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Spectral Line VLBI

Spectral Line VLBI. Alison Peck SAO/SMA Project. Galactic and Extragalactic Line Absorption – H I , OH. Extragalactic Megamasers – H O, OH. 2. Galactic Masers – SiO, H O, OH, methanol. 2. Spectral Line Observations. Similarities to Continuum Observations.

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Spectral Line VLBI

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  1. Spectral Line VLBI Alison Peck SAO/SMA Project Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  2. Galactic and Extragalactic Line Absorption – HI, OH Extragalactic Megamasers – HO, OH 2 Galactic Masers – SiO, HO, OH, methanol 2 Spectral Line Observations Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  3. Similarities to Continuum Observations Scheduling an experiment, you will need: • At least one “fringe finder” • One amplitude calibrator • One bandpass calibrator And possibly: • One phase reference calibrator (very nearby) • (if using phased VLA) one VLA phase calibrator Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  4. Similarities to Continuum Observations Amplitude calibration: use measured Tsys recorded in telescope logs, and a priori measurements of antenna gain (APCAL) (Can also use auto-correlation spectra) Estimate initial delay and rate errors using bright calibrator (FRING, KRING), (but can’t record pulse cal signal) Solve for residual delays and rates using target source (if bright enough) or phase reference calibrator (“global” FRING) Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  5. Spectral Line Observations • Correlators are intrinsically spectral line, so • can do imaging and spectroscopy simultaneously • Spectral resolution is a function of bandwidth and number of lags • Maser components are frequently very • narrow (0.5 km/s) while absorption features • can be very broad (>300 km/s) so need • wide range of spectral resolution and bandwidths • Hanning smoothing might be required to prevent • “ringing” on sharp features Peck et al 2002 Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  6. Main Differences Bandpass calibration more important When looking for weak lines toward strong continuum source Doppler tracking Need to remove frequency offset introduced at each station by rotation and revolution of Earth Continuum Subtraction Only necessary if strong continuum present Fringe fitting Different techniques if no strong continuum source present Giant data sets Need to keep short integration times to avoid time smearing Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  7. Galactic HI Absorption Can determine small scale structure of HI in our Galaxy 3C138 • Several lines of sight through • gas on AU scales • Can estimate density based on optical depth Faison et al 1998 Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  8. Extragalactic HI Absorption • Circumnuclear torus seen in HST image also detected in absorption on one EVN baseline • Absorption appears only toward counterjet • First scientific observation processed at the new JIVE correlator image courtesy Ylva Pihlstroem Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  9. Extragalactic HI Absorption Peck and Taylor 2001 Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  10. Masers lie in a ring of radius r~22pc • Enclosed mass m~7x10 M 6 solar • Velocity field confirms rotation • Absorption lies outside maser ring OH Emission and Absorption 18 cm observations toward IIIZw35 (Pihlstroem et al 2001) image courtesy Ylva Pihlstroem Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  11. Bandpass Calibration Linear fit across bandwidth on strong calibrator (BPASS) • Use cross-correlation spectra to correct for phase as well as amplitude, (do fringe-fitting first to remove residual errors) • Need strong continuum source at same frequency as target source • Better to use unresolved calibrator, but can use model of CLEAN components if calibrator is resolved • Can also do polynomial fit, but risky… (CPASS) • Could also use auto-correlation spectra, but doesn’t correct phase • Watch out for RFI! Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  12. ±30 km/s ±0.5 km/s plot courtesy Mark Reid Doppler Tracking Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  13. Continuum Subtraction • Can be done in u,v plane using line-free channels on both sides of line features (UVLSF, UVLIN) • Can also be done in imageplane using model of clean components made from averaged line-free channels (might be preferable if source structure complicated) • Not necessary for many maser sources Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  14. VLBI Maser Studies • Extragalactic megamasers in accretion disks and a few jet-cloud interactions • Galactic masers in star forming regions and • photospheres of AGB stars • Lines can be very narrow but cover large velocity range • Masers can be spread over large field • Frequently very little continuum emission in target sources Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  15. plots courtesy Mark Reid Fringe fitting Removing residual delays and rates (FRING) Can use phase referencing to phase reference calibrator or strong, narrow spectral feature Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  16. Fringe rate mapping • Large fields with many spectral • channels can be very CPU intensive • to image • Fringe rate mapping permits determination of the location of the emission without having to image the whole field (FRMAP) qy (arcsec) • Fringe fitting in one spectral channel at an instant on one baseline constrains the location of the dominant emission feature to a “line” on the sky qx (arcsec) • Can use one scan on several baselines or several scans on one baseline, so also useful for pinpointing origin of • emission in single baseline observations Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  17. Molecular Accretion Disks • Considered best evidence of • a supermassive black hole • Can estimate central mass • Can estimate distance to • host galaxy image courtesy Lincoln Greenhill (see Miyoshi et al 1995 Herrnstein et al 1999) Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  18. Calibration using Auto-correlation Spectra Use template from most sensitive telescope (ACFIT) Pros: • Relative amplitude calibration good to within 1% • corrects for pointing errors on individual telescopes Cons: • Absolute calibration depends on accuracy of flux density scale for template • Need a strong line source for good SNR • Requires some “off” time, but can use scans on continuum calibrators plot courtesy Mark Reid • RFI can be a big problem Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  19. Motions of Water Masers near IRAS 05413-0104 • Young stellar object at a distance • of 450 pc • Observations made in four epochs • over 10 weeks Claussen, Marvel, Wooten & Wilking 1998 image courtesy of Kevin Marvel Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  20. AGB Stars Maser emission provides a means to detect these shells when star is obscured by dust • Can probe dynamics of the gas • in shells • Can also determine Galactic dynamics • using very accurate positions and • velocities (i.e. VERA project) image courtesy Lorant Sjouwerman Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  21. TX Cam Movie 44 VLBI observations over nearly 2 years (Diamond and Kemball 2002) movie courtesy Athol Kemball and Phil Diamond Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  22. 6.7-GHz methanol in G339.88-1.26 image courtesy Chris Phillips (Phillips et al 2002) • Methanol masers observed using Australian VLBI • network • Masers probably tracing shock rather than disk Alison Peck, Synthesis Imaging Summer School, 20 June 2002

  23. Spectropolarimetry Information about magnetic fields image courtesy Crystal Brogan Alison Peck, Synthesis Imaging Summer School, 20 June 2002

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