Two Different Gamma-Ray Bursts Struck Four Minutes Apart, and Nobody Can Fully Explain Why

Astronomers have reported a pair of gamma-ray bursts that does not match anything seen or predicted before: a short burst and a long burst, separated by only four minutes. According to ScienceAlert, the finding, described in a paper in Nature Astronomy, may force scientists to rethink how these explosions are classified and where they come from.

The event, named GRB 160425A, was detected on April 25, 2016, by the Neil Gehrels Swift Observatory in space. Gamma rays are blocked by Earth’s atmosphere, so a space-based instrument is needed, while ground-based telescopes then followed the fading afterglow, ScienceAlert reported.

What are gamma-ray bursts, and why are two types a big deal?

Gamma-ray bursts are among the most energetic cosmic events since the Big Bang. Astronomers sort them by duration. Short ones last less than two seconds and are thought to come from collisions and mergers of compact objects such as neutron stars and black holes. Long ones last more than two seconds and are typically linked to core-collapse supernovae, the deaths of massive stars.

Until now, a single event has not been seen to combine both kinds, with a quiet gap in between.

What exactly did the telescopes record?

The first burst, called G1, lasted 1.7 seconds and is associated with a merger origin. The second, called G2, lasted 51.2 seconds and is associated with a collapsing star. The host galaxy, possibly a pair of merging galaxies, is so distant that the light took roughly 5.5 billion years to reach Earth, according to ScienceAlert.

The outlet reported that, if the energy were spread evenly in all directions, the first burst would have released billions of years’ worth of the Sun’s output and the second roughly three times more. Gamma-ray bursts actually release energy in focused beams, however, which substantially changes the math, corresponding author Bing Zhang of the University of Hong Kong said.

Could it just be a coincidence?

That is the first of five explanations the researchers considered, and they describe each as challenging. In this scenario, the two bursts came from separate host galaxies that happened to line up in space and time, both pointing their jets toward Earth.

ScienceAlert reported that the odds of this are about 2 in a trillion for any one moment and patch of sky. Across all the years Swift has been watching, the odds are only around 1 in 100,000 to 1 in 10 million. Zhang said he would think the bursts were independent, but that the chance of such an alignment is so low that the pair has to share an origin.

What are the other explanations?

The second idea is that both bursts came from the same galaxy but from different places. According to the report, that is even less likely once Swift’s years of observations are factored in.

The third is a rare version of a short burst, with a big blast followed by a weaker tail. The long quiet gap, and the fact that the second burst is far too energetic to be a tail, make that unlikely.

The fourth is a single long burst from a dying massive star, with a smaller early emission before the main explosion. That also looks doubtful, because such a process would probably produce a weaker first signal.

The fifth is the most unconventional: a single system with an undetermined type and number of celestial bodies that produced an intertwined blast, through a mechanism that cannot yet be specified.

Why does it matter?

For readers, this is a story about how science handles a surprise. The standard picture of gamma-ray bursts, with short ones from mergers and long ones from collapsing stars, has served astronomers for years. One event that does not fit either box is a signal that the picture may be incomplete.

The result is a single observed event and the explanations are still open, so it is too early to say which, if any, is right. Zhang, who has worked on gamma-ray bursts as a theorist for more than two decades, said all the scenarios have “severe difficulties” and that this one really challenged him. He added that the researchers ran out of possibilities.

Around Prague, based on reporting by ScienceAlert, Universe Today