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Science with the Long Wavelength Array

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Science with the Long Wavelength Array Exploring the Last Electromagnetic Frontier •• • • • with the Long Wavelength Array Namir E. Kassim1, A.S. Cohen1, P.C. Crane1, R. Duffin1, C.A. Gross1, B. C. Hicks1, W.M. Lane1, J. Lazio1, E. J. Polisensky1, P. S. Ray1, K. Stewart1, K.W. Weiler1, T. E. Clarke2, The University of New Mexico H.R. Schmitt2, J. M. Hartman3, J.F. Helmboldt3, J. Craig4, W. Gerstle4, Y. Pihlstrom4, L.J. Rickard4, G.B. Taylor4, S.W. Ellingson5, L.R. D’Addario6, R. Navarro6 1NRL, 2NRL/Interferometrics, 3NRL/NRC, 4UNM, 5VA Tech, 6JPL http://lwa.unm.edu Vision: Nearly three decades ago, the VLA first opened the cm-wavelength radio sky to detailed study. Recently, the VLA 74 MHz system has provided the first sub-arcminute resolu- tion view of the meter-wavelength radio universe. This technical innovation has inspired an emerging group of much more powerful instruments, including the Long Wavelength Array (LWA), the Low Frequency Array (LOFAR), and the Murchison Widefield Array (MWA). Located in New Mexico near the VLA, the LWA will be a versatile, user-oriented electronic array which will open the 20-80 MHz frequency range to detailed exploration. With a collecting area approaching a million square meters (at 20 MHz), the 400 km LWA’s milli-Jansky sensitivity and arc-second resolution will surpass, by 2-3 orders of magnitude, the imaging power of previous low frequency interferometers. Because it will explore one of the last and most poorly investigated regions of the spectrum, the potential for unexpected new discoveries is high. Pathfinder LWA Science with the Long Wavelength Demonstrator Array Pathfinder LWA science programs were initiated in 2007 near the center of the VLA with the Long Wavelength Demonstrator Array (LWDA - below left). The LWDA was a technology path- finder covering 60-80 MHz, and producing 2 fully independent steerable beams each of 1.6 MHz bandwidth. The 16 element station consisted of full polarization active dipole antennas, analog gain stages, flexible FPGA-based digital receivers, and a digital beam former. All-sky data taken by the LWDA are currently being analyzed to search for bright radio transients, while a program to monitor the secular flux density decrease of the supernova remnant (SNR) Cassiopeia A with the LWDA backend is ongoing. Photograph of the LWDA with the VLA in the background in its compact “D configuration” Left: The LWDA digital receiver and data acquisition system allowed us to cross cor- relate each of the 120 pairs of antennas in the array to produce low-resolution all-sky images. The image at left is the first frame from an LWDA all-sky movie made at 74 MHz. Emission from the bright sources Cas A and Cyg A can be seen, along with ex- tended emission associated with the first quadrant of the Galactic plane. (Green labels are Galactic coordinates.) Science with the Long Wavelength Array First Science: The Secular Flux Density Decrease of Cas A Future Science In 2007, outlier LWA prototype antennas utilizing the LWDA backend as a correlator began The LWA science case is described in our Activity White Paper for the Astro2010 decadal panel conducting high Signal-to-Noise measurements on many known discrete radio sources. These (http://www.ece.vt.edu/swe/lwa/memo/lwa0162.pdf). The key science drivers are summarized below (left), together measurements were originally intended to verify the performance of LWA prototype antennas with two graphical illustrations encompassing Cosmic Evolution, Relativistic Particle Acceleration, Extra-solar Plan- compared to computer simulations. A serendipitous scientific result emerged by combining the ets, & Discovery Science. LWA & Clusters: Particle Acceleration, Dark Matter, Dark Energy measurements of Cas A with data from the 74 MHz VLA, establishing a much more accurate Below: A521 radio halo (center) & relic (left) at 240 MHz (red), superimposed The LWA will be uniquely sensitive to the merger-driven shocks & turbu- model for its long term secular flux density decrease, together with a verification of previous on Chandra X-rays (blue) (Brunetti et al. 2008, Nature, 455, 944). lence that heat the ICM & compress its magnetic fields. For example, the Key LWA Science Drivers steep spectrum radio halo Abell 521 (left) is barely detectable at 1400 suggestions for relatively short term variability (Helmboldt & Kassim, 2009, Astronomical MHz. Recent work (e.g. Cassano et al. 2008, A&A, 480, 687) suggests A521 may be the “tip of an iceberg” of a population of radio halos that Journal in press, arXiv:0903.5010). Constraining the temporal flux density behavior of Cas A Cosmic Evolution emerge ONLY at low radio frequencies and would be easily detected with is required to understand supernova blast wave evolution and their interaction with the ISM. High Redshift Universe LWA. Evolution of Large Scale Structure, Below: Left - LWDA interferometer based measurements of LWA antenna gain patterns as a function of altitude and azimuth as By tracing merger-driven emission from thousands of clusters over a range measured towards 4 bright radio sources. Right - our improved model of the secular flux density decrease of Cas A - the data Dark Matter & Dark Energy of redshifts, the LWA will trace cluster number density & merger fre- also confirm previously suggested short-term variability, including a hint of non-stochastic behavior. The results motivate con- quency, placing tight constraints on the cosmological evolution of the larg- est Dark Matter halos in the Universe. LWA maps will also differentiate tinued monitoring. Our measurements are referenced to Cygnus A - a large, powerful, and distant lobe-dominated radio galaxy Acceleration of Relativistic Particles magnetic-field configurations & particle-acceleration mechanisms. whose flux density (set on the Baars et al. scale) is assumed constant. In SNRs in normal galaxies at energies 15 X-ray observations of clusters probe the Dark Energy density and equa- up to 10 eV. tion of state by measuring the baryonic mass fraction of the Universe, but In radio galaxies & clusters at energies depend on the identification of a large, relaxed cluster sample, a costly and 19 often ambiguous task at optical and X-ray wavelengths. LWA observations up to 10 eV. will be an efficient method for distinguishing a relaxed sample of galaxy In ultra high energy cosmic rays at 21 LWA & Extra-solar planets energies up to 10 eV & beyond Jupiter’s Radio Spectrum 10-18 LWA -19 ) 10 1 Plasma Physics & Astrophysics - z H Io- and 2 - non-Io-DAM Ionospheric waves & turbulence m 10-20 W ( U A Solar and Planetary Science 1 -21 10 t a f y f t i o The Interstellar Medium (ISM) and t s u n -22 e 10 C D F x L beyond u l c i r F -23 e 10 h p s o DIM n o Opportunity: Discovery Science 10-24 I 10-1 100 101 102 103 104 The greatest discoveries in astrophysics Frequency (MHz) have coupled key technical innovations Left: Schematic of Jupiter’s decametric emission (courtesy Imai Lab., Kochi National College of Technology); Right: Zarka & Kurth 2005, Sp. Sci. Rev. 116, 371. Radio transients offer a new frontier for next-generation electronic telescopes such as the LWA. The most exciting anticipated with new windows on the EM spectrum transients may be extra-solar planets. All solar system giant planets generate radio emission from the interaction between their Potential New Horizons: transients, magnetic fields and the solar wind. If planetary magnetic fields are typical, extrasolar planets may be detectable. Jupiter’s decametric radio bursts, with their characteristic upper frequency cutoff ~40 MHz, are the prototype for such studies (see fig- extra-solar planets, coherent emission ures above). Current extra-solar planet searches below 100 MHz have been sparse, with observational limits at the optimistic Helmboldt & Kassim, 2009, Astronomical Journal in press, arXiv:0903.5010) Hartman 2009 (LWA memo155 - http://www.ece.vt.edu/swe/lwa/memo/lwa0155a.pdf) sources - what else? extreme of theoretical predictions. Allowing for anticipated variations of the magnetic fields and internal compositions, the LWA may directly detect them. Radio-traced Cosmic Ray-shielding planetary magnetospheres could be a prerequisite for life. 2008: Bmax ~72 km, Ae~103 m2, 73.0 < ν < 74.6 MHz Tomorrow: Bmax ~400 km, Ae~105-6 m2, 20 < ν < 80 MHz Single LWA Station (inset: CDR-ready LWA prototype antenna) ~53 LWA Stations across New Mexico The State of the Art: 74 MHz VLA + Pie Town With baselines up to 73 km, these VLA + Pie Town link 74 MHz images represent the most recent state of the art in high angular resolution, low frequency radio astronomy. For comparison, 330 MHz images are also shown. These images are being used to study evolution (e.g. via synchrotron aging) and self-absorption processes (e.g. via synchrotron self-absorption) and acceleration in radio galaxies (Cyg A and Her A), and for studying shock acceleration and ejecta in SNRs (Cas A). Cygnus A Hercules A Cassiopeia A Lazio et al. (2006) Gizani et al. (2005) T. Delaney, PhD Thesis, UMN Kassim et al. (2007) 74 MHz VLA A-configuration 74 MHz VLA: A-config θ ~ 25'' (~35 km) θ ~ 25'' 74 MHz θ ~ 25'' VLA: A+PT 74 MHz VLA - Pie Town Link θ ~ 8'' (~72 km) θ ~ 8'' 325 MHz VLA: A-config θ ~ 6'' (~35 km) θ ~ 6'' θ ~ 8'' ~100 m The LWA project is led by the University of New Mexico, and includes the Naval Research Laboratory, Jet Propulsion Laboratory, Los Alamos National Laboratory, Virginia Tech, and U. Iowa, with cooperation from the National Radio Astronomy Observatory. Funding for the LWA is managed by the Office of Naval Research. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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