From Jets to Mergers: Multiwavelength Signatures of Synchrotron Transients

This plot compares the kinetic energy and outflow speed of synchrotron transients, spanning non-relativistic to highly relativistic events. It shows where the unusual radio transient ASKAP J0055–2558 sits relative to known populations such as supernovae, gamma-ray bursts, and tidal disruption events, highlighting its mildly relativistic outflow and energetics. Derived from radio observations, the figure illustrates how different explosion geometries and evolutionary stages map onto the broader landscape of cosmic explosions.

Abstract

Synchrotron transients arise from some of the most energetic explosions in the Universe and trace the interaction of relativistic or sub-relativistic outflows with their surrounding environments. These interactions generate shocks that accelerate particles and amplify magnetic fields, producing broadband synchrotron emission. While the underlying emission mechanism is broadly similar across sources, differences in progenitor systems give rise to a wide range of outflow velocities, energies, and light-curve behaviours. Broadband observations are therefore a powerful tool for probing the physics of these extreme events, from compact object interactions to stellar explosions.

In this talk, I present results from three complementary approaches to studying synchrotron transients: targeted wide-field radio surveys, untargeted extreme transient searches, and rapid response follow-up. I discuss late-time ASKAP searches for radio afterglows from compact object mergers detected by LIGO–Virgo, the discovery and detailed characterisation of the luminous and long-lived extragalactic radio transient ASKAP J0055–2558, and a comprehensive radio-to-X-ray study of the exceptionally bright GRB 240825A. Together, these studies demonstrate how broadband observations enable robust constraints on relativistic outflows and establish radio surveys and electromagnetic follow-up as a critical tool for exploring the extreme transient Universe in the SKA era.