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Mapping the U.S. Scientific Future in VLBI. VLBI Future Committee:

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mapping the u s scientific future in vlbi

Mapping the U.S. Scientific Future in VLBI

VLBI Future Committee:

Shep Doeleman (Haystack Obs.) Dave Hough (Trinity College) Shri Kulkarni (Caltech) Colin Lonsdale (Haystack Obs.) co-chair Alan Marscher (Boston Univ.) Chris O'Dea (STScI) Greg Taylor (NRAO) co-chair David Wilner (Harvard-Smithsonian CfA) Joan Wrobel (NRAO)


Very Long Baseline Array (VLBA)

Dedicated in 1993

  • Frequencies ranging from 330 MHz to 86 GHz
  • Angular resolution to 100 microarcseconds at highest frequency

Very Long Baseline Interferometry

  • Radio interferometry with elements (antennas) separated by hundreds to thousands of kilometers.
  • Can’t be connected-element interferometry
  • Each antenna has it’s own frequency and time reference
  • Data and time stamps recorded on magnetic tape (600 GBy)
  • Tapes brought together and correlated at a central site
  • VLBI technique has been around for 30 years
  • VLBA is culmination of project to provide VLBI capabilities in a more easy-to-use, more flexible, always available telescope.
free free absorption in 1946 708
Free-free absorption in 1946+708

Peck & Taylor (2001)

Spectral index map from 1.3/5 GHz VLBI observations

free-free optical depth:

tff ~ T-3/2 ne2n-2 d

Ne ~ 3 x 1022 cm-2

ionization ~ 10%

evla and new mexico array
NMA proposal being reviewed by AUI
  • EVLA can provide correlator upgrade for VLBA
EVLA and New Mexico Array

resolution at 5 GHz: 10” 1” 0.1” 0.01” 0.001”


Mechanisms for High Brightness Radio Emission

  • Synchrotron / gyrosynchrotron emission from electrons in magnetic fields
    • quasars, extragalactic radio jets and lobes
    • x-ray binaries (Sco X-1)
    • flare stars (AD Leo)
    • colliding winds (WR stars)
    • SNe
    • GRBs
  • Maser emission from molecules
    • star forming regions
    • circumstellar shells in late-type stars
    • supernova remnants

VLBI Epochs

50 mJy

10 mJy

1 10 20 days

VLA Light Curves (Berger et al 2003, submitted)


Resolving the Afterglow

4th Epoch – May 19


Beam is 0.67 x 0.24 mas

Jet component at 0.28 +/- 0.05 mas

Not consistent with standard model

prediction of 0.12 mas expansion

average expansion velocity of 19c


HI absorption in 1946+708Peck & Taylor (2001)“Global” VLBIobservationscore:t ~ 0.2FWHM = 350 km/sNH = 3 x 1023 cm-2for Tspin = 8000 KM ~ 108 Msun


Astrometry Example:

  • Pulsar Proper Motions
  • parallax ok out to 10 kpc


100 --- 10000 AU


100s of AU



SiO Masers and Dust Condensation Zone


A few stellar radii


TX Cam

Masers around an evolved



Discussion Questions

  • In what areas of research are you currently active? What types of data do you use, or are relevant to your research?
  • Do you currently use VLBI in your research? If so, how, and if not, why not?
  • Do you now, or have you in the past, used the VLBI results of others to enhance or motivate your work? Please elaborate.
  • This request for input was accompanied by a summary of present and future VLBI technical capabilities. Were you aware, in terms relevant to your research, of the capabilities and limitations of the VLBI technique before? Might it make a difference to you?
  • Also accompanying this request for input was a brief account of the multiple ways in which present-day VLBI is being used to address astronomical and astrophysical issues. Were you aware of these ways? Does the versatility of the technique pique your interest? If not, what about in 5 or 10 years based on the projected capabilities of VLBI?

6. Is lack of funding (e.g. graduate student support) a significant impediment to including VLBI observations in your own research program?

7. In general, if you wanted to get VLBI data and results, would you make VLBI observations yourself, or would you pursue a collaboration? Why?

8. What is your perception of the accessibility of the VLBI technique?

9. Based on your view of the future of your field, and the new instruments and capabilities expected in coming years, do you see potential synergies developing with VLBI where none exist today?

10. Please share any additional insights you may have on the state and future of VLBI in the U.S.