A golden era of time-domain and multi-messenger astronomy is upon us as new, panchromatic observatories such as SKA and Rubin become operational, and upgrades to LIGO and Virgo expand our capacity to detect gravitational wave events. We expect these observatories to identify hundreds of thousands of transients and open up a tremendous discovery space. UVEX’s rapid imaging and spectroscopic follow-up will provide a vital new window on the dynamic universe by probing the short-lived UV emission from merging neutron stars, performing spectral follow-up of the first hours of core collapse supernovae, and providing a community resource for target-of-opportunity observations.

UVEX at the frontier of gravitational wave physics

The Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, and the Kamioka Gravitational Wave Detector (KAGRA) will be online with major upgrades in 2028. These gravitational wave interferometers are sensitive to fluctuations in gravitational strain due to violent, short-lived mergers of compact objects such as black holes and neutron stars. When one of the merging stars is a neutron star, the merger produces copious amounts of emission in the form of light across the electromagnetic spectrum. For example, in its initial observing run in 2017, LIGO detected a merger of two neutron stars in the GW170817 event. Follow-up observations of this merger event with ground- and space-based telescopes yielded a scientific bonanza: independent constraints on the Hubble constant and thereby present-day expansion rates of the universe, neutron star mergers as sites of heavy element production in the universe, and the connection of short gamma-ray bursts to merging neutron stars.

GW170817 was just the beginning. UVEX will chase prominent early-time UV emission from neutron-star-containing mergers up to 850 Mpc in NUV and 450 Mpc in FUV. This early UV emission, as seen in GW170817, is strikingly bright and peaks in the region of ~10,000 K. Although the origin of this emission is still uncertain, several theories exist: it may be radiatively powered by merger ejecta, or driven by a shock from the interaction of merger ejecta and a relativistic jet. UVEX’s 70% instantaneous sky coverage, 3 hrs average response time, and large field of view for tiling gravitational wave confidence regions will enable the early time capture of UV emission for a planned sample of 20 neutron star mergers. This will help discover precise composition, mass and velocity of early-time ejecta for a host of events ultimately answering questions such as: how rapidly does a remnant black hole collapse, what mechanism generates the UV emission, and of what exotic, ultra-dense matter are neutron stars made? 
 

The core-collapse supernovae revolution

Massive stars perish in core-collapse supernovae. These explosions shock and heat up the material surrounding the supernova to ~10,000K while also propeling ejecta from the star at tremendous velocities. The shocked ejecta and the circumstellar material glow with continuum UV emission and line emission from CIV, HeII, and NIV, before rapidly expanding and cooling. By capturing the early-time UV spectra from up to 2 days post-supernova, we can gain unique insight into the mass-loss history of the star in the years leading up to its demise, a time machine into the final years of its life.

Most current supernova studies have been carried out in the optical, but optical spectroscopic observations are limited in their ability to detect bright emission lines, with far more spectral transition lines being available in the UV. Additionally, the earliest optical spectroscopic observations obtained to date are limited to ~15 hrs. UV spectra can reveal the chemical composition and kinematics of core-collapse supernovae by detecting spectral lines inaccessible to optical spectroscopy. UVEX will provide a unique opportunity to obtain the earliest supernova spectra in UV, tracking the evolution of the event from as low as 3 hours after a trigger, a window which remains inaccessible to the current generation of UV observatories.

UVEX: a community resource for exploring the dynamic sky

Almost all of the explosive transient events in the universe, such as black holes and neutron stars in outburst, relativistic supernovae, and tidal disruption events (TDEs), peak in the ultraviolet. UVEX’s capacity for performing rapid spectroscopic follow-up of these events will open a new window on the dynamic universe and provide a key community resource for target-of-opportunity observations. Specifically, UVEX will devote 8% of its prime mission observing time to spectroscopic target-of-opportunity observations triggered by the community. This will allow astronomers to:

• Capture the geometry and kinematics of outflows through measuring the absorption features in UV spectra of TDEs, an essential input to a prominent unification theory and an entirely new glimpse into accretion disk formation. 
• Resolve the absorption features of disk winds from outbursting black holes, dwarf novae and neutron stars, providing an unprecedented way of looking at accretion physics at the end of the stellar life cycle.
• Track the evolution of the most extreme cosmic explosions, from gamma-ray bursts to Fast and Blue Optical Transients (FBOTs), to discover how different stellar properties lead to a dramatic variety of explosive outcomes.