A very brief cosmic explosion, followed by another much longer one in the same region of the sky: astronomers are still trying to understand what happened.
On April 25, 2016, NASA's Swift satellite recorded a gamma-ray burst. These flashes of radiation are among the most energetic phenomena in the Universe. They generally last only a few seconds or minutes, but can release a considerable amount of energy.
Artist's impression of a gamma-ray burst.
Credit: ESO/L. Calçada — CC BY 4.0.
That day, the instruments detected something unusual. A first burst lasted 1.7 seconds. After about four minutes of calm, a second lasted 51.2 seconds. Both came from the same small region of the sky.
At the time, the observations had not made it possible to resolve this mystery. Ten years later, a team reanalyzed data from the satellite and several telescopes. It discovered that the two phenomena appear to belong to two different categories of gamma-ray bursts.
Short bursts are generally linked to the collision of two extremely dense objects, such as neutron stars. Long bursts, on the other hand, often appear when a very massive star collapses. Astronomers distinguish these events by their duration, but also by their energy and the shape of the radiation emitted.
Yet the first burst, dubbed G1, displays several characteristics of a short burst. The second, G2, looks more like a long burst. This is therefore not simply an explosion that was interrupted for a few minutes before resuming. The signals appear to be linked to two different physical phenomena.
The researchers considered the possibility that two independent explosions had occurred by chance in the same direction. But such a coincidence would be extremely improbable. They also studied the possibility of a single event producing two phases. None of the known mechanisms easily explains both the long pause and the difference between the two bursts.
The event, named GRB 160425A, occurred in a galaxy so distant that its light traveled for about 5.5 billion years before reaching us. Astronomers were able to observe the afterglow left behind by the explosion, without identifying with certainty the objects that caused it.
For now, the researchers propose several possibilities, including scenarios involving an unusual system of celestial objects. But no answer currently stands out. This observation could lead to a reassessment of how gamma-ray bursts are classified and to the search for other comparable events in archival data.