High Sensitivity VLBI Sheperd Doeleman MIT Haystack Observatory

BW and Continuum Sensitivity Bandwidth much cheaper than steel. New technical developments – –Disk based VLBI systems –Digital wideband backends VLBA now: 256Mb/s sustainable RateBWBW increaseSens increase 2 Gb/s250 MHzx 8x Gb/s1.25 GHzx 40x Gb/s8 GHzx 250x 16 EVLA WIDAR Correlator and software correlators can process 8GHz stations.

cm UVLBI Array Arecibo, Jodrell Bank (Lovell), Effelsberg, Westerbork, GBT Rms map sensitivities: (2 hours with Arecibo) 1 Gb/s2 Gb/s4 Gb/s L Band Array 2.3  Jy1.8  Jy1.3  Jy C Band Array 3.1  Jy2.4  Jy1.7  Jy

Motivation for wideband DBE VLBI data rates up by only x4 since 1980’s: –Moore’s Law ~10,000 –SNR~(BW) 0.5 * (Diam) 2 Modern FPGAs give increased performance at small fractions of Mark4/VLBA cost (sample IF, filter digitally). For continuum obs. widebanding now cost effective vs. larger dishes: 512Mb/s to 4Gb/s same as 42m VLBA dishes. DBE systems are portable. Wideband obs. important for key science: –SgrA*, GRB afterglows, ULIGs, Gravitational Lenses, Pulsar Astrometry, Astronomical Masers Industry driven growth path: DSP, storage media, high speed data protocols (10GbE).

DBE prototype Current modes: 15 channels, each 32MHz, 2-bit = 1920Mb/s 15 channels, each 16MHz, 2-bit = 960Mb/s H Maser

Prototype DBE Sampler boards iBOB Hardware developed by Berkeley Space Sciences Lab (CASPER) iBOB board and iADC sampler. Haystack/CASPER collaboration on Firmware. COST: $7.5K (includes purchase of Virtex2Pro FPGA) for 4Gb/s.

16MHz vs 32MHz Channels 8 vs 4 filter taps 16MHz Channel 4 taps 32MHz Channel 8 taps

Disk Based Recorders Mark5B: 2Gb/s using VSI interface. Mark5C: 4Gb/s using 10GbE Next Gen: 8-16Gb/s using 10GbE (Commercial Off The Shelf – COTS) Final piece of the system Next stop 16Gb/s

230GHz VLBI 2x 2Gb/s = 4Gb/s SMTO-JCMT: uas fringe spacing. Tcoh~60sec

Stellar UVLBI Stars exhibit radio activity all over HR diagram - at various stages of stellar evolution. Non-thermal radio emission, due to magnetic activity – VLBI scales. Magnetic fields are critical in Pre Main Sequence stellar evolution with energetic particles emitting both Xrays and gyromagnetic radio. Radio follow up of identified PMS stars from Spitzer surveys. Use UVLBI to differentiate between stellar flares, magnetospheres, star-disk interfaces. Brown Dwarfs: mysterious mechanisms.

xray/radio correlation Dwarf LP Violates this relation by 4 orders of magnitude.

Central Gravitational Lens Images Lens theory predicts ‘odd’ number of images, but almost all systems have 2 or 4: a mystery. H B ‘Missing’ images are faint and close to lensing galaxy: can’t see them in the optical. Only one central image has been detected so far, but UVLBI sensitivities should be sufficient to detect ~50%. Statistical studies of central regions of galaxies possible.

Central Image Winn et al 2004 J Free-free absorption in lensing galaxy, 200pc from core.

Density and BH mass Detection or limits on central image flux density constrains core size and steepness of density profile. SMBH at center of lensing galaxy can create a 4 th image (central) that can be used to directly estimate mass of BH. - 30% flux density of first central image. - ~20mas separation Boyce et al 2006

Planned Observations Nov 2007 – 8 asymmetric double lens systems. Arecibo – GBT at 4Gb/s (  <2  Jy/beam) –factor of x10 improvement in limits. VLBA at lower bit rate to model bright images and subtract from AR-GBT data. Goal: probe cores of z~0.3-1 galaxies on same scales as HST does for local galaxies. High resolution and high sensitivity required. –SKA ideal instrument –AR-anchored VLBI arrays available now (>0.1SKA)

Pulsar Astrometry New VLBI instrumentation allows record- time gating of pulsar data: wide bandwidths but conserving of media. Astrometry complements pulsar timing: –VLBI observes all pulsar types (not just MSP). –Supernovae core collapse (pulsar proper motions) –NS-supernovae associations. –Timing vs. VLBI: ties together extragalactic and solar system ref. frames.