Multiwavelength analysis of AT 2023sva: a luminous orphan afterglow with evidence for a structured jet

Abstract

We present multiwavelength analysis of ZTF23abelseb (AT 2023sva), an optically discovered fast-fading ( Δmr=2.2 mag in Δt=0.74 d), luminous ( Mr∼−30.0 mag), and red ( g−r=0.50 mag) transient at z=2.28 with accompanying luminous radio emission. AT 2023sva does not possess a γ -ray burst (GRB) counterpart to an isotropic equivalent energy limit of Eγ,iso<1.6×1052 erg, determined through searching γ -ray satellite archives between the last non-detection and first detection, making it the sixth example of an optically discovered afterglow with a redshift measurement and no detected GRB counterpart. We analyse AT 2023sva's optical, radio, and X-ray observations to characterize the source. From radio analyses, we find the clear presence of strong interstellar scintillation (ISS) 72 d after the initial explosion, allowing us to place constraints on the source's angular size and bulk Lorentz factor. When comparing the source sizes derived from ISS of orphan events to those of the classical GRB population, we find orphan events have statistically smaller source sizes. We also utilize Bayesian techniques to model the multiwavelength afterglow. Within this framework, we find evidence that AT 2023sva possesses a shallow power-law structured jet viewed slightly off-axis ( θv=0.07±0.02 ) just outside of the jet's core opening angle ( θc=0.06±0.02 ). We determine this is likely the reason for the lack of a detected GRB counterpart, but also investigate other scenarios. AT 2023sva's evidence for possessing a structured jet stresses the importance of broadening orphan afterglow search strategies to a diverse range of GRB jet angular energy profiles, to maximize the return of future optical surveys.