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March 30, 2007

Prototype for Long Wavelength Array Sees First Light
University of New Mexico is lead institution in revolutionary new radio telescope

Astronomers at the Naval Research Laboratory (NRL) have produced the first images of the sky from a prototype of the Long Wavelength Array (LWA), a revolutionary new radio telescope to be constructed in southwestern New Mexico. The images show emissions from the center of our Galaxy, a supermassive black hole, and the remnant of a star that exploded in a supernova over 300 years ago.

Not only a milestone in the development of the LWA, the images are also a first glimpse through a new window on the cosmos. “First light” is an astronomical term for the first image produced with a telescope. It is a key milestone for any telescope because it indicates that all of the individual components are working in unison as planned.

The University of New Mexico, the lead institution for the LWA, will supervise all aspects of the siting, design, construction and operation says Greg Taylor, Interim Director for the LWA and associate professor of Physics and Astronomy at UNM.

“We hope to present the U.S. astronomical community with a powerful and unique instrument for exploring the universe at long wavelengths,” said Taylor. “At the same time, we will use the LWA to investigate the nature of the Earth’s ionosphere.”

Once completed, the LWA will provide an entirely novel view of the sky, in the radio frequency range of 20–80 MHz, currently one of the most poorly explored regions of the electromagnetic spectrum in astronomy. The LWA will be able to make sensitive high-resolution images, and scan the sky rapidly for new and transient sources of radio waves, which might represent the explosion of distant, massive stars, the emissions from planets outside of our own solar system or even previously unknown objects or phenomena.

“The LWA will allow us to make the sharpest images ever possible using very long wavelength radio waves,” said Namir Kassim, an NRL astronomer in the Remote Sensing Division and LWA Project Scientist. “This newly opened window on the universe will help us understand the acceleration of relativistic particles in a variety of extreme astrophysical environments including from the most distant supermassive black holes. But perhaps most exciting is the promise of new source classes waiting to be discovered.”

According to Taylor, LWA scientific frontiers include distant radio galaxies and clusters – tools for understanding the earliest black holes and cosmological evolution of dark matter and dark energy, respectively; acceleration, propagation and turbulence in the interstellar medium, including the space-distribution and spectrum of Galactic cosmic rays and supernova remnants; planetary, solar and space science, including space-weather prediction, ionospheric measurements and extra solar planet searches; and the radio transient universe including Gamma Ray Burst’s, ultra-high energy cosmic rays and new sources of unknown origin.

The current prototype, referred to as the Long Wavelength Demonstrator Array (LWDA) to differentiate it from the larger LWA project, completed installation on the Plains of San Agustin in southwestern New Mexico in the fall of 2006. Funded by NRL and built by the Applied Research Laboratories of the University of Texas, Austin, the telescope consists of 16 antennas connected to a suite of electronics that combine the signals from each antenna. Each antenna is only four feet tall and acts much like an old style television antenna, receiving radio waves from many different directions simultaneously. When combined, the data from the individual antennas is comparable to that from a more traditional dish style telescope with a diameter of 70 feet.

Although radio astronomy was discovered at low frequencies (near 20 MHz, corresponding to wavelengths of 15 meters), well below the current FM band, astronomers quickly moved up to higher frequencies (centimeter wavelengths) in search of higher resolution and to escape the corrupting effects of the Earth’s ionosphere, a region of charged particles between about 50 and 600 miles above the surface.

The ionosphere, which can "bend" radio waves to produce long-distance reception of AM and short-wave radio signals, causes distortions in radio telescope images. Ionospheric effects become much worse at low frequencies, but new imaging techniques developed at NRL and elsewhere have allowed the "ionospheric barrier" to be broken and enabled high-resolution astronomical imaging at these low frequencies for the first time.

These new imaging techniques provide an improved view of not only the astronomical sky, but the Earth's ionosphere as well. The full LWA will generate richly detailed measurements of the ionosphere that will complement other ionospheric data sources. Understanding the ionosphere is critically important to the Department of Defense because of its effects on communications and navigation systems.

“Radio astronomy got its start at low frequencies (below 100 MHz) with the work of Karl Jansky and Grote Reber, but Reber and others quickly moved to higher frequencies where the ionosphere was less problematic,” says Taylor. “The ionosphere distorts the radio waves at low frequencies and causes radio sources to shift around on the sky, much like stars twinkle due to passage through the atmosphere.  The phase distortions induced by the ionosphere are particularly challenging to interferometers, and can prevent them from achieving useful results. 

“Astronomers (including members of the LWA collaboration) have recently developed techniques that can allow us to remove these phase fluctuations and recover faithful images of the sky. At the same time these phase variations provide us with important insights into the nature of the ionosphere, such as minute density variations.”

The antenna design, which resembles a household ceiling fan, with blades that have drooped down at an angle of 45 degrees, was conceived to allow the array to see the full sky and cover a wide range of frequencies with a single antenna “The sophisticated digital electronics used in the LWDA allow it to change observing frequency or point in a new direction in an instant, and even allow it to look in two directions at the same time,” says Dr. Paul Ray, an astrophysicist at NRL who is overseeing the overall performance of the LWDA.

When completed, the LWA will operate in a similar manner, but on a much grander scale. Plans call for over 13,000 individual antennas, divided into 50 stations. These stations will be spread over a 250-mile area across New Mexico, and possibly beyond.

“I think the possibility for detecting entirely new classes of astrophysical objects is excellent,” said Taylor. “Consider two of the past Nobel prizes in radio astronomy for the detection of pulsars and the cosmic microwave background. These great discoveries happened when we opened up new frequency windows on the universe. The LWA will open up the lowest frequency window available from the ground (before the ionospheric cutoff kicks in at 5-15 MHz) at dramatically higher sensitivity and resolution than we have had before. 

“At these low frequencies we have the possibility to detect coherent emission from gamma-ray bursts, from extra solar planets, and from sources of yet unknown origin. In combination with the wealth of scientific studies that we know will be possible by extraction from higher frequencies the deployment of the LWA will begin a very exciting era for astronomy.”

In addition to Taylor, other UNM researchers involved in the project include, all from Physics and Astronomy, Professor Jack McIver, Associate Professor Patricia Henning, Assistant Professor Ylva Pihlstrom, Adjunct Professors John Dickel and Helene Dickel, Computer Engineering Professor Walter Gerstle and Electrical Engineering Associate Research Professor Christopher Watts. 

UNM will be establishing a Project Office that will help Taylor manage the project. The LWA will also provide a training ground for radio astronomy instrumentation at UNM. Several UNM students have already been involved with work on the LWDA including its siting, construction, and radio-frequency interference evaluation and mitigation.

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