We give here a brief overview of the scientific areas of interest for LT2. A more detailed discussion of the science case for LT2 can be found in our white paper, which is available here.
Supernovae Type Ia are of great importance as standard candles for the measurement of intergalactic distances, to constrain cosmological parameters and test models of dark energy, and all classes of supernovae are of interest as probes of exotic, high-energy physics. Galaxy evolution is also heavily influenced by the radiative, kinetic energy, nucleosynthetic outputs and the birthrates of SNe Ia with time. The model behind SN Ia explosions, in particular the transition from deflagration to detonation, is still a matter of intense debate, and the existence of 'super-Chandrasekhar' SNe Ia, which have early-time spectra resembling those of other SNe Ia but with a luminosity of about twice the average, potentially compromises the use of SN Ia as standard candles. This problem of the super-Chandrasekhar SNe is tied into the uncertain nature of the progenitors: while the majority of the evidence favours single-degenerate scenarios, some observations support the double degenerate paradigm. If there are multiple routes to a SN Ia explosion then the mix of progenitors could potentially change with time. Interest in core collapse supernovae has increased in recent years because of the connection with long-duration gamma ray bursts (GRB). Despite progenitor models for the two classes of object being very similar, the relationship between 'ordinary' SN Ib/c and GRBs is unclear. Possibly many GRBs do not produce enough nickel to be accompanied by an optically bright supernova, or their may be a difference in the stellar explosion mechanism such that a large fraction of associated supernovae are underluminous. Obscuration by dust may also be a significant factor.
Our understanding of supernovae is set to rapidly increase as we progress further into the era of synoptic surveys. As well as providing more statistically complete samples, these surveys are revealing a much greater diversity in the population than was previously observed. The cadences of these all-sky surveys means there is an increasing number of SNe discovered at very early times, which is revealing more about the nature of the progenitors. The diversity of the population suggests that there are many parameters which influence the explosion, such as the initial mass, metallicity, and rotation rate of the progenitor, and the presence and properties of binary companions or magnetic fields.
The major limitation for current supernovae programmes is the lack of follow-up spectra. It is estimated that only 10 per cent of PTF transients receive a spectral classification, let alone any scientific exploitation. This problem will get much worse in the LSST era: there will be many examples of the rarer subclasses, as well as transients at the extreme ends of the luminosity spectrum, which will require a greater physical understanding in terms of explosion parameters and physics. There will be rare ultrabright events, and SNe in unusual environments such as low-luminosity, metal poor hosts. We will also potentially discover new types of stellar explosion by combining optical data with observations from SKA, CTA, gravitational wave and neutrino experiments.
Recently the Public ESO Spectroscopic Survey of Transient Objects has shown that dedicating large amounts of 4 metre time to spectroscopic follow-up of transients can be extremely scientifically productive. Surveys of this nature are well suited to robotic telescopes like LT2, since large numbers of objects can be observed efficiently, with automated scheduling allowing observing cadences to be optimised for each individual object. The instrument would be an intermediate resolution spectrograph with a wavelength range covering the optical and the near infrared, in order to probe to deeper redshifts.
Gamma ray bursts (GRBs) are the most energetic explosions in the universe and have featured prominently in the scientific literature since the launch of the Swift satellite. About 70 per cent of GRBs are classed as long bursts, with burst durations in excess of 2 seconds. The observational evidence strongly suggests that long GRBs are associated with the core collapse of massive stars. The origin of the short bursts, which have durations of less than 2 seconds and make up the remaining 30 per cent of the population, is less clear. The current evidence suggests they do not have a supernova origin, but instead are associated with the mergers of two compact objects, such as neutron stars or stellar mass black holes.
Moving into the next decade, one major goal is to increase the number of GRBs with well-determined high redshifts. GRBs could then potentially be used to derive reliable determinations of cosmological parameters, as well as contribute to our understanding of reionisation and the star formation history of the universe. One associated goal is direct detection of the first stars in the universe, with the identification of GRBs with population III progenitors. This work will be performed with 8 metre class telescopes and the ELTs. However, this is also still much work to be done on smaller telescopes with long GRBs of low to intermediate redshift. Detailed study of the associated supernovae will be critical to the advancement of our understanding of the physics. The number of observed GRB-supernova associations is small, and needs to be increased. There are also many open questions regarding the physical process at work during the initial, prompt phase of the GRB, in terms of particle acceleration and radiation processes. The role of magnetic fields is also an important area. The polarisation measurements have shown that early-time observations of the reverse shock are a key diagnostic of the magnetic structure. The limitation at this point is the number of such measurements.
Our understanding of short GRBs, and in particular the nature of their progenitors, has fallen behind compared to the long GRB population. This is mainly due to a paucity of good quality observations of short GRB afterglows, which limits the statistical analyses. While it is clear that sGRBs are not core collapse events, the binary merger model is not yet confirmed by the observational evidence. Determining the nature of the binary components in a sGRB progenitor (NS-NS or NS-BH) is a key goal. Additionally, it appears likely that the sGRB population is not homogeneous, with some small fraction produced by giant flares from soft gamma repeaters: a class of neutron star which emits irregular bursts of X- and gamma-rays.
Generally as astronomical technology has advanced, the drive has been to greater apertures, in order to capture more photons. Gamma-ray bursts afterglows, and other fast-fading transients present an interesting example where the reaction time of the telescope; incorporating slew time, mirror settling time, instrumental overhead and so on; is of an equal or greater importance. As the size of the telescope increases it is harder to maintain a rapid reaction time: this is part of the reason why the Liverpool Telescope has maintained a leading role in GRB follow-up, despite a relatively modest aperture of 2 metres. Rapid reactions are also of course an inherent boon of robotic observing. The ideal facility for GRB afterglow work in 2020+ therefore improves on both factors: a greater aperture and a more rapid reaction time.
Another key element in the Liverpool Telescope's success in this area is its diverse instrument suite. The automatic response of the telescope to a GRB trigger is to take an initial image, gauge the brightness of the afterglow, then obtain photometry, spectroscopy or polarimetry as appropriate. Polarimetry is an area in which the LJMU team has been particularly productive, through the construction and use of the innovative RINGO series of imaging polarimeters. It has been shown by the GROND team that a simultaneous multiband imager can be a powerful tool for SED modelling, and an instrument of this type on LT2, covering optical and infrared bands, is a serious consideration. Rapid follow-up is reliant on a wide field GRB detector in space supplying alerts for ground based follow-up. Happily, there are strong prospects for a Swift-successor facility in the next decade, in particular the proposed Franco-Chinese mission SVOM.
The era of 'multi-messenger' astronomy, in which transients will be detected via non-electromagnetic emission, has begun. The first direct detection of a gravitational wave source in Autumn 2015 by the Advanced LIGO detector has opened a new window on the universe. By 2022 the LIGO detectors, working in partnership with other facilities such as Advanced VIRGO will reach full sensitivity. The source of the first event was a merging black hole - black hole binary, however as the sensitivity of the detectors increase the population of detected events is expected to be dominated by neutron star - neutron star or neutron star - black hole mergers. Such events should have an electromagnetic counterpart, and detection of this counterpart by conventional telescopes is the next milestone in this story. The components of the electromagnetic signature are the prompt emission and afterglow, which will fade very rapidly (minutes) and will be harder to detect the further we lie off axis. Their may also be a 'kilonova' - a supernova-like, isotropic component powered by radioactive decay of heavy elements synthesised in ejecta. This will be visible on timescales of hours to days. A rapid response is therefore required for optical follow-up. A complication is the localisation of the gravitational wave source, which, like the detector sensitivity, will improve with time. By 2022 the median uncertainty will be reduced to a few square degrees. This is much smaller than the hundreds of square degree uncertainty of current detections, but still a challenge for surveys and wide field telescopes such as LSST. The role of LT2 would be the next stage in the process, responding rapidly to the electromagnetic counterpart detection and obtaining the first spectra, for verification and exploitation.
Another multi-messenger route is the search for astrophysical neutrino sources. The IceCube detector in Antarctica is operational and beginning to make detections, and will be joined by KM3Net in the Mediterranean Sea. The majority of IceCube detections will be in the northern hemisphere, where it has the best sensitivity (the Earth serves as a filter for atmospheric muons). Likely detections are neutrinos coincident with gamma-ray bursts and supernovae, so these detectors synergise well with our proposed science programme for LT2.
The next decade will see a large number of new ground and space based facilities which will work in the time domain. Some of these will open temporal windows in new regions of the electromagnetic spectrum. Others will detect large populations of new objects in well-studied regimes. All of which will benefit from ground-based optical/infrared follow-up from a time domain facility like LT2.
The Gaia satellite launched in 2013 and began science operations in 2014, with the first interim data release eagerly awaited in 2016. It will have an operational lifetime of 5 years, with the final catalogue published in 2020. There will therefore be no overlap with LT2, and so LT2 will not be involved in the Gaia alerts programme. However the catalogue, which will contain a billion stars with incredibly precise positions and distances, will be an incredible resource for LT2 to exploit. This catalogue will contain millions of variable stars and binaries, providing statistically complete samples. Today there are many phases of stellar or binary evolution in which only a very small number of objects are known: in the Gaia era the numbers will increase so that these states can be studied in terms of populations of objects. However, Gaia will provided limited photometry and very limited spectroscopy: ground based follow-up will be essential in order to properly realise the potential of the catalogue.
The Square Kilometre Array Phase 1 will begin full science operations in 2020, with phase 2 following in 2024. Pathfinders like LOFAR are exploring temporal variation in the radio sky, with the LOFAR Transients Key Science Project focuses on exploring and understanding the explosive and dynamic universe by observing transient and variable radio sources. Many of the expected sources are optically bright: Synchrotron emission will be detected in jets from CVs, X-ray binaries, GRBs, SNe and AGN; and coherent emission will be detected from flare stars, brown dwarfs and hot Jupiters. However, the characteristics of the radio transient sky - the range and origin of phenomena - are currently quite unclear.
Construction of the Cherenkov Telescope Array will begin in 2018, with both a northern and southern site. The northern site will be co-located with LT and LT2 at the ORM on La Palma. The arrays will consist of three different types of telescope, with a combined energy range from 10s of GeV to 100 Tev, and a 5-10 improvement in sensitivity over HESS, MAGIC and VERITAS. The northern site will focus on AGN, GRBs and clusters, and the southern site will focus on Galactic sources such as supernova remnants, pulsars, star forming regions and X-ray binaries. Opening the temporal window at this energy range will bring surprises, but the optical is sure to continue to contribute to the study of these classes of object.
The study of extra-solar planets has been a productive area for the Liverpool Telescope, thanks to the RISE instrument: a fast readout camera for the precision measurement of transiting exoplanet timing. The study of transiting exoplanets has greatly advanced in recent years thanks to NASA's Kepler mission, however one limitation is that Kepler planet host stars have tended to be quite faint, with typical magnitudes of 13.5 or greater. This has limited the potential for detailed ground-based follow-up. This is an issue the next generation of planet finders (NGTS, TESS and PLATO) intend to address, by concentrating on stars with visual magnitudes of between 8 and 13. These facilities will find tens of thousands of Neptunes, super-Earths and Earths. Exoplanet science is also now entering the phase when the study of planetary systems will be routine, rather than individual planets. NGTS is now operational at ESO Paranal, TESS is approved by NASA for a 2017 launch, and PLATO is the ESA M3 mission for a ~2022 launch. These missions all fit well with the LT2 timeline, and the planets they find will benefit from detailed optical follow-up. For example, transmission spectroscopy (the measurement of transit depth as a function of wavelength) is a proven technique for probing exoplanet atmospheres, and is increasingly being successfully performed with spectrographs on ground-based facilities. As this technique is refined over the coming decade, we will begin to probe the atmospheres of Earth-sized planets for water and biomarkers.