VLBI: The telescope the size of the planet
What the VLBA can do for you Amy Mioduszewski (NRAO) What VLBI is
good for Resolution Geodesy Astrometry 5-0.1 mas
Watch objects evolve (e.g., SS433 movie) Geodesy Earth rotation and
orientation Tectonic plate motions Astrometry Fundamental reference
frame Parallax, proper motions (e.g., TTauSb) SS433 Movie X-ray
binary with precessing relativistic jet
Daily snapshot observation with the VLBA at 20 cm for 40 days (~1/4
of precession period). 250 AU Mioduszewski, Walker, Rupen &
Taylor What VLBI is good for Resolution Geodesy Astrometry 5-0.1
mas
Watch objects evolve (e.g., SS433 movie) Geodesy Earth rotation and
orientation Tectonic plate motions Astrometry Fundamental reference
frame Parallax, proper motions (e.g., TTauSb) Distance from Germany
to Massachusetts
Baseline Length Baseline transverse 10 cm GSFC Jan. 2000 What VLBI
is good for Resolution Geodesy Astrometry 5-0.1 mas
Watch objects evolve (e.g., SS433 movie) Geodesy Earth rotation and
orientation Tectonic plate motions Astrometry Fundamental reference
frame Parallax, proper motions (e.g., TTauSb) Parallax of TTauSb
over a year
Observations every 2 months for a year with the VLBA at 4 cm
Astrometric accuracy of of 0.2 mas Very Long Baseline Array
Ten radio antennas operating as dedicated VLB interferometer Pie
Town, NM Los Alamos, NMKitt Peak, AZ Fort Davis, TXOwens Valley,
CANorth Liberty, IA Brewster, WA Hancock, NH Mauna Kea, HI St.
Croix, VI 25 meter dishes Frequencies ranging from 330 MHz to 86
GHz Angular resolution to 100 microarcsec at highest frequency How
is it different from connected element interferometry
Not fundamentally different, just issues that lead to different
considerations while calibration Phase variations and gradients
caused by Separate clocks Independent atmospheres Inaccurate source
positions, station locations and Earth orientation, which are
difficult to know to a fraction of a wavelength Solve by fringe
fitting Calibrators not ideal All a little bit resolved Compact
sources tend to be variable Solve by using Tsys and gains to
calibrate amplitudes More serious issues Only sensitive to a
limited set of scales
i.e., you can easily resolve out structure e.g., at 4 cm with the
VLBA structures larger than ~37 mas will not be measured. You have
to be very careful when measuring spectral indices Only solution is
more short baselines MERLIN, NMA Lack of sensitivity Only sensitive
to non-thermal processes
~108 K brightness temperature limit Mechanisms for High Brightness
Radio Emission Synchrotron/ gyrosynchrotron emission (electrons in
mag fields) quasars, extragalactic radio jets and lobes,x-ray
binaries,flare stars, colliding winds (WR stars), supernova Maser
emission from molecules star forming regions, circumstellar shells
in late-type stars, supernova remnants Coherent emission processes
pulsars sensitivity depends on collecting area (size and number of
telescopes), quality of receivers, time on source, bandwidth and
sampling rate (1 or 2 bit sampling) Data rate=2*bandwidth*sampling
rate normal VLBA data rate=128 Mbits/sec (64MHz band at 1
bit/sample) For spectral line and phase referencing 2 bit sampling
is generally a good idea, so dont be afraid to ask for 256
Mbits/sec To improve sensitivity (realistically in the near
term)
Use a higher data rate, i.e., a wider bandwidth Only useful for
continuum experiments The VLBA can do 512 Mb/sec with their tape
based system but it is logistically difficult MkV (disk based
recording), installed on the EVN, can reach 1 Gb/sec but it is
limited by the number of disks available.The VLBA is going to go to
MkV slowly over the next few years. Going from 256 Mb/sec Gb/sec,
only gains a factor of two in sensitivity and widening bandwidth
can cause problems Use bigger telescopes (HSA) e.g., for 4 hours on
source at 256 Mb/s at 4cm VLBA only: thermal noise = 47 mJy/beam
VLBA + GBT + Y27 + EF + AR: thermal noise = 4.5 mJy/beam Useful web
site, the EVN sensitivity calculator: So why use the VLBA?
Dedicated array, long multi epoch obs. With identical array SN1993J
Fast response (ToO) Cyg X-3 Phase referencing WR140 Astrometry
Pulsars Polarization Rotation measure Frequency Agile 3C84 Ease of
use Calibrated array Expansion of SN1993J Observations at 8
GHz
Global VLBI with VLBA as backbone So why use the VLBA? Dedicated
array, long multi epoch obs. With identical array SN1993J Fast
response (ToO) Cyg X-3 Phase referencing WR140 Astrometry Pulsars
Polarization Rotation measure Frequency Agile 3C84 Ease of use
Calibrated array Curved one-sided jet in X-ray binaryCygnus X-3,
gone one week after outburst
2 days after outburst 4 days after outburst Mioduszewski, Rupen
& Hjellming So why use the VLBA? Dedicated array, long multi
epoch obs. With identical array SN1993J Fast response (ToO) Cyg X-3
Phase referencing WR140 Astrometry Pulsars Polarization Rotation
measure Frequency Agile 3C84 Ease of use Calibrated array
Wolf-Rayet O star binary: WR140
10 mas 6 AU VLBA at 8 GHz Colliding wind shock interaction region
Beasley et al. So why use the VLBA? Dedicated array, long multi
epoch obs. With identical array SN1993J Fast response (ToO) Cyg X-3
Phase referencing WR140 Astrometry Pulsars Polarization Rotation
measure Frequency Agile 3C84 Ease of use Calibrated array Parallax
and proper motion of Pulsars
Chatterjee et al. So why use the VLBA? Dedicated array, long multi
epoch obs. With identical array SN1993J Fast response (ToO) Cyg X-3
Phase referencing WR140 Astrometry Pulsars Polarization Rotation
measure Frequency Agile 3C84 Ease of use Calibrated array Variable
rotation measure of quasar 3C279
VLBA polarization observations at 10 frequencies High rotation
measure near core may be because sometimes the flux from the core
passes through the narrow line region (accretion disk) of the
quasar So why use the VLBA? Dedicated array, long multi epoch obs.
With identical array SN1993J Fast response (ToO) Cyg X-3 Phase
referencing WR140 Astrometry Pulsars Polarization Rotation measure
Frequency Agile 3C84 Ease of use Calibrated array NGC1275 (3C84)
Free-free Absorption
Walker et al. Ap.J. 530, 233 NGC1275 (3C84) Free-free Absorption So
why use the VLBA? Dedicated array, long multi epoch obs. With
identical array SN1993J Fast response (ToO) Cyg X-3 Phase
referencing WR140 Astrometry Pulsars Polarization Rotation measure
Frequency Agile 3C84 It is easy to use, reliable and calibratable
The VLBA has turned VLBI into a scientific tool rather than a toy
for black belt engineers