This paper is highly cited because it reports the first observational evidence of the existence of a strong correlation between the energy radiated by Gamma-Ray Bursts (GRB) under the assumption of isotropic emission (conventionally named Eiso) and the photon energy at which these events are most luminous (called "peak energy"). This correlation has several implications for the physics and geometry of GRB emission and for the understanding of different classes of GRBs.

“This paper reports the discovery of the Ep-Eiso correlation in Gamma-Ray Bursts (GRB), i.e., the correlation between the photon energy at which these events are most luminous (peak energy, Ep) and their radiated energy assuming they are spherical sources (isotropic equivalent energy, Eiso).”

Moreover, based on our work, it was found that, by adding a third observable, the Ep-Eiso correlation tightens and becomes useful for standardizing GRBs and uses them for the estimate of cosmological parameters, in a way similar to Type Ia supernovae. Thus, many theoretical and observational papers on GRB discuss this correlation and refer to our paper.

This paper reports the discovery of the Ep-Eiso correlation in Gamma-Ray Bursts (GRB), i.e., the correlation between the photon energy at which these events are most luminous (peak energy, Ep) and their radiated energy assuming they are spherical sources (isotropic equivalent energy, Eiso).

This correlation was found by us based on GRBs detected by the Italian-Dutch satellite "BeppoSAX" and was later confirmed and reinforced by observations of other satellites, including the now operating "Swift" satellite. In the paper we also introduced the methodology for the computation of Eiso in a common cosmological rest-frame energy band by transforming the observed energy spectrum into the GRB rest-frame spectrum.

GRBs are the most powerful transient events in the universe. They are sudden and unpredictable bursts of hard X-ray/soft gamma-ray radiation (most of the photons have energies from 1-2 keV up to 1-2 MeV) emanating from random directions in the sky, showing a huge intensity and lasting from a tiny fraction of a second up to a thousandth of a second. They are detected by low Earth orbit satellites at a rate of about 1 event/day. Their study started in 1973 and, despite the enormous observational progress occurred in the last 10 years, we are still far from a satisfactory comprehension of these phenomena. We know that most of them come from "cosmological distances" (i.e., from one to several billions of light years), that their released energy is huge and that the longer ones (>2s) are likely associated with peculiar supernova explosions, while the shorter (<2s) ones are likely produced by the coalescence of stellar binary systems composed by two neutron stars or a neutron star and a black hole.

In particular, the physics behind the emission of X/gamma-ray photons is still far from being settled and several emission mechanisms may play a role. Thus, a "forest" of models has grown, and the complexity of the GRB phenomenon makes it difficult to choose among them and to constrain the free parameters of each of them.

The peak energy (Ep-Eiso) correlation discovered by us is one of the most robust observational evidences and must be directly linked to the physics of the emission. Thus, it is a strong test for GRB emission models, and it is commonly used as an input or a test output for GRB synthesis models.

Also, it has been found that the Ep-Eiso plane is a powerful tool for the identification and understanding of peculiar sub-classes of GRBs. For instance, particularly soft and weak events (called X-Ray Flashes) are consistent with the correlation, showing that they likely have the same origin of normal GRBs. Instead, the shorter GRBs (those lasting less than 2 seconds) do not follow the Ep-Eiso correlation: a further evidence of a different nature of short and long GRBs.

Finally, it has been found that, with the addition of a third observable—jet opening angle or the "high signal" time—the Ep-Eiso correlation becomes tighter and useful for the estimate of cosmological parameters. This is because, if the correlation holds and is accurate enough, we can estimate the radiated energy from the spectral peak energy and compare this value with that estimated from the distance and the measured flux (Luminosity = 4*pi*distance^2).

The distance is computed from the measured redshift of the optical counterpart and/or the host galaxy of the GRB by assuming a set of values for cosmological parameters (e.g., curvature, density of baryonic matter, density of "dark matter," cosmological constant, or "dark energy" density). Therefore, the comparison between the GRB luminosity, or radiated energy, obtained through the Ep-Eiso correlation and that obtained from flux and distance allows us to estimate the values of cosmological parameters.

I was first involved in the BeppoSAX mission as a Ph.D. student in 1996, working on the calibration and data analysis of the Gamma-Ray Burst Monitor (GRBM) on-board this satellite. This was the very exciting period of the revolutionary BeppoSAX discoveries on GRBs—first accurate localizations, discovery of X-ray afterglow emission and consequent discovery of first optical counterparts and estimates of distance and energetics.

Thus, I was in the right place at the right moment and I could participate in the analysis and interpretation of this unprecedented data on GRBs. Towards the end of the mission, in 2001, as an active member of the BeppoSAX GRB team, I was in charge of coordinating the analysis of the energy spectra of GRBs detected by both the WFC (X-ray detectors) and the GRBM (soft gamma-rays detector). By exploiting the redshift estimates available for 12 of these GRBs and the excellent data, we searched for correlations between spectral parameters and total radiated energy, also developing a method to compute the radiated energy in a common cosmological rest-frame energy band.

In this way we arrived at the discovery of the Ep-Eiso correlation as reported in Amati et al. (2002) and confirmed by measurements from other satellites later on. I still continue to actively work on this topic (e.g., Amati, Monthly Notices of the Royal Astronomical Society, 2006, and Amati et al., Astronomy & Astrophysics, 2007).

None, directly. More generally, the technologies developed for X and gamma-rays astronomy have a stronger impact on practical applications for the development of sophisticated diagnostic tools in the fields of medicine, the arts, and security.