As described above, the key science goals for the Transients KSP are to discover, identify and report (e.g. to other facilities) variable and transient radio phenomena. In doing so we will be undertaking a census of energetic feedback and particle acceleration in the universe, completing the most comprehensive northern hemisphere survey for radio pulsars, and uncovering the range and origin of coherent radio phenomena from nearby extrasolar planets to very distant extragalactic radio bursts.
Synchrotron emission, particle acceleration and cosmic feedback: essentially all explosive events which inject energy into the ambient medium result in particle acceleration and/or compression/enhancement of magnetic fields, resulting in synchrotron emission. Such emission is likely to be initially self-absorbed at LOFAR frequencies, with a rise time corresponding to the timescale for the source to expand and become optically thin in a given band. This expansion timescale is proportional to the initial size of the emitting region divided by the expansion velocity. In approximate order of increasing rise time, sources which will be associated with such emission are jets from CVs, X-ray binaries, GRBs and SNe, and finally AGN. Under most conditions, synchrotron emission has a maximum brightness temperature of 1012K.
Figure 2. Evolution of a radio outburst from the X-ray transient CI Cam, with data from the Ryle Telescope (15 GHz), the Green Bank Interferometer (2 and 8 GHz) and the Westerbork Synthesis Radio Telescope (0.8 and 0.3 GHz). The radio signal is due to synchrotron emission from an expanding source of accelerated electrons. By the time of the first radio observations (`externally triggered' by X-ray observations) the radio source is already optically thin at the highest frequency, 15 GHz. As the source expands, the flux density at different frequencies increases as the optical depth at that frequency decreases, but then subsequently decays once in the optically thin regime as expansion results in energy losses. The emission at 330 MHz, just above the LOFAR band, was detectable within a few days of the outburst, but did not peak until 20—30 days later. Such behaviour will be characteristic of explosive outburst events associated with e.g. relativistic jets, supernovae, GRB afterglows (as indicated by the cartoons to the right of the figure), with the rise and decay timescales being an increasing function of the luminosity of the event.
Note that many of the types of object responsible for explosive particle acceleration, e.g. X-ray binaries, Gamma-ray bursts, are of intense interest to the high-energy astrophysics. LOFAR detections of such events, especially with the Radio Sky Monitor (see below), will undoubtedly therefore be used as triggers for optical / infrared / X-ray / gamma-ray follow-up. Taken as an ensemble, observations of the synchrotron sources will provide a complete time-resolved census of particle acceleration in the local universe, shedding light on the energization of ambient media and sites of cosmic ray acceleration.
Coherent emission: several different types of radio emission with high brightness temperatures often resulting from very short durations, are lumped together under the title of `coherent' radio emission. This is generally taken to mean groups of electrons moving together en masse, and may often be highly anisotropic (e.g. maser emission). Several different classes of coherent radio emitters are likely to be detected by LOFAR, e.g.
1. Flare stars, brown dwarfs and active binaries are likely to be present in almost every LOFAR beam, giving off highly circularly-polarised radio bursts from coherent emission processes. Potential targets include M dwarf flare stars, active binaries like RS CVn and low mass L and T dwarfs.
2. LOFAR will also study radio emission from planets both within and beyond the Solar System. This includes imaging Jupiter's magnetosphere at high spatial and time resolution, imaging Jupiter's radiation belts, and studying planetary lightning from the other planets within the Solar System. It is also predicted that radio bursts from nearby so-called 'hot Jupiter' exoplanets might also be detected, and we will carry out a detailed survey for such objects. If successful, we would have an inclination independent catalogue of extrasolar planets, including possible diagnostics of their magnetic fields and rotation periods.
3. LOFAR may detect extragalactic radio bursts, such as that reported by Lorimer et al. (Science, 2007), to very large distances, possibly as far as redshift ~7, providing a unique probe of the properties of the intergalactic medium (via their dispersion measure). Such events may be even be associated with neutron star - neutron star mergers, in which case the radio identification of events detectable by advanced LIGO may be possible. Since such mergers are predicted to provide independent distance measurements based on their gravitational wave signatures, identification of the electromagnetic counterpart would provide a unique test of gravity on cosmological scales, as well as an independent test of the redshift-distance relation.
In addition to all this, a major survey of classical radio pulsars will be undertaken, as well as the study of related objects such as Anomalous X-ray Pulsars (AXPs) and Rotating Radio Transients (RRATs). The LOFAR pulsar survey is expected to discover more than 1000 new pulsars (see Figure 3), which will provide the majority of pulsars for the Pulsar Timing Array in the northern hemisphere. Such a survey also has a fair chance of turning up the first pulsar - black hole binary. In addition, LOFAR will provide the sensitivity to allow us to study the individual pulses from an unprecedented number of pulsars including millisecond pulsars and the bandwidth and frequency agility to study them over a wide range which will provide vital new input for models of pulsar emission. This will provide us with an unparalleled survey of the population of massive star end-products within our galaxy.