SN 2023lcr: the first supernova from an orphan afterglow
Résumé
Dirty fireballs are on-axis gamma-ray bursts (GRBs) with moderately relativistic jets, unable to create the bright flash of gamma-ray radiation characteristic of GRBs. Up to 2023, only 3 dirty fireball candidates had been observed. In June 2023 ZTF reported a new fast fading transient known as AT2023lcr, consistent with a GRB afterglow at z~1.03. Thanks to constraining pre-explosion imaging from GOTO, it was possible to tightly constrain the time window of the explosion, when the gamma-ray prompt emission should have happened. Despite continuous coverage, no gamma-ray emission was seen by any of the GRB observatories, such as INTEGRAL and Fermi, allowing us to set tight limits on the prompt gamma-ray luminosity of this event. In terms of prompt vs afterglow emission, these limits make AT2023lcr one of the most inefficient GRBs jets ever observed to date. In this talk, we will present the results from an intensive multi-wavelength campaign from radio to X-rays carried out to study the nature of this observationally rare GRB case. These observations are consistent with an on-axis afterglow at redshift 1.027, making AT2023lcr the best "dirty fireball" candidate observed to date. Late monitoring of AT2023lcr with VLT/HAWK-I, GTC, JWST and HST revealed the supernova associated with the orphan afterglow confirming its association with the collapse of a massive stripped star, as seen in typical GRB-SN associations.