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Spectral Line Sources

Spectral Line VLBI
Chris Phillips JIVE The Netherlands

· VLBI High brightness temperatures · Masers:
Galactic
· OH, H2O, SiO, CH3OH

Extragalactic
· OH, H2O

· Extra-galactic HI absorption

Image courtesy of Lincoln Greenhill, CfA VLBI Group See also: Hernstein etal, 1999, Nature, 400, 539 Miyoshi etal, 1995, Nature, 373, 127

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SiO masers in TX Cam

Movie courtesy of Phil Diamond, Jodrell Bank

SiO masers in Orion BN/KL

Greenhill etal, 1998, Nature, 396, 650

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6.7-GHz methanol in G339.88-1.26

De Buizer etal, ApJ In press

HI absorption towards NGC 3894

Peck & Taylor, 2000, EVN Symposium 2000

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Spectral Line Correlation
· Galactic masers typically 1-20" · 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 Short integration time (Time Smearing)

· Hanning smooth · Scalar and vector averaging:
Scalar - Noise bias Vector - Small synthesised beam

ATCA Data

(6.7 GHz Methanol)

VLBI Data

(12.2 GHz Methanol)

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Calibration
· Basically the same as for continuum
Estimate time dependent antenna Tsys Estimate residual delay and rate

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

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

Amplitude Calibration
· Can do normal Tsys calibration · 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

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

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· Measure residual delay by measuring phase slope · 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

Doppler Correction
· · · · · 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

Self-calibration
· 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

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

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

Water masers in W49N

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

Walker etal 1982, ApJ

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Scheduling
· Choose enough bandwidth for velocity coverage · Calculate required spectral resolution
(Allow for Hanning smoothing)

· · · · ·

Find correct velocity (and ref frame!) Find close (enough) delay calibrator Choose a strong bandpass calibrator Turn off phasecal! Consider over sampling

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