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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

GÑmez 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
.