A New Window for Exploration
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 10 and 90 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 most distant objects in the Universe to
the earth's ionosphere. Five key science areas have been identified
for the LWA: cosmic evolution, transient objects, the interstellar
medium of the Milky Way Galaxy, solar and extra-solar planets, and
ionospheric, solar and space weather sciences. Our confidence that
the LWA can explore these science topics has been increased by 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.
The Early Universe
The LWA will be able to take advantage of the generally steep spectral
slope of distant radio objects to probe the Universe throughout its
evolution. This includes detecting the first radio galaxies and black
holes in the high redshift, or very young, universe, and tracing out
the large scale structure of the universe via cluster radio haloes,
relics, and radio galaxies which can be observed from present day out
to the very earliest times in the universe. Issues of galactic evolution
will be addressed through studies of the nonthermal gas associated with
massive star-forming regions in the interstellar medium (ISM) of normal
nearby galaxies. And finally, it will be possible to study the epoch
of reionization with a search for HI 21cm absorption against the most
distant radio-loud quasars and galaxies identified using the LWA.
Radio Transient Sky
In comparison to higher energies (X- and gamma-ray), the radio transient
sky is poorly explored, primarily because of the lack of wide-field
radio telescopes. Nonetheless, the variety of known radio transients
suggests that the radio sky may be quite dynamic. With its wide field
of view, the LWA will be ideal for exploring the radio transient sky.
Expected sources of emission include millisecond-period pulsars, black-hole
neutron star binaries, and other similar exotic systems. The LWA will
also be able to study the variable but coherent radio emission from
SNe, GRBs, and AGN, helping to better define the emitting mechanisms
for these sources. Finally, the LWA will be able to detect radio emission
from ultra-high-energy cosmic ray air showers, and may detect new
classes of radio transients.
Interstellar Medium
The LWA will provide an excellent probe of the interstellar medium
(ISM) of the Milky Way Galaxy. It will trace Galactic electron cosmic
rays, from their presumed origin in supernova remnants to their three-dimensional
spatial distribution as measured by thermal absorption to objects
of known distance. Using the broadband capacity of the LWA it will
also be possible to study the energy distributions of the cosmic rays.
Using interstellar scattering, the LWA will also probe the possible
coupling between dilute relativistic gas formed by the Galactic electron
cosmic rays and the more substantial warm ionized medium. A complete
census of supernova remnants will be made and targeted for study of
their interaction with the Galactic environment. Finally, thermal
absorption measured over a variety of optical depths will provide
excellent radial distance information.
Solar System
Within our solar system, it is known that Jupiter emits bursts of
nonthermal emission at very long wavelengths, which would be visible
to the LWA for detailed study, helping to pinpoint their location and
origin. Extra-solar planets which, like Jupiter, have strong coherent
cyclotron emission would be directly delectable with the LWA as well.
Ionosphere, Sun, and Space Weather
A final area of interest for LWA measurements is ionospheric, solar,
and space weather science. The ionosphere will necessarily contribute
phase turns to every observation made by the proposed instrument. Thus
the LWA will be a sensitive probe of ionospheric turbulence, and especially
traveling ionospheric disturbances (TIDs). 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 and scintillations.
With the use of an appropriate transmitter, this could possibly be extended
to solar radar experiments to predict geomagnetic storms.
Serendipitous Discoveries
The LWA will be able to address all of these science issues and
more. Due to the relatively poorly explored spectral regime in which
it will operate, serendipitous discoveries are to be expected. The
design of the instrument will be flexible enough that it can be easily
adapted to new scientific aims as they become known, allowing the
instrument to become a long-lived contributor to radio-astronomical
science.
