A long lasting and unexpected stellar explosion



Calar Alto has participated in the follow-up observations of a stellar explosion which lasted more than a minute and which cannot be explained with the current theoretical models of such bursts.

Gamma-ray bursts (GRBs) are the most energetic phenomena in the Universe, detectable even if they occur in galaxies millions of light-years away. They are classified as short or long GRBs, whether they last less or more than two seconds. Their duration is associated with their origin: long outbursts happen when very massive stars die, while short outbursts are related to the merger of two compact objects, such as neutron stars, black holes, or both.The recent detection of a one-minute GRB produced by the collision of two compact objects complicates the scenario, as it shows that the classification of these bursts according to their duration does not fully respond to the reality and this opens new scenarios for the death of the stars.

"When studying the outburst called GRB211211A, we observed clear hints that pointed to a kilonova, which is produced by the merger of two neutron stars, and not to a supernova, the explosion of very massive stars when their lives end," -- 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 color 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 both in light and in gravitational waves."

Neutron stars are very compact, rapidly rotating objects that occur when a very massive star ejects its envelope in a supernova explosion. We know that the neutron star merger 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 chemical elements -- actually, most of the gold and platinum on Earth is believed to have formed during ancient kilonovae.

The characteristic signature of kilonovae is their brightness in the near-infrared, much higher than their brightness in visible light. This difference is due to the fact that the heavy elements ejected by the kilonova block the visible light but not the infrared, which has a longer wavelength. “Still, observing in the near infrared is technically challenging and few ground-based telescopes can do it well. This finding has been possible thanks to the twin Gemini telescopes, which showed us that we were facing a merger of neutron stars”, says Jillian Rastinejad, a researcher at Northwestern University (USA) who is leading the work.

The conclusions of the scientific team, who also used data from other telescopes, including the Hubble Space Telescope, the Gran Telescopio Canarias (La Palma) and the 2.2-meter telescope at Calar Alto Observatory (Almeria), coincide with the ones obtained by another group which, after studying the outburst with different approaches and observations, also concluded that the outburst was due to a kilonova. "We were able to observe this event in detail only because it was very close to us," said Castro-Tirado, co-author of a second paper (led by Eleonora Troja), which also includes data obtained by the 2.2m Calar Alto telescope. “Only after ruling out other possibilities did we realize that our decade-long paradigm had to be revised.

In addition to contributing to our understanding of kilonovae and GRBs, this finding provides a new way to study the formation of heavy chemical elements in the Universe. Until recently, there was a disagreement with what is known as the r-process (or fast process), which takes place in explosive stellar events and which is responsible for the production of half the elements heavier than iron, among them, uranium and gold. Although at first it was thought that supernovae were the source of these elements, the latest studies favor neutron star mergers as the main producers of the heaviest chemical elements.





J. C. Rastinejad et al. “A Kilonova Following a Long-Duration Gamma-Ray Burst at 350 Mpc”. Nature 612, 223


E. Troja et al. "A nearby long gamma-ray burst from a merger of compact objects". Nature 612, 228



Institute of Astrophysics of Andalusia (IAA-CSIC)

José Feliciano Agüí Fernández feli @ iaa.es

Calar Alto Observatory

Jorge Iglesias  jiglesia @ caha.es

Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI)


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