James Freeburn: A fast-cadenced search for gamma-ray burst orphan afterglows with the DWF programme

The relativistic outflows that produce long gamma-ray bursts (LGRBs) can be described by a structured jet model where prompt 𝛾-ray emission is restricted to a narrow region in the jet's core. Viewing the jet off-axis from the core, a population of afterglows without an associated GRB detection can be predicted. In this work, we conduct an archival search for these "orphan" afterglows (OAs) with minute-cadence, deep (𝑔∼23) data from the Dark Energy Camera (DECam) taken as part of the Deeper, Wider, Faster programme (DWF). We introduce a method to select fast-evolving OA candidates within DWF data that comprises a machine learning model, based on a realistic synthetic population of OAs. Using this classifier, we recover 51 OA candidates. Of these candidates, 42 are likely flare events from M-class stars. The remaining nine possess quiescent, coincident sources in archival data with angular profiles consistent with a star and are inconsistent with the expected population of LGRB host galaxies. Therefore, these are likely Galactic events.