Last year, on September 5, 2021, light from a very energetic gamma ray burst - or GRB - reached our planet. This was a rare event because it came from a distant cosmic source at redshift above 6, which means that it traveled for over 12.8 billion years, starting when the Universe was just 880 million years old. It was first detected in the gamma-ray energy-band by the BAT instrument aboard the Neil Gehrels Swift Observatory in orbit around the Earth, a prolific hunter of GRBs, and it was also detected by Konus-WIND, a wide energy band GRB mission operating in interplanetary space. As usual, the event was named after the date of the first observations: GRB 210905A. In the following months, a team of astronomers led by Andrea Rossi, researcher at the Italian National Institute of Astrophysics (INAF), continued to observe this event, or rather its afterglow - the residual which glows at lower energies and lasts for weeks or even months after the flash. Most of the astronomers involved in the international team that conducted the study are members of the STARGATE collaboration, which brings together all those active in GRB follow-up with ESO facilities. The results are now published in Astronomy and Astrophysics.

“This GRB is full of interesting properties. It is one of the most distant GRBs ever and one of the most energetic”, says co-author Dmitry Frederiks, researcher at the Ioffe Institute in St. Petersburg. Moreover, “because of its slow evolution in the last phases both in the optical band and in the X-rays, its afterglow is one of the most luminous ever. Finally, thanks to a last observation with the Hubble space telescope we detected light from its host galaxy, making it one of the 4 GRB hosts known to date at those distances, and also one of the brightest”, explains co-author David Alexander Kann, researcher at the Institute of Astrophysics in Granada (Spain).

Yet, despite these very exceptional findings, the study shows that its properties are consistent with those of more nearby bursts. In particular, this GRB follows the relationship between the frequency at which the spectrum peak occurs and the energy radiated in the gamma-ray band, both assuming an isotropic emission or correcting for the opening angle of the jet. Similarly, the relationship between the energy of the gamma-ray burst emission and the subsequent X-ray emission is similar to that found for other GRBs. Finally, the opening angle of the jet turns out to be the highest at these redshifts, but in line with the angles obtained by observing GRBs closer to us. “Thanks to our observations we can conclude that the mechanism responsible for GRBs does not evolve with the Universe” says Rossi.

As for the cosmic objects that could be the central engine of the extremely large observed energy, the most favored among suggested scenarios propose either a magnetar (a rapidly rotating neutron star with a strong magnetic field) or a black hole. In both cases they arose from the collapse of a massive star, as is the case for a long GRB like this, unlike shorter GRB, which are usually associated with the collision of compact objects such as neutron stars. “The high energy leads us to exclude that the energy needed for the explosion was extracted from a magnetar, while the mechanism that invokes the black hole easily explains the observed energies” says Rossi.

These results were obtained thanks to observations over a total of 8 months, most of them concentrated between September and November 2021, with the last observation obtained with the Hubble telescope in April. Swift and Konus-WIND contributed for the gamma-rays part, Swift and Chandra for the X-rays part, and in optical and infrared ESO’s Very Large Telescope (VLT), Las Cumbres Observatory (LCOGT), the ESO 2.2m telescope with GROND, the INAF-managed Rapid Eye Mount telescope (REM) and the Hubble Space Telescope. In particular, the observations obtained by the STARGATE collaboration with the X-Shooter spectrograph mounted on VLT was important to determine the distance. The infrared imager HAWK-I on VLT was important to follow-up the final trend of the optical emission, while REM was used to monitor the early phase. Thus, this study involved telescopes around the world and in space, and a great international collaboration. “Once again we have shown that when dealing with transient phenomena you need to be able to act quickly and have the right tools”, adds Rossi. “So both observing the phenomenon when it is still bright to obtain a clear and unequivocal result, and having access to those facilities that allow you to cover a large wavelength range, from gamma-rays to X-rays, optical and radio.”

But this is not the end of the story: the spectra obtained with VLT are of great quality and we expect more results to come, revealing the environment where this GRB exploded. Moreover, the host galaxy of this GRB is the brightest of its kind at very high redshift, and is therefore one of the best candidate for observations with the James Webb Space Telescope which has just begun to demonstrate its incredible capabilities, promising to unveil the characteristics of the environment where the massive star at the origin of this GRB was born.

Optical image of the field where the GRB exploded obtained with the acquisition camera of the X-shooter spectrograph mounted on VLT. The region in false colors is obtained by combining the z-band observation in the background with r- and I-band images. The afterglow of GRB 210905A stands out for its red color, which indicates the very high redshift nature of this event. Credits: ESO/STARGATE/A. Rossi - INAF.

Reference:

• the paper published in Astronomy and Astrophysics A blast from the infant Universe: the very high-z GRB 210905A by Rossi, A. search by orcid ; Frederiks, D. D. ; Kann, D. A. ; De Pasquale, M. ; Pian, E. ; Lamb, G. ; D'Avanzo, P. ; Izzo, L. ; Levan, A. J. ; Malesani, D. B. ; Melandri, A. ; Nicuesa Guelbenzu, A. ; Schulze, S. search by orcid ; Strausbaugh, R. ; Tanvir, N. R. ; Amati, L. ; Campana, S. search by orcid ; Cucchiara, A. search by orcid ; Ghirlanda, G. ; Della Valle, M. ; Klose, S. ; Salvaterra, R. search by orcid ; Starling, R. ; Stratta, G. ; Tsvetkova, A. E. ; Vergani, S. D. ; D'Ai, A. ; Burgarella, D. search by orcid ; Covino, S. ; D'Elia, V. search by orcid ; de Ugarte Postigo, A. ; Fausey, H. ; Fynbo, J. P. U. ; Frontera, F. ; Guidorzi, C. ; Heintz, K. E. search by orcid ; Masetti, N. ; Maiorano, E. ; Mundell, C. G. search by orcid ; Oates, S. R. ; Page, M. J. search by orcid ; Palazzi, E. ; Palmerio, J. ; Pugliese, G. ; Rau, A. ; Saccardi, A. ; Sbarufatti, B. ; Svinkin, D. S. search by orcid ; Tagliaferri, G. ; van der Horst, A. J. ; Watson, D. ; Ulanov, M. V. ; Wiersema, K. ; Xu, D. ; Zhang, J.