GRBs and Relativistic Transients in the 2040s

TL;DR

Relativistic transients provide extreme-physics laboratories and cosmic lighthouses, and the 2040s will yield an enormous discovery rate from LSST, THESEUS, and third-generation GW detectors. The paper argues for a dedicated time-domain facility with a collecting area of $10{-}30$ m, robotic scheduling, and optical-NIR spectroscopy at $R\sim 2{,}000{-}10{,}000$ to characterize dozens of events per night down to $m\sim 23{-}25$, enabling rapid redshift, composition, and velocity measurements. It outlines key open science questions spanning jet launching, structure, environment, high-redshift probes (e.g., $z>6$ GRBs), and nucleosynthesis, and prescribes automatic data pipelines and multi-wavelength coordination to maximize scientific return. Together, this infrastructure would enable population-level constraints on jet physics, progenitor environments, and heavy-element production across cosmic time, transforming our understanding of relativistic transients.

Abstract

Relativistic transients such as gamma-ray bursts (GRBs), jetted tidal disruption events, luminous fast blue optical transients, and fast X-ray transients, represent the brightest explosions in the Universe and serve dual roles as laboratories for extreme physics and as cosmic lighthouses probing the earliest epochs of the Universe. The 2040s will bring transformative capabilities: wide-field optical surveys discovering tens of thousands of optical transients nightly, proposed high-energy missions like THESEUS providing 10-100x improved high-energy monitoring, and third-generation gravitational wave detectors identifying $\mathcal{O}(10^5)$ compact object mergers annually, many accompanied by relativistic jets. This industrial-scale discovery rate will enable population studies addressing fundamental questions such as jet launching mechanisms, nucleosynthesis, the first stars, and how progenitor environments shape these transients across cosmic time. However, realizing this science requires overcoming a critical bottleneck: these transients evolve on timescales of seconds to days, with their physics encoded in rapidly-changing multi-wavelength signatures demanding immediate spectroscopic characterization down to m ~ 25. Current facilities, optimized for classical/queue scheduling, do not provide the rapid, flexible, multi-target response necessary for industrial-scale follow-up. This white paper demonstrates that without a dedicated large-aperture (10-30 m effective collecting area) time-domain facility with robotic scheduling and optical-NIR spectroscopic capabilities, the transformative potential of relativistic transient science in the 2040s will be considerably limited.