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