long wavelength array
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Long Wavelength Array
Joseph LazioNaval Research Laboratory
High Angular Resolution, Long-Wavelength Radio
Astronomy
• An Historical OverviewWhy now?
• The Long Wavelength Array– Science – Technology
Early Days: Telescopes
Jansky
Clark Lake TPT
UTR-2
• Jansky first detected celestial radio emission at 20 MHz.
• Long wavelength astronomy stimulated much of modern astronomy.
Non-thermal emission, Pulsars, Quasars, …
• Large telescopes built.
Early Days: Science
Jansky
Clark Lake TPT
UTR-2
• Ultra-high Energy Cosmic Rays: 45 MHz (~ 1965)
• Pulsars: 80 MHz (1967)• VLBI: (1967)
What happened?
Ionospheric Phase Effects
• If antennas are close together, << 1 radian Imaging possible
• If antennas are far apart, > 1 radian Imaging possible only if phase effects can be corrected
CorrelationPreserved
CorrelationDestroyed
> 5 km<5 km
Ionosphere
= reNe
Ionosphere Refraction
• Both global and differential refraction seen.
• Time scales of 1 min. or less
• Equivalent length scales in the ionosphere of 10 km or less
Confusion
~ 1´rms ~ 3 mJy/beam
~ 10´rms ~ 30 mJy/beam
= /D
NRL-NRAO 74 MHz Very Large Array
• Early 1980s: development of self-calibration
– Data driven– Solve for N antenna phases
using N(N-1)/2 observed interferometric phase differences
• Early 1990s: 8-antenna prototype
• 1998: All 27 antennas outfitted
> 5 km<5 km
NRL-NRAO 74 MHz Very Large Array
74-MHz VLA is the world’s most powerful long-wavelength
interferometer.
First Sub-arcminute Imaging74 MHz VLA
(d) (e)
(b)(a)
Crab(Beitenholz et al. 1996)
Cas A(Kassim et al. 1995)
M87(Kassim et al. 1995)
Hydra A(Lane et al. 2004)
Approaching Arcsecond ImagingVLA+PT
Cygnus A: A Long-Wavelength Resolution of the Hot Spots (Lazio et al.)Highest angular resolution imaging at wavelengths longward of 3 m ( < 100 MHz)
VLA
PT antenna, 70 km
distant
~ 10" angular resolution
VLA Low-frequency Sky Survey
Summary• Image 3π sr north of = 30°
95% complete
• Frequency = 74 MHz (4 m)• Resolution = 80" (FWHM)
VLA B configuration
• Noise level ≈ 0.1 Jy beam-1
• Point-source detection limit 0.7 Jy• Nearly 70,000 source catalog
Methodology Survey region covered by 523 individual
pointings TOS: 75 minutes per pointing Each pointing is separated into five, 15-
min. observations spread out over several hours Data reduced by completely automated
pipeline Once reduced and verified, all data
posted to the Web
Correcting the Ionosphere
Self-Calibration Field-Based Calibration
Field-Based Calibration Take snapshot images of bright sources in the field and compare to NVSS positions. Fit to a 2nd order Zernike polynomial phase delay screen for each time interval. Apply time variable phase delay screens
Field-Based Calibrationdeveloped by J. Condon & W. Cotton
2.5°
VLSS Image Gallery
Imaging Parameters: RMS noise level: ~0.1 Jy/beam Resolution: 80 ''
5'
Gallery ofunusually large objects
Long Wavelength ArrayA New Window on the
Universe
Long Wavelength Array Long Wavelength Array
Current Capabilities
LWA
Angular resolution Sensitivity
LWA Science Case1. Acceleration of Relativistic Particles
• Supernova remnants (SNRs) in normal galaxies (E < 1015 eV)
• Radio galaxies & clusters at energies (E < 1019 eV)• Ultra-high energyc cosmic rays (E ~ 1021 eV?)
2. Cosmic Evolution & the High-z Universe• Evolution of Dark Matter & Energy by differentiating
relaxed and merging clusters• Study of the 1st black holes• H I during the Dark Ages?
3. Plasma Astrophysics & Space Science• Ionospheric waves & turbulence• Acceleration, Turbulence, & Propagation in the interstellar
medium (ISM) of Milky Way & normal galaxies• Solar, Planetary, & Space Weather Science
4. Radio Transient Sky
Pulsars at Long Wavelengths
• 4C 21.53W recognized as steep spectrum source.
• Later identified as PSR B1937+21.
• A high dynamic range, long-wavelength instrument may find interesting pulsars.– PSR B0809+74 is steepest
spectrum source in pilot VLSS observations.
– Viz. PSR J0737-3039 (S1400 ≈ 5 mJy).
PSR B0809+74
Long Wavelength Array
• 20–80 MHz• Dipole-based array stations• 50 stations across New Mexico• 400-km baselines arcsecond resolution
400 km
Long Wavelength Demonstrator Array
• 60–80 MHz• 16-element dipole station + 1 outlier• At VLA site in NM
Long Wavelength Demonstrator Array
• Dual-polarization dipole + active balun• Cable to (shielded!) electronics hut• Receiver (reconfigurable FPGA) selects frequency,
digitizes, time-delays, filters to 1.6 MHz bandwidth• Beamforming or all-sky imaging
LWDA First Light Movie
Cas A
Cyg A
Galactic plane
LWDA First Light Movie
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
LWDA First Light Movie
Cas A
Cyg A
Galactic plane
Cyg A = 17 kJy @ 74 MHz
cf. STARE program found no transients above 27 kJy at 610 MHz
RFI Environment
RFI Environment
Frequency (MHz)
FM radioTV audio and video carriers
HF COMM
LWA Progress
• Several candidate antennas being field tested
• Site testing around New Mexico• Program office at the U. New
Mexico• Southwest Consortium
– UNM, NRL, ARL:UT, LANL– U.Iowa
• Multi-year funding through Office of Naval Research
• Target is first, full LWA station, LWA-1, in 12–18 mon.
• LWA Science and Operations Center in New Mexico in ~ 3 yr
LWAPhased Development
Time Phase Description
1998-present 0 Existing 74 MHz VLA
2005–present ILong Wavelength Development Array
(funded by NRL/ONR)
2007–2010 II9-station Long Wavelength Intermediate
Array
2010–2012 III LWA Core
2012–2014 IV High-Resolution LWA
2009– V LW Operations & Science Center
SUMMARY
• LWA will open a new, high-resolution window below 100 MHz one of the most poorly explored regions of the spectrum
• Key science drivers:– Particle Acceleration– Cosmic Evolution & the High-z
Universe– Plasma Astrophysics & Space
Weather– Radio Transient Sky
• Long Wavelength Demonstrator Array (LWDA) already demonstrating potential for transient surveys.
• Rapid progress being made toward Long Wavelength Array deployment
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