A blast from the past

Do you see that small red spot? That’s an extremely distant explosion in the early universe imaged by the X-Shooter instrument at ESO’s Very Large Telescope (VLT). This light is from a gamma-ray burst (GRB), one of the most luminous and puzzling phenomena in the universe. 

In September 2021, NASA's Neil Gehrels Swift Observatory detected a bright source of gamma rays in this area of the sky. Once the initial bright flash of a GRB has died down, the afterglow shines at longer wavelengths like visible or infrared light. But they fade very quickly, so astronomers must react fast! A team of astronomers led by Andrea Rossi at INAF in Bologna observed the aftermath of the GRB with a number of telescopes around the world, including several ESO instruments on the VLT and the robotic telescopes REM and GROND hosted at ESO’s La Silla Observatory.

Besides taking images with X-Shooter, the team also used this instrument to obtain spectra. This was key to discover that the burst originates from an extremely distant galaxy, when the universe was only 6% of its current age, making this one of the most distant GBRs ever found. 

The origins of gamma-ray bursts however remain a bit of a mystery. According to Rossi’s team, this particular GRB put out so much energy that it was probably powered by material falling onto a black hole or (less likely) a magnetar –– a neutron star with a very strong magnetic field. With ESO’s upcoming Extremely Large Telescope, properties of bursts like these and their progenitors can be studied in greater detail, and their elusive origin can be uncovered.