VLBI/e-VLBI

An Introduction for

Networkers

Tasso Tzioumis, ATNF, CSIRO

Outline

• Caveat: A Basic introduction– Not a review of the latest techniques and

results

• Radio Interferometry & VLBI

• Why: Science with VLBI

• How: Technology

• e-VLBI: Impact

Radio Interferometry

Simple InterferometerSimple Interferometer

Fringe PatternFringe Pattern

• Baseline B: Distance between antennas

• “Virtual” telescope of diameter B.cos (projected baseline)

• Fringe pattern provides information on structure and position of the radio source

Earth Rotation Aperture Synthesisuv- diagram

• Baseline length and orientation (as viewed from the source) changes as the Earth rotates new information on source structure.• uv-diagram - an indicator of imaging “quality” of an array of antennas

• more antennas filled uv better image fidelity

Connected Element Arrays

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VLA

• Baselines up to 10s of kms• “Real-time” detections• Full-time operation

ATCA

MERLIN

• Baselines 5-200 kms• Radio-linked “real-time” Operation• e-MERLIN - fibre links under construction

VLBI Arrays

LBA

x New Norcia

x NTD

VSOP

Resolution Resolution = Observing wavelength / Telescope diameterAngularResolution

Optical (5000A)Diameter Instrument

Radio (4cm)Diameter Instrument

1′ 2mm Eye 140m GBT+1″ 10cm Amateur Telescope 8km VLA-B0.″05 2m HST 160km MERLIN0.″001 100m Interferometer 8200km VLBI

Jupiter and Io as seen from Earth1 arcmin 1 arcsec 0.05 arcsec 0.001 arcsec

Simulated with Galileo photo

Atmosphere gives 1" limit without corrections which are easiest in radio

Why?

Unique VLBI ScienceZooming in!

Cen A

Active Galactic NucleusModel

Hercules A

3C236

Superluminal motion

Wide-field Imaging - surveying

Gravitational lensingDouble-quasar 0957+561 - VLBA+EVN, 18cm

PKS1830-211Einstein RingMERLIN, 5cm

NGC 4258

• Galaxy with disk• Radio continuum jet• Jet on one side obscured• H2O masers• Continuum amplifies maser

emission (in green)• Tangential to disk maser

emission – faint red & blue spots at Keplerian (point mass) rotation

• First real measurement of nuclear Black Hole mass

• Add time dimension (4D): geometric distance!

– Image courtesy: Lincoln Greenhill

M82 Starburst

SN1993J

Bartel et al

Mira Variable TX Cam• 43 GHz SiO maser

emission around a star

• No continuum emission at all

• Actually many frequencies with masers; composite

• movie!– Image courtesy: Phil Diamond

& Athol Kembal

SS433• X-ray binary

• Precessing jets

• 0.26c jet speed

• Baryonic jets

Model

Movie of VLBIObservations

Astrometry & Geodesy

Baseline Length

Baseline transverse

10 cm

10 cm

1984-1999

• Fundamental reference frames – International Celestial Reference

Frame (ICRF)– International Terrestrial Reference

Frame (ITRF)– Earth rotation and orientation relative

to inertial reference frame of distant quasars

• Tectonic plate motions measured directly• Earth orientation data used in studies of

Earth’s core and Earth/atmosphere interaction

• General relativity tests – Solar bending significant over whole

sky

SGR A* - The Galactic CentreMeasures rotation of the Milky Way Galaxy

Spacecraft Navigation• Astrometry relative to background radio

sources• Can measure micro-arcsecond position

differences

Huygens spacecraft at Titan:–January 2005, VLBI tracking–Determine probe position during descent–Accuracy ~1 km–Fringes detected but data still being reduced

How? - current systems• Data recorded on

tape or disk systems at < 1 Gbps

• Transported (slow!) to correlator facilities

• Data processing by correlator

VLBI Recorders - Mkx

S2 VLBI RecorderLBA Disk system

The Future

e-VLBI

e-VLBI global data transport

e-VLBI Science Impact• Real-time operation (cf ATCA, VLA,…)

– Fast response to “targets of opportunity” like flaring or exploding stars

– Monitoring of evolving events– Immediate assessment of results

• Higher sensitivity from high data rates– imaging sources at the edge of the universe– Very high quality imaging– (very efficient way to increase sensitivity cf building larger

antennas)

• Support high data rates of processed data – Facilitate “wide-field” imaging– Utilise GRID computing? – Database “mining”

SKA - an e-VLBI array?

1Tbit/s data transportrequirement in inner array

100 Gbit/s data transportrequirement in outer array

SKA

Operational Impact• Flexible scheduling

– Fast response to astronomical events

• Robust operation– Real-time performance monitoring

• Immensely easier data transport

• Remote antenna operation

• Remote data analysis

• Distribution of processed data to users

Lower operating costs?

Summary

• VLBI offers a unique look at the universe - a “zoom” camera

• e-VLBI will revolutionise VLBI operations

• e-VLBI will enhance VLBI sensitivity, response time and flexibility

• SKA will be an e-VLBI array• Networking is the future for VLBI

VLBA

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The VLBAThe VLBA

Ten 25m Antennas, 20 Station Correlator327 MHz - 86 GHz

National RadioAstronomy Observatory

A Facility of the National Science

Foundation

EVN

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Kx recorders