LWA Science Summary

The greatest discoveries in astronomy have been the result of technological innovations that open new windows of the electromagnetic spectrum. The Long Wavelength Array (LWA) will explore the relatively neglected frequency regime between 20 and 80 MHz with unprecedented angular resolution and sensitivity, making it uniquely suited to serendipitous discovery, and also able to address a variety of scientific problems ranging in scale from the earth's ionosphere to the most distant objects in the Universe. Three key science areas have been identified for the LWA: cosmic evolution, the acceleration of relativistic particles, and plasma physics. Confidence that the LWA can successfully explore these science topics has been increased by the success of the 74 MHz system at the Very Large Array (VLA), which, although much less capable, has acted as a pathfinder for both the technical and scientific aspects of the LWA.

Cosmic Evolution

As matter accretes onto supermassive black holes it creates the powerful radio emission which is the signature of radio galaxies. Detection and study of radio galaxies at high redshift therefore probes the epoch in which these sources were first assembled and the galactic bulges in which they are found. Because they are thought to be a major source of energy input to the Universe at certain epochs, an understanding of the redshift evolution of supermassive black holes is necessary to test cosmological predictions and to interpret observations of other objects in their near environments. Additionally, samples of radio galaxies observed at low frequencies are important for work on unification schemes because the lobe emission is uncontaminated by beaming effects. The emission spectra of radio galaxies have a typically steep slope at wavelengths shorter than 30cm in the rest frame. Due to Doppler shifting, searching for steep spectrum objects at long wavelengths is an efficient way to detect high redshift radio galaxies. Given the very long wavelengths at which it operates, the high sensitivity, and the large field of view which will make it possible to observe and derive spectra for many sources simultaneously, the LWA will be an ideal instrument for efficiently detecting very high redshift radio galaxies.

Current models for the reionization of the Universe suggest that star formation may have begun as early as z ~ 20. The radio afterglow from gamma-ray bursts as these early stars collapsed to black holes, along with any very early radio galaxies, would provide natural background sources against which to measure highly-redshifted 21-cm absorption from the still largely neutral (i.e., H I) intergalactic medium. The LWA will not only be able to find these high-redshift background sources, but the good instantaneous bandwidth and spectral resolution will provide an efficient means to search a range of redshifts for HI 21-cm absorption profiles, which provide information the distribution, kinematics and temperature of any neutral gas detected.

Finally, the assembly of large-scale structures in the Universe is thought to proceed in an hierarchical manner, with smaller structures merging to form larger structures. The most energetic of these mergers in the present-day Universe involve merges of groups and clusters of galaxies. At least a fraction of these are marked by cluster halos and relics, which are characterized by diffuse, steep-spectrum emission not associated with any galaxy in the merging cluster. The current census of cluster halos and relics is considered incomplete due to the low sensitivity of existing long-wavelength instruments. The LWA will have the sensitivity at long enough wavelengths to detect less energetic and more distant mergers, and it is estimated that there may be thousands of them. A complete census of diffuse emission in merging clusters would make it possible to not only trace the dark matter potentials which govern mergers, but also to define a non-merging cluster sample which would provide the undisturbed systems necessary to study the dark energy equation of state through determination of the baryonic mass fraction in massive clusters.

The Acceleration of Relativistic Particles

Numerous aspects of the mechanisms by which particles are accelerated remain problematic or poorly understood nearly a century after the discovery of cosmic rays. The LWA will have the potential to probe shock acceleration across the full range of particle energies, from 10¹² eV to 10²⁰ eV and above. At energies below roughly 10¹⁵ eV, the majority of the particles are thought to be protons and nuclei accelerated in Galactic supernova remnants (SNRs), a conclusion strengthened with the identification of nonthermal X-ray emission from a small number of Galactic SNRs. Comparing SNR shock acceleration models to observations, however, requires high-resolution, low-frequency observations, which the LWA will be able to provide. The LWA will allow sensitive, resolved spectral index studies to be conducted for hundreds of Galactic SNRs in the Milky Way and in nearby galaxies. Moreover, at low frequencies, H II regions become opaque, meaning that the emission measured toward them is solely the result of the synchrotron emissivity along the line of sight. By measuring the synchrotron emissivity to hundreds and potentially thousands of H II regions at known distances throughout the Galaxy, the LWA will be able to produce a three-dimensional map of the synchrotron emissivity, and potentially the cosmic-ray distribution itself, within the Galaxy. By combining this information with low energy gamma-ray measurements, it becomes possible to derive information about the three-dimensional geometry and strength of the Galactic magnetic field.

At energies between roughly 10¹⁵ and 10¹⁹ eV, particle acceleration is thought to occur in the jets and lobes of powerful radio galaxies. Current studies and models of these phenomena usually rely on extrapolations of source spectra from observations at shorter wavelengths, to the long wavelengths where the low energy electrons which contain the bulk of the energy in the systems emit. The combination of sensitivity, long wavelengths, and resolution offered by the LWA will greatly advance our understanding of these objects by accurately measuring their long wavelength fluxes, allowing us to model the physical processes involved. LWA measurements will probe emission from electrons with Lorentz factors (γ < 1000) that dominate pressure and energy dynamical calculations, including the electron population responsible for interpreting inverse Compton emission. Accurate spectral index maps can also differentiate between primary and secondary acceleration mechanisms in radio galaxies which lie in clusters, and constrain magnetic field strengths and geometry (Feretti et al. 2004) .

Finally, at the highest energies, above roughly 10¹⁹ eV, particle propagation should be limited by scattering off the cosmic microwave background or the so-called GZK mechanism. The presence of particles with these energies suggests either the presence of nearby and unidentified accelerators or possibly physics beyond the Standard Model of particle physics. As these ultra-high energy cosmic rays strike the atmosphere, they produce a pulse of coherent emission, with a radius of a few hundred meters at LWA wavelengths. By detecting these coherent emission pulses at the individual LWA stations, one can turn the entire atmosphere into a cosmic ray detector, with a much larger effective area than any terrestrial detector.

Plasma Physics

The LWA will probe plasmas ranging from the earth's ionosphere to the inter-stellar medium, to the inter-galactic medium. First, the earth's ionosphere will necessarily contribute phase turns to every observation made by the proposed instrument, allowing us to study it for free. Thus the LWA will be a sensitive probe of ionospheric turbulence, including ionospheric disturbances (TIDs), and can be used to improve global ionospheric models. Second, the LWA will be able to study both the quiet sun and the bright active sun, including measurements of Coronal Mass Ejections, solar bursts, interplanetary shocks, scintillations, and all aspects of the sun-Earth connection which include manifestations of various Space Weather effects. With the use of an appropriate transmitter, this could possibly be extended to solar radar experiments to predict geomagnetic storms.

Finally, all Galactic and extragalactic radio sources are observed after their radiation has propagated through the Galactic plasma. By measuring thermal absorption towards SNRs, the LWA will obtain distances to these nonthermal sources, coupling them to the HI-derived Galactic kinematic distance scale. Variations in the ISM plasma density produce refractive index fluctuations, scaling as ν ^-2, which in turn scatter the radiation. In addition to their corrupting effects, interstellar propagation effects are a powerful sub-parsec probe of the interstellar plasma, can provide a tracer of energy input into the ISM, and may be linked to cosmic ray propagation. The LWA will make long-wavelength observations of compact sources to provide a powerful diagnostic of propagation effects from the ISM. The LWA will also be capable of detecting scattering caused by clouds in the inter-galactic medium, a phenomenon so far unobserved.

Conclusions

Wilkinson et al. (2004) have described how radio astronomy has had a successful track record of discovering new phenomena, ranging from the discovery of nonthermal emission processes to pulsars to the indirect detection of gravitational radiation to the first extrasolar planets. General purpose instruments, such as the LWA is envisioned to be, have played an important role in these discoveries. The range of possible discoveries for the LWA is large, with coherent emission processes---from classes of sources both known (eg. Jupiter-like planets) and unknown---being an obvious example of what the LWA might discover. The design of the instrument will be flexible enough that it can be easily adapted to additional scientific aims, allowing the LWA to become a long-lived contributor to radio-astronomical science.