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(Spectral Line) VLBI
Chris Phillips CSIRO ATNF

Quest for resolution
· Maximum resolution of an instrument is proportional to length of longest baseline

Quest for resolution

Quest for resolution

R e so An gular R eso lutio n 1 1 0.05 0.001

lutio n = O bserving w avelength / T O ptical (5 00 0 A ) D ia m eter Instru m en t 2m m Eye 10 cm A m ateur T elescope 2m HST 100m Interfero m eter

elesco pe diameter R ad io (4 cm ) D iam eter Instrum ent 140m GB T + ATCA 8km 6km VLA-B 160km ME R L IN 8200km VLB I

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

1 arcmin

Jupiter and Io as seen from Earth 1 arcsec 0.05 arcsec 0.001 arcsec

Simulated with Galileo photo

Courteous Craig Walker

VLBI Targets
· VLBI is only sensitive to the most compact structure · Need high brightness temperatures
Compact and bright Plus physics to drive

Continuum Targets
· · · · Radiation from jets and cores in galaxies Gravitational lenses Supernova remnants in nearby galaxies Pulsars

GСmez et al. Science 289, 2317 McDonald, et al., 2001, MNRAS, 322, 100

VLBI Science
· Standard imaging
Spectral cubes Polarimetry

· Proper motion and parallax studies
Speed of Gravity

· Astrometry & Geodesy
Tectonic plate motions and EOP's
Edward Fomalont & Sergei Kopeikin

image courtesy Crystal Brogan
. . .

Dodson, Legge et al, ApJ, 2003

Spectral Line Sources
· Requirement of high brightness temperatures means no thermal emission · Masers:
Galactic Extragalactic
· OH, H2O · OH, H2O, SiO, CH3OH

· Galactic & extra-galactic HI absorption

VLBI arrays
· VLBA (Very Long Baseline Array)
Dedicated US array of ten 25m telescopes 330 MHz 90 GHz

· EVN (European VLBI Network)
Formal collaboration of radio telescopes in Europe, Asia and South Africa ~15 telescopes 15m to 100m 330 MHz 22/43 GHz

VLBI arrays
· LBA (Long Baseline Array)
Parkes, Mopra, ATCA, Hobart, Ceduna and Tidbinbilla 1.4 GHz 22 GHz

· APT (Asia Pacific Telescope
LBA +
· Hartebeesthoek (SA), Kokee Park (Hawaii) · Nobeyama and Kashima in Japan · Shanghai and Urumqi in China

· Global EVN + VLBA · Space VLBI: VSOP mission · CMVA: US + European at 3mm

VLBI Correlation
· There are no fundamental difference in processing VLBI data
Need to synchronise tapes Digital fringe rotation

· Modern digital correlators intrinsically spectral line · Spectral resolution function bandwidth & number of lags (or size of FFT) · Maser components are very narrow
High spectral resolution is needed

VLBI Calibration
· Basically the same as for ATCA
Estimate time dependent antenna gain
· Tsys · Residual delay and rate

· Also correct for bandpass Assume time and frequency corrections are independent

Amplitude Calibration
· Usually use Tsys measurements
No secondary calibrator

· For spectral line, optionally use auto correlations:
Gives very good results (in principle) Corrects for pointing errors at telescope Only gives relative calibration
· Depends on amplitude calibration of template spectrum · Fails on extended sources

Fringe Fitting
· Need to estimate residual delay and rate · Residual delay seen as shift in lag domain, so a slope of phase across the bandpass in the frequency domain · Residual rate seen as slope of phase in time (in both frequency and lag domain)

· Estimate residual delay and rate using 2D FFT & least squares "self-cal"
Usually obtain antenna based correction

· Only a couple of channels per feature for spectral line
Cannot measure residual delay

· Continuum delay calibrator must be observed every hour or so · Residual rates obtains from a bright spectral feature

Phase referencing
· Can phase reference observations in a similar way to ATCA observations
Need much shorter cycle time

· Fringe fit and then phase (& amplitude) selfcal calibrator apply to target · Image sources too weak to detect in coherence time · Obtain accurate (relative) positions · Need v. compact strong source close by
Good catalogues in North, difficult for South

Bandpass Calibration
· Need relatively strong continuum source · Must observe at same frequency · Can use auto-correlations, but cannot correct phase · Cross-corr allow phase correction
Need enough S/N on calibrator Need to fringe fit first

Self-calibration
· As normal for continuum
Beware poor UV coverage

· Spectral line,many separate components at different velocities and position · Cannot selfcal data set as a whole · Cannot run self cal on each frequency channel separately · Selfcal strong (compact) feature and apply calibration to rest of channels

Continuum Subtraction
· No need for Galactic masers · Do after all calibration for HI absorption
Image negative hole in image POSSM/UVSPEC plots show as emission

Scalar/Vector Averaging
· Visibilities are complex vector + noise · Estimate average:

|| ||

Vector Scalar

· Vector averaging sensitive to uncalibrated phases · Scalar averaging noise bias

ATCA Data

(6.7 GHz Methanol)

VLBI Data

(12.2 GHz Methanol)

Imaging
· Nothing special but...
Large maps with many frequency points yields large data cubes

Scheduling
· Observe a fringe finder · If using tied ATCA include regular observation of strong compact calibrator
Calculate FOV of ATCA beam

· Seriously consider phase referencing · Consult a local expert

Spectral Line Scheduling
· Find close (enough) delay calibrator · Choose a strong bandpass calibrator · Choose enough bandwidth for velocity coverage · Calculate required spectral resolution
(Allow for Hanning smoothing)

· Find correct velocity (and ref frame!) · Turn off phasecal! · Consider over sampling

Doppler Correction
±0.5 km/s

±30 km/s

· · · · ·

Each station at different velocity Need to correct to standard rest frame Observe at fixed frequency Fringe rotation at correlator does some Further velocity correction in software
Application depends critically on design of correlator

Fringe Rate Mapping
· Galactic masers sometimes large (>10")
Often many sources in beam Large data cube

· Wide velocity width · Use fringe rate mapping to find where emission is · Also gives absolute position · FRMAP in AIPS tricky to use
.