Astronomy Tea Talk

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. We therefore conclude that these are likely Galactic events. We calculate an upper limit on the rate of OAs down to 𝑔<22 AB mag of 7.46 deg−2yr−1 using our criteria and constrain possible jet structures. We also place upper limits of the characteristic angle between the 𝛾-ray-emitting region and the jet's half-opening angle.

Gravitational wave and electromagnetic observations offer complementary insights into the lives of compact stellar binaries as they form, evolve, and die. With even the very formation of compact binaries poorly understood, I will first introduce a novel technique using mid-infrared data to reveal common envelope ejection leading to LISA-detectable white dwarf binaries that ultimately end as Type Ia supernovae. Next, I will discuss a search using TESS aimed at identifying the poorly-localized mergers of neutron star and black hole binaries which are detected by current GW observatories like LIGO, to directly probe their final fates. Finally, I will describe upcoming Hubble and JWST time-domain surveys of globular clusters, predicted to be prolific dynamical factories of these systems; binaries discovered in these environments will help to untangle the histories of LIGO binaries, and could themselves be LISA sources. Together, these observations explore the evolution of compact binaries from formation to merger.