A stellar collision that shone for almost a minute complicates the stellar explosion scenario

Gamma-ray bursts (GRBs) are the most energetic phenomena in the universe, detectable even if they occur in galaxies billions of light-years away. They are classified as short or long depending on whether they last longer than two seconds, and their duration is associated with their origin: long bursts occur with the death of very massive stars, while short bursts are related to the merger of two compact objects, such as neutron stars, black holes, or both. Now, the detection of an almost one-minute GRB produced by the collision of compact objects complicates the scenario, as it shows that the classification of these bursts according to their duration does not entirely correspond to reality and opens up new scenarios in the death of stars.

“When studying the outburst, called GRB211211A, we observed clear indications that pointed to a kilonova, produced in the fusion of two neutron stars, and not to a supernova, the explosion with which very massive stars end their lives", says José Feliciano Agüí Fernández, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participates in the study, published in Nature. “In fact, the luminosity, duration and colour of the kilonova are similar to another well-known event that occurred in 2017, a neutron star merger that was the first observation of a cosmic event in light and gravitational waves”.

Neutron stars are very compact, rapidly rotating objects that arise when a very massive star ejects its envelope in a supernova explosion. We know that the merger of neutron stars will produce a short gamma-ray burst (GRB), gravitational waves and a kilonova, a phenomenon similar to supernovae but whose energy comes partly from the decay of radioactive species and which produces large amounts of heavy elements -indeed, most of the gold and platinum on Earth is thought to have formed as a result of ancient kilonovae.

The characteristic signature of kilonovae is their brightness in the near-infrared, which is much higher than their brightness in visible light. This difference is due to the fact that the heavy elements ejected by the kilonova block visible light but not infrared light, which has a longer wavelength. "However, observing in the near-infrared is a technical challenge and few ground-based telescopes are able to do so. This finding was made possible by the Gemini telescopes, which showed us that we were looking at a neutron star merger", says Jillian Rastinejad, the lead researcher at the University of Northwestern (USA).

The conclusions of the scientific team, which also used data from other telescopes, including the Hubble Space Telescope, the Gran Telescopio Canarias (La Palma) or the 2.2-metre telescope at Calar Alto Observatory (Almería), are in agreement with those obtained by another group that, after studying the outburst with different focus and observations, also concluded that the outburst was produced by a kilonova. "We were able to observe this event only because it was very close to us -says Alberto Castro-Tirado, IAA-CSIC researcher and co-author of this second paper, which also includes data from the 2.2-metre Calar Alto telescope-. Only after ruling out other possibilities did we realise that our decade-long paradigm had to be revised".

In addition to contributing to our understanding of kilonovae and GRBs, this discovery provides a new way to study the formation of heavy elements in the Universe. Until recently there was disagreement about what is known as the r-process (or fast process), which takes place in explosive stellar events and is responsible for the production of half of the elements heavier than iron, including uranium and gold. Although supernovae were originally thought to be the source of these elements, recent studies favour neutron star mergers as the main producers of the heavier elements.