“One every 10,000 years so strong.” Report the birth of a black hole – Corriere.it

Something like this happens (maybe) once every 10 thousand years. On Sunday 9 October 2022 the satellites detected a high energy radiation recognized as the
gamma-ray burst
(in English gamma-ray burst, abbreviated to Grb) brightest in history. Of the approximately 7,000 gamma-ray bursts observed so far, last October’s was the brightest, thanks to a particular combination. Despite being occurred in a galaxy 1.9 billion light-years away from us, is one of the closest GRBs ever observed and it has been calculated that such a bright one could only occur once every 10,000 years. The study was published in Astrophysical Journal Letters
.

From the explosion of a star that generates a black hole

Gamma-ray bursts are short pulses generated by the explosion of a massive star in a distant galaxy, possibly leading to the formation of a black hole. What differentiates GRBs from normal supernova explosions is the formation of two bursts of radiation fired at nearly the speed of light in two opposite directions. If one randomly points in our direction, we can also observe it from Earth. GRBs are among the most powerful phenomena in the entire Universe.

Clouds of gas and dust

But this episode has another characteristic that makes it unique, studied by an international group led by Andrea Tiengo, associate professor of the Iuss University School of Pavia. The radiation emitted by the distant galaxy crossed the plane of the Milky Way, our galaxy, encountering dense clouds of gas and dust along the way. A part of this radiation, in the X-ray band, bounced off the dust grains, changing direction and therefore traveling a longer distance. The X-rays therefore arrived a few hours later than the gamma rays, whose first impulse lasted about 2 seconds. Each cloud of gas and dust produces a perfect circle around the GRB’s location. X-ray observations have indeed shown a series of concentric rings with increasingly larger radii.

Like a tomography of the Milky Way

Thanks to this, scientists were able to locate and study twenty dust clouds at a distance of between one thousand and 60 thousand light years in our galaxy. «I never imagined that the brightest GRB in history would allow us to make such a detailed tomography of the Milky Way» said Tiengo. The work is based on data collected by the European satellite XMM-Newton in observations carried out two and five days after the gamma ray burst. “Each ring was produced by interaction with a specific cloud at a particular time. The brightest ring was generated by a cloud about 2,380 light years from us, which was therefore hit by the GRB when Alexander the Great was born», explains the professor.