Transients and Time-Domain Astronomy – Probing the dynamic Universe in real time
The Universe is not static. Explosive events, rapid variability, and short-lived phenomena play a fundamental role in the life cycle of stars and galaxies. Transient sources — such as supernovae, tidal disruption events, and compact-object mergers — provide direct insight into stellar evolution, nucleosynthesis, and extreme physical processes.
By combining high angular resolution, integral-field spectroscopy, and rapid response capability, HARMONI on the ELT will offer a transformative view of transient phenomena, enabling spatially resolved studies that are impossible with current facilities.
Spatially resolved spectroscopy of explosive events
Many transient phenomena occur in complex and crowded environments, such as star-forming regions or galaxy nuclei. Disentangling the transient signal from its surroundings requires both high spatial resolution and spectral information.
HARMONI will enable:
- Spatially resolved spectroscopy of supernovae and their immediate environments
- Separation of transient emission from host-galaxy background
- Mapping of velocity fields, excitation conditions, and chemical abundances
This capability is essential to link transient properties to their progenitor systems and local environments.
Supernovae and their environments
Supernovae are key drivers of chemical enrichment and feedback in galaxies. However, the nature of their progenitors — particularly for core-collapse supernovae — remains incompletely understood.
With HARMONI, astronomers will:
- Probe supernova ejecta and circumstellar material
- Characterise local stellar populations at explosion sites
- Constrain progenitor mass, age, and metallicity
- Study asymmetries and velocity structure in expanding ejecta
These observations provide direct constraints on stellar evolution models and explosion mechanisms.
Compact-object mergers and exotic transients
The discovery of gravitational-wave sources has opened a new window on the transient Universe. Electromagnetic counterparts to compact-object mergers are often faint, rapidly evolving, and embedded in complex host environments.
HARMONI will contribute by:
- Providing high-sensitivity spectroscopy of faint transient counterparts
- Measuring host-galaxy properties and local conditions
- Constraining the sites of heavy-element (r-process) nucleosynthesis
Such observations link time-domain astronomy to fundamental questions about the origin of the elements.
Nuclear transients and extreme environments
Transient events in galaxy nuclei, including tidal disruption events and nuclear supernovae, probe extreme gravitational and radiative environments.
HARMONI’s angular resolution allows:
- Separation of nuclear transients from surrounding stellar light
- Detailed spectroscopic characterisation of emission-line regions
- Studies of the interaction between transients and central black holes
These observations provide insight into the interplay between stellar evolution and galactic nuclei.
Rapid response and time evolution
A defining feature of transient science is the need to observe rapidly and repeatedly. HARMONI is designed to support efficient follow-up and time-resolved spectroscopy, enabling:
- Monitoring of spectral evolution on timescales of days to weeks
- Tracking changes in velocity structure and ionisation conditions
- Building three-dimensional views of transient evolution (space, wavelength, time)
Integral-field spectroscopy is particularly powerful in this context, capturing the full spatial and spectral information in a single observation.
Synergies in the time-domain era
HARMONI will operate in an era of major time-domain surveys and multi-messenger astronomy. Its role is to provide detailed physical characterisation following discovery.
Key synergies include:
- Wide-field transient surveys providing discovery and classification
- Space-based observatories offering complementary wavelength coverage
- Gravitational-wave and neutrino detectors identifying extreme events
Together, these facilities enable a complete, multi-scale view of transient phenomena.
A dynamic pillar of HARMONI science
Time-domain and transient astronomy represents a key pillar of HARMONI’s scientific programme. By enabling spatially resolved spectroscopy of rapidly evolving events, HARMONI will:
- Reveal the physical mechanisms driving stellar explosions
- Link transient events to their progenitors and environments
- Advance our understanding of feedback, chemical enrichment, and extreme physics
HARMONI will thus play a central role in exploring the dynamic Universe, where change, violence, and variability shape cosmic evolution.