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VLBI: The telescope the size of the planet

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Presentation on theme: "VLBI: The telescope the size of the planet"— Presentation transcript:

1 VLBI: The telescope the size of the planet
What the VLBA can do for you Amy Mioduszewski (NRAO)

2 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)

3 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

4 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)

5 Distance from Germany to Massachusetts
Baseline Length Baseline transverse 10 cm GSFC Jan. 2000

6 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)

7 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

8 Very Long Baseline Array
Ten radio antennas operating as dedicated VLB interferometer Pie Town, NM Los Alamos, NM Kitt Peak, AZ Fort Davis, TX Owens Valley, CA North 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

9 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

10 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

11 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 don’t be afraid to ask for 256 Mbits/sec

12 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:

13 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

14 Expansion of SN1993J Observations at 8 GHz
Global VLBI with VLBA as backbone

15 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

16 Curved one-sided jet in X-ray binary Cygnus X-3, gone one week after outburst
2 days after outburst 4 days after outburst Mioduszewski, Rupen & Hjellming

17 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

18 Wolf-Rayet O star binary: WR140
10 mas 6 AU VLBA at 8 GHz Colliding wind shock interaction region Beasley et al.

19 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

20 Parallax and proper motion of Pulsars
Chatterjee et al.

21 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

22 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

23 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

24 NGC1275 (3C84) Free-free Absorption
Walker et al. Ap.J. 530, 233 NGC1275 (3C84) Free-free Absorption

25 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

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