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

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

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

4. Alison Peck, Synthesis Imaging Summer School, 20 June 2002 arities to Continuum Observations 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) (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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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