks to the quick reflexes of a new generation of telescopes, astronomers have finally been able make an unprecedented and detailed study of one of the most violent and energetic occurrences in the universe, a gamma-ray burst (GRB). This event is now providing new clues to what causes a GRB and how its energy in produced.

Gamma-ray bursts are some of the most luminous explosions astronomers can observe, but they are quick and fleeting and therefore mysterious, because no one knows for sure what the sources of GRBs are. They could be caused by massive supernovae, or they could be from a dying star collapsing to become a black hole.

GRB 160625B was one of the brightest bursts in recent years, and in about 40 seconds it released as much energy as the sun will emit over its entire lifetime, which happened by chance to be directed in a tight beam of gamma rays toward Earth.

Two NASA satellites that monitor the sky for such phenomena, the Fermi Gamma-ray Space Telescope and the Swift Gamma-Ray Burst Mission, detected the GRB event. These telescopes slewed quickly to the location of the burst to make observations and also immediately relayed the GRB’s position to several automated ground-based telescopes, which then began their observations.

Thanks to this quick reaction, said Nathanial Butler from Arizona State University in a statement, “we were able to measure this one's development and decay almost from the initial blast.”

“We think the gamma-ray emission is due to highly energetic electrons, propelled outward like a fireball.”

A team of 31 astronomers combined their findings in a new paper published in Nature, providing strong evidence that this particular GRB came from a young black hole.

“Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun,” Eleonora Troja, an assistant research scientist from the University of Maryland and lead author of the paper, remarked in a statement. “If you ranked all the explosions in the universe based on their power, gamma-ray bursts would be right behind the Big Bang.”

At least once a day, gamma-ray bursts are detected as brief but intense flashes of gamma radiation, the most energetic form of light. They come from all different directions in the sky, and they last from tens of milliseconds to about a minute, making it hard to observe them in detail.

Gamma rays are invisible to the human eye, but gamma-ray telescopes — if they are fast enough — can catch at least part of the burst, or what’s called the afterglow of light that lingers from the blast. Additionally, there are sometimes additional wavelengths of light emitted, and astronomers can garner other details, especially if there is an optical component associated with the GRB.

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.”