A comprehensive broadband analysis of the high-redshift GRB 240218A

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Context. 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. Unfortunately, they are rather rare, with only a dozen of them identified so far. Aims. We present here the discovery of the GRB with the second highest spectroscopic redshift measured to date, GRB 240218A at z = 6.782, and the broadband analysis of its afterglow. Following the detection by high-energy satellites, we obtained multi-epoch and multi-wavelength photometric follow-up observations, from 68 s to ∼48 d after the detection. These data allow us to perform a comprehensive study of the emission and physical properties of this event. We also compare these properties with GRBs observed at high and low redshift. Methods. We built the X-ray, near-infrared, and radio light curves and studied their temporal evolution. Moreover, we investigated the spectral energy distribution (SED) at different times to trace 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 GRB (LGRB) population. Results. The SED analysis reveals a typical afterglow-like behaviour at late times. The origin of the early-time emission is uncertain, with the probable presence of an additional contribution on top of the afterglow emission. From the broadband physical modelling of the afterglow, we identify a narrow Gaussian jet seen slightly off-axis, θv = 2.52−0.29+0.57 deg, and pinpoint the presence of a possible jet break ∼0.86 d after the trigger. Conclusions. The results of the analysis and the comparison with other high-z GRBs reveal that we can consider GRB 240218A as a ‘standard’ high-redshift LGRB: the prompt phase properties, the X-ray luminosity, and the optical extinction are consistent with the values derived for the LGRB population. The jet opening angle is narrower but compatible with those of high-z bursts, possibly pointing to more collimated jets at high redshift.

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