Spectral lags as a new window on GeV–TeV emission and particle acceleration in ultra‑relativistic jets

Spectral lags are time delays between emission peaks observed at different photon energies in the same high‑energy transient or variable source. In simple terms, they quantify the fact that “hard” (high‑energy) and “soft” (low‑energy) photons do not always arrive at exactly the same time, and thus provide a direct handle on how emission regions evolve and how particles are accelerated and cooled in these regions.

In gamma‑ray bursts, spectral lags have been measured for decades and are known to correlate with luminosity, but they have rarely been used to constrain detailed prompt‑emission models, largely because these models remain highly degenerate and difficult to formulate self‑consistently. In active galactic nuclei, by contrast, even the simplest time‑dependent one‑zone leptonic scenarios for GeV–TeV flares predict energy‑dependent spectral delays, making lags a promising — yet still under‑exploited — diagnostic of jet structure and particle acceleration. In pulsars, phase lags between radio and gamma‑ray peaks, and between multiple gamma‑ray peaks, have long been recognized as key probes of magnetospheric geometry and emission zones, but the corresponding energy‑dependent (GeV–TeV) lag phenomenology remains largely unexplored.

This workshop will bring together observers and theorists to assess the potential of spectral‑lag measurements as a new observational window on GeV–TeV emission and particle acceleration in ultra‑relativistic jets and pulsar environments.