short gamma-ray burst
The other clump in the gamma-ray burst durations is the short bursts — flashes lasting less than about two seconds, sometimes just a few hundredths of a second. They are harder to study precisely because they are over so fast, but they turned out to come from something completely different than their longer cousins: not a single dying star, but the violent marriage of two dead ones.
The favored cause is the merger of two neutron stars (or a neutron star and a black hole). Two ultra-dense stellar corpses, each more massive than the Sun but only about the size of a city, spiral together over millions of years, then collide in a final fraction of a second. The collision can form a black hole ringed by a hot disk that launches brief, fast jets — producing a short gamma-ray burst. Because the engine runs out almost instantly, the flash is short. And because such mergers can happen long after star formation has ended, short bursts are often found in older, quieter galaxies, unlike the long bursts.
Short gamma-ray bursts became one of the most important objects in modern astrophysics in 2017, when a neutron-star merger was caught simultaneously in gravitational waves and as a short burst. That single event confirmed the merger model, showed that such collisions forge heavy elements like gold and platinum, and let astronomers measure the speed of gravity against the speed of light. The short burst was the spark; the gravitational waves were the proof of what made it.
On 17 August 2017, gravitational-wave detectors felt two neutron stars spiral together; 1.7 seconds later, gamma-ray satellites caught a short burst from the same patch of sky. For the first time, a short gamma-ray burst had a confirmed cause we could also feel through the fabric of spacetime.
Sub-two-second flashes from two neutron stars merging — confirmed by gravitational waves in 2017.
Not every short burst is fully pinned down — duration alone can mislead, and some come from neutron-star and black-hole pairings rather than two neutron stars. But the 2017 multi-messenger event made the merger picture the firmly favored explanation.