Supernovae (SNe) are stellar explosions. Their catastrophic nature make them powerful probes of the evolution and life cycle of different types of star. Type Ia SNe are also used as standardisable candles to measure cosmic distances and as a probe of dark energy. Supernova science is a core activity for the time domain community and the Liverpool Telescope (LT) receives more proposals based on the follow-up of supernovae and related stellar explosions than any other science topic.
Distribution of LT proposals based on science area showing that supernovae are the most popular science topic.
The term 'supernova' covers a wide, and growing, variety of stellar explosions which can be distinguished according to their explosion mechanism. The four boxes below summarise the key supernovae of interest to the NRT project, including the 'exotic' subclass which include new and exciting sources which are predicted to be discovered by the cadences and depths provided by new survey facilities.
- White dwarf and another star
- Apparent homogeneity
- Mature and direct probe of dark energy and accelerating Universe
- Observationally diverse
- Multiple progenitor scenarios
- End of life for massive stars
- Wide range of subclasses
- Affected by metallicity, rotation rate, binary companions
- Early time spectra can probe shock breakout phase
- Asymmetricity common; can be probed using polarimetry.
- Relatively new class of extremely luminous transients
- Variety of explanations for mechanism of energy conversion
- Models include ejecta-circumstellar collisions, fast-spinning neutron stars or large 56Ni masses.
- New surveys detect previously unknown variable sources
- ATLAS source AT2018cow first of a new class of fast and blue transient
- First spectrum obtained by LT
- Rapidly growing class
- Defy current SNe models
To date, two gravitationally lensed supernovae (LSNe) have been discovered. These sources present an exciting opportunity to probe cosmological parameters such as the dark energy content of the Universe and its expansion history. Multiple images of transient events are not observed simultaneously due to their different paths through the Universe and lens; these time delays between images can be used to measure the Hubble constant. Rubin is forecast to discover ~50 type Ia SNe per, however, the observing cadence does not yield sufficiently many LSNe with accurate and precise time delays, therefore there is a critical need for high cadence, multi-band monitoring of future Rubin LSNe, along with confirmation spectroscopy to determine characterisation. The NRT is ideally suited to this task.
The modern survey era has seen the rate of discovery of new SNe jump to thousands per year, providing scope for large scale and systematic approaches to classification and follow-up for population studies. There has also been the discovery of new and exotic phenomena which challenge existing explosion and progenitor models, such as superluminous supernovae. Nightly-cadence surveys such as ZTF and ATLAS are currently revealing a population of fast and blue transients; heralding the promise of the 'faint and fast' Rubin era. These searches cannot operate effectively in isolation as they provide only limited photometric information, a limited cadence and, in some cases, sacrifice spatial resolution for field of view. Multicolour light curves are crucial for classification purposes: without detailed photometry to provide supernovae luminosities and colours, the physical parameters of the supernova explosions cannot be mapped, and the large samples for cosmological studies cannot be assembled.
The NRT will provide essential photometric follow-up of these survey candidates, even as the cadences of the surveys themselves improve. The Vera Rubin Observatory's Legacy Survey of Space and Time will image the entire Southern sky every few days, but the observing cadence in the same filter will be typically much longer.