VLBI: The telescope the size of the planet

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

What VLBI is good for • Resolution • 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

10 cm Baseline Length 1984-1999 Baseline transverse 10 cm Distance from Germany to Massachusetts GSFC Jan. 2000

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, 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

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 don’t 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 1 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: http://www.evlbi.org/cgi-bin/EVNcalc

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 So why use the VLBA?

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

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

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

Parallax and proper motion of Pulsars Chatterjee et al.

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

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

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 So why use the VLBA? The VLBA has turned VLBI into a scientific tool rather than a toy for black belt engineers

VLBI: The telescope the size of the planet