Star merger generates 2nd largest gamma ray explosion – 03/03/2024 – Science

An article in Nature magazine on February 21 this year, with the participation of Brazilians, provides further evidence that a gamma-ray burst observed in March 2023 must be the result of a rare cosmic event: the merger of two neutron stars or a star of this class and a black hole.

The collision and subsequent union of such dense celestial objects is called a kilonova. Considered the second most luminous and energetic gamma ray burst ever recorded, last year’s occurrence had been the subject of previous work, also published in Nature, but carried out by another group of astrophysicists, which presented results similar to those of the most recent study.

The explosion associated with a kilonova generates gigantic emissions of light, especially in the wavelength of gamma rays, which are invisible to the human eye. Another signature resulting from the union of two neutron stars is the production of chemical elements from the rare earth group, also called lanthanides, which are heavier than iron.

In space, only the explosion associated with a kilonova would be capable of acting as a rare earth factory. Lanthanides are forged from a process known as neutron capture, which involves radioactive decay and emissions of gamma rays, the most energetic form of light (photon).

“We found evidence that this kilonova would have produced more stable lanthanides than we initially expected,” says astrophysicist Clécio De Bom, from the Brazilian Center for Physical Research (CBPF).

He is one of three Brazilians who signed the article, written by an international group of researchers and coordinated by Eleonora Troja, from the University of Rome Tor Vergata. The other two are Martín Makler, also from CBPF, and Felipe Navarete, an astronomer who works on the Soar telescope, an observation instrument installed in Chile, operated by Brazil and American universities.

In a kilonova, the gamma ray emissions resulting from the merger last for seconds, but there is a release for days or weeks of a residual glow, the so-called afterglow, in other wavelengths, such as optical and infrared. Therefore, even telescopes that do not operate at gamma-ray frequencies, such as Hubble and James Webb, can be activated to monitor the “afterglow” of extremely energetic events. “We observed this kilonova with Soar at five different times”, says Navarete.

The second most intense gamma-ray burst was first recorded on March 7, 2023 by NASA’s Fermi satellite, dedicated to observations in this wavelength range. The event was named GRB 230307A. The first three letters are an acronym for the name of the phenomenon in English, gamma–ray burst (GRB). The numbers refer to the year (23), month (03) and day (07) in which the event occurred. The “A” means it was the first explosion of its kind seen on that date.

In addition to its enormous luminosity, the phenomenon aroused immediate interest in the astrophysicist community due to two other peculiarities: its atypical duration and the region of space from which the explosion originated. The strong pulse of gamma rays lasted for around 35 seconds, which classifies it as a long-lasting explosion, according to the area’s classification. Events of this type that last less than 2 seconds are considered short; Those that exceed this time limit are considered long.

The problem is that kilonovae have always been associated with short GRB, while longer-lasting gamma-ray pulses are often produced by supernovae. The death of very massive stars, a process that also generates a strong explosion, is called a supernova. However, astrophysicists from the two independent groups who analyzed GRB 230307A did not find any massive dying stars where the event occurred.

This region is practically empty of objects and is about 130 thousand light years from the nearest galaxy. Such a bleak scenario led researchers to propose an explanation for this uncomfortable situation: a pair of neutron stars had broken away from the mother galaxy and migrated to a neighboring area. In the middle of nowhere, the stars merged and generated an astonishing burst of gamma rays lasting more than half a minute.

According to Italian astrophysicist Eleonora Troja, a kilonova and its developments have never been observed for more than a few days. “You have to watch for weeks and months [como eles fizeram] to discover which metals were forged in the explosion”, said the researcher, in a press release about the study. As in the place associated with GRB 230307A there is evidence of the formation of lanthanides, the researchers defend the idea that there was a fusion of two objects there very dense and compact, at least one of which was a neutron star.