Radio Astronomy – VLBI Technique Maria J
Advanced Topic in Astrophysics, Lecture 4!
Radio Astronomy – VLBI technique Maria J. Rioja, ICRAR, UWA Outline!
What is VLBI?!
From connected interferometry to VLBI!
Spatial Resolution with VLBI !
Sensitivity with VLBI! Radio Telescopes: Resolu on
• Resolving power (how small of a thing you can “see”) depends on the size of the telescope and the wavelength of the light
For radio waves, this λ is large… size So this must also be large
• “Size” = diameter of telescope for single dish; maximum distance between telescopes for arrays Radio Telescopes: Resolu on
Green Bank Telescope, WV Very Large Array, NM
Size
Size
Single Dish Connected Arrays European VLBI Network!
Very Long Baseline Interferometry!
p.5 VLBI array Beyond Earth limita ons: Space VLBI – VSOP What is VLBI? (Very Long Baseline Interferometry)
• Radio interferometry with unlimited baselines – For high resolu on – milliarcsecond (mas) or be er – Baselines up to an Earth diameter for ground based VLBI – Can extend to space (VSOP) • Tradi onally uses no IF or LO link between antennas – Atomic clocks for me and frequency– usually hydrogen masers – Disk based systems to record data – Delayed correla on, a er media for data storage shipped – Real me over fiber is a reality now! • Can use antennas built for other reasons • Not fundamentally different from linked interferometry
VLBI ‐‐ Craig Walker Synthesis Imaging Summer 7 School 2002 What is VLBI?!
DELAY
D Processing of VLBI observations! What is VLBI?! (Very Long Baseline Interferometry)!
• Radio interferometry with unlimited baselines – For high resolu on – milliarcsecond (mas) or be er – Baselines up to an Earth diameter for ground based VLBI – Can extend to space (HALCA) • Tradi onally uses no IF or LO link between antennas – Atomic clocks for me and frequency– usually hydrogen masers – Disk based systems to record data – Delayed correla on, a er media for data storage shipped – Real me over fiber is a reality now! • Can use antennas built for other reasons • Not fundamentally different from linked interferometry
VLBI ‐‐ Craig Walker Synthesis Imaging Summer 10 School 2002 DATA REDUCTION! VLBI vs LINKED INTERFEROMETRY! • VLBI is not fundamentally different from linked interferometry • Differences are a ma er of degree – Separate clocks allow rapid changes in instrumental phase – Independent atmospheres give rapid phase varia ons and large gradients • Different source eleva ons exacerbate the effect – Sources bright enough to be both easily detectable and compact to VLBI are small, highly energe c, and variable • There are no flux calibrators • There are no polariza on posi on angle calibrators • There are no good point source amplitude calibrators – Model uncertain es are can be large • Source posi ons, sta on loca ons, and the Earth orienta on are difficult to determine to a small frac on of a wavelength – O en use antennas not designed for interferometry. Not very phase stable
VLBI ‐‐ Craig Walker Synthesis Imaging Summer 11 School 2002 VLBI arrays like the EVN & VLBA have most of their telescopes located in the Northern hemisphere. The uv-coverage for sources in the southern sky (-ve declinations) is poor. The EVN can make use of telescopes located in the Southern hemisphere at European longitudes e.g. South africa: UV Coverages = Fourier sampling!
At lower source declinationsEVN + (< VLBA +15 degrees)! the coverage of the EVN + VLBAEVN array (“Global+ SouthAfrica VLBI”) becomes! foreshortened (above left), resulting in an increasingly elongatedEVN beam and poorer uv-coverage. For the EVN this can be improved in the N-S direction by including Hartebeesthoek, in South Africa (above right). UV coverage for VSOP observations!
Space Baseline length: 400 x 106 x 6 cm = 24000 km
Resolu on = 1/ (400 e6) rad = = 0.5 mas VLBI provides…..
HIGHER SPATIAL RESOLUTION: High fidelity imaging and high precision astrometry – What they look like and where they are
TRADE‐OFF WITH SENSITIVITY: Targets: Very bright, compact e.g.: Ac ve Galac c Nuclei (AGNs), astrophysical masers THE QUEST FOR RESOLUTION
Atmosphere gives 1" limit without corrections which are easiest in radio
Jupiter and Io as seen from Earth 1 arcmin 1 arcsec 0.05 arcsec 0.001 arcsec
Simulated with Galileo photo Radio jets & Black Hole physics Cygnus A
VLA
Jet = collima ng ejecta that have opening angles =< 15o
VLBI offers highest resolution! What is Sensi vity & Why Should You Care? • Measure of weakest detectable radio emission • Important throughout research program – Technically sound observing proposal – Sensible error analysis in publica on • Expressed in units involving Janskys – Unit for interferometer is Jansky (Jy) – Unit for synthesis image is Jy beam‐1 • 1 Jy = 10‐26 W m‐2 Hz‐1 = 10‐23 erg s‐1 cm‐2 Hz‐1 • Common to use milliJy or microJy
J.M. Wrobel ‐ 19 June 2002 17 SENSITIVITY Sensi vity – Antenna Performance
• One antenna
– System temperature Tsys – Gain G • Overall antenna performance is measured by “System Equivalent Flux Density” (SEFD):
SEFD = Tsys /G – Units Jy
€Credit: J.M. Wrobel Interferometer Sensi vity • Antennas performance
– SEFDi = Tsysi / Gi and SEFDj = Tsysj / Gj – Each antenna collects bandwidth • Interferometer built from these antennas has – Accumula on me , system efficiency – Sensi vity
Credit: J.M. Wrobel Interferometer Sensi vity
• For SEFDi = SEFDj = SEFD drop subscripts
[Units Jy]
Image Sensi vity
. Interferometer sensi vity
• No. of interferometers
• Sensi vity of synthesis image:
Units Jy beam‐1
How affects Image Sensi vity?
• Eg: VLBA con nuum, 8.4 GHz – Observed ‐1 • T: Ipeak = 2 milliJy beam • B: Gaussian noise = 90 microJy beam‐1
– Posi on error from sensi vity?
• Gaussian beam = 1.5 milliarcsec • Then = 34 microarcsec • Other posi on errors dominate
Credit: J.M. Wrobel Radio Telescopes: Sensi vity • Sensi vity (how faint of a thing you can “see”) depends on how much of the area of the telescope/array is actually collec ng data – VLA B‐array: Total telescope collec ng area is only 0.02% of land area • More spread‐out arrays can only image very bright, compact sources Brightness Temperature Sensi vity
• Tb sensi vity = Tbs × Filling Factor
– Tbs = Tb sensi vity of equivalent area single dish
Filling factor ∝1/ D2 so VLBI can only see very “Bright” sources
Credit: C.Walker
VERA : VLBI Explora on of Radio Astrometry Construc on completed in 2002 Regular observa ons from 2004
Mizusawa
Iriki
Ishigaki‐jima Ogasawara
Target sources : Galac c masers (H2O@22GHz, SiO@43GHz) New aspect: dual‐beam for phase‐referencing Very Long Baseline Array (VLBA)
• Larger arrays give you be er and be er resolu on • Trade‐off with sensi vity (collec ng area stays the same Size while diameter increases) END!
Next lesssons! Monday next week:! - VLBI Science, by Phil Diamond (2pm - 3pm)! - VLBI Astrometry, by Maria Rioja (3pm – 4pm)! Location: ICRAR/UWA!
March 29 ,Board room of the ICRAR/Curtin Brodie-Hall ! Building at 1 Turner Ave, Technology Park, Bentley!