Rome, 26 June 2023 - A team of researchers, which includes the National Institute of Astrophysics (INAF), has observed the very first stages of a gamma-ray burst (GRB), which turned out to be the brightest in the optical bands detected until now. GRBs are explosive transient phenomena at the centre of continuous scientific revolutions. INAF is engaged both on an observational-interpretative level and with the participation in large missions from space to detect and study GRBs. Gamma-ray bursts are among the most violent events in the universe, billions of light-years away from us. Their energy is transferred into mighty collimated jets, which emit the radiation we observe. The researchers studied GRB220101A, whose signal - as the acronym says - was detected for the first time in January 2022.

The astronomers, led by researchers of the Purple Mountain Observatory (in China), used a new method developed to derive reliable photometry from sources "captured" by the Ultraviolet and Optical Telescope (UVOT), one of the three instruments aboard the Swift Observatory.

Stefano Covino, a researcher at INAF and the only Italian among the authors of the study, explains that "this discovery helps to reveal the origin of extremely energetic ultraviolet/optical flares and demonstrates the need for high temporal resolution observation in the first instants of the evolution of the phenomenon". And he adds: “Each GRB event shows original behaviours, but in general, we find that even the most extreme cases still fall into the same phenomenology. GRB220101A is no exception. It is therefore not a new category of GRB but plausibly an extreme case among those already known".

Why, then, is it a “monstre” case? Covino observes that “the reason is probably twofold. On the one hand, simply by accumulating more observations, it is possible to identify rarer cases that normally there would be a low probability of being able to observe. And in addition, there is a technical question which consists of having defined a procedure to obtain reliable information from satellite observations even when, as in this case, the data are saturated. This allowed us to have information in the first phase of this event and identify the impressive peak in brightness we are talking about".

Swift, Fermi and Agile have observed GRB220101A. “As always, when Swift identifies a GRB, the on-board small field of view telescopes, such as UVOT, are repointed, and data is obtained within seconds after the identification of the high energy event (the actual GRB). An excellent result for an instrument that has been flying since 2004! As soon as the identification alert arrived on the ground, the "ground-based" telescopes also began to observe. The Chinese 2.2m Xinglong telescope obtained the distance measurement via a spectrum, resulting in the remarkable value of z=4.6. At the time of the event that generated this GRB, the universe was just over a billion years old,” Covino says.

The researcher underlines the tremendous technical work done on this GRB: “First of all, we have to imagine that any optical telescope receives the luminous radiation from a celestial object and converts it into an image on its detector. Now, what happens is that, depending on the characteristics of the telescope, the image that is created for a point object, such as stars or even a GRB at cosmological distances, has a precise mathematical form (technically, it is the PSF). We can imagine a pointed hat with a point at the top and wide brims around it. Making "photometry" means measuring well this hypothetical hat's extension and height! However, in practice, for such brilliant events, the central part of the "hat" is erased, as if cut, and therefore it is impossible to obtain the necessary information. However, there are precise relationships between the height of the "hat" and the faults, which depend for telescopes in space (i.e. without the effect of the atmosphere) only on the technical characteristics of the telescope itself. With very detailed work, we measured the parameters of these relationships and then reconstructed the shape of the "hat" in retrospect to obtain complete photometric information. This too can be an example of how, even with an instrument that has been flying since 2004, we never stop improving”.

Despite decades of study, GRBs continue to show surprises. Covino concludes: “It almost seems they are an inexhaustible reservoir of extreme behaviours. They show how certain combinations of parameters lead to the prodigious optical luminosity observed in the real world. This has significant consequences for evaluating GRBs' impact on their host galaxies' environment.

Co-author of the paper, Hao Zhou, and first author, Zhi-Ping Jin, of the Purple Mountain Observatory, have a solid connection to Italy. Jin was a postdoc in Milan with Covino, while Zhou, a young man at the end of his doctorate, is currently visiting the INAF headquarters in Milan, where he works with Covino.

Related journal article:

“An optical/ultraviolet flare with absolute AB magnitude of -39.4 detected in GRB 220101A", Zhi-Ping Jin, Hao Zhou, Yun Wang, Jin-Jun Geng, Stefano Covino, Xue-Feng Wu, Xiang Li, Yi-Zhong Fan, Da-Ming Wei and Jian-Yan Wei, Nature Astronomy.

Contacts:

INAF press office - Marco Galliani, ufficiostampa@inaf.it, +39 3351778428