Speaker
Description
The detection and follow-up observations of high-redshift (z > 6) gamma-ray bursts (GRBs) provide a unique opportunity to explore the properties of the distant Universe. GRB\,240218A, discovered by Swift/BAT, is one of the few identified so far, and with a redshift of $z=6.782$ is the burst with the second highest spectroscopic $z$ measured to date. Following the detection by high-energy satellites, several ground-based observations from 68 s to $\sim48$ d after the detection have been secured, from the optical to the radio band.
I will present these broad-band, multi-epoch observations, which allowed us to perform a comprehensive study of the emission
and physical properties of this event. Specifically, we studied the temporal evolution of the X-ray, NIR and radio light curves, and we investigated the spectral energy distribution (SED) at different times to trace the possible spectral evolution. We also compared the prompt phase properties, X-ray luminosity and optical extinction of GRB\,240218A with those of the long-duration GRBs (LGRBs) population, both at high and low redshift.
The SED analysis revealed a typical afterglow-like behaviour at late times. The origin of the early-time emission is uncertain, with the possible presence of an additional contribution on top of the afterglow emission. The broad-band physical modelling of the afterglow is consistent with a narrow Gaussian jet seen slightly off-axis, with $\theta_v=2.52^{+0.57}_{−0.29}$ deg, and with the presence of a jet break $\sim0.86$ d after the trigger. In addition, we identified a narrow jet opening angle consistently with other high-z bursts, possibly pointing to the presence of more collimated jets at high redshift.
This discovery increased the limited sample of $z>6$ GRBs discovered to date, and gave the opportunity to compare them with the population of bursts at any distances. Moreover, it highlights the unique role of high-redshift GRBs like GRB\,240218A in probing the properties of the early Universe, offering critical insights into jet structure, energetics, and the evolution of GRBs across cosmic time.
