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1. Introduction: Transforming VLBI Across the Radio Spectrum

1.1. Modern Digital Solutions to Historical Bandwidth Challenges

The resolving power of VLBI has led to numerous important scientific advances, from the study of superluminal jets and ultraluminous infrared mergers in extragalactic objects, to Galactic micro-quasars and the central massive black hole in the Milky Way. But, historically, VLBI sensitivity has been challenged by a stark mismatch between broadband front-end receivers used at radio telescopes and the relatively narrow band backends and data recorders used for storing VLBI data. Another challenge involves phasing up the considerable collecting area of radio arrays in the mm/submm bands for use as high frequency VLBI stations. Previous phasing solutions have been implemented in the analog domain with designs tailored for specific local array architectures. The result is that transferring solutions from one array to another has been impractical, and cost considerations have led to bandwidth restrictions for these systems.

Recent advances in VLBI technology are capitalizing on the availability of increasingly high performance commercial products to significantly increase VLBI data rates and, hence, array sensitivities. The Mark5 VLBI data recorder, developed at Haystack, has shifted VLBI away from using high cost magnetic tapes to a modest cost hard disk architecture that increases standard recording rates by at least a factor of four to 2Gbit/sec. Widespread acceptance of the Mark5 as the new global VLBI standard has meant much lower storage media cost, much reduced overhead during correlation, and higher reliability. The advance of FPGA signal processing capabilities, in turn, has enabled Haystack, in partnership with the Space Sciences Lab at UC Berkeley, to develop a low-cost fully digital wideband (4Gbit/sec) VLBI backend prototype (DBE1) that is aimed at replacing analog filterbank systems, which have been in use for over 25 years. With performance of the DBE1 now proven in numerous technical and scientific experiments, Haystack is currently partnering with NRAO and UC Berkeley to develop a next generation DBE2 (also called the VLBA-DBE, or VDBE) that will serve as permanent upgrade for the VLBA and High Sensitivity Array (HSA) VLBI arrays.

1.2. Main Technical Innovations in this Proposal

Building on these advances, this proposal describes three main technical projects that, when combined, will result in dramatic increases in VLBI sensitivity from cm to submm wavebands. The first among these is development of the Mark5C VLBI recorder that will record at 4Gb/s over a modern high speed data protocol (10GbEthernet) line. The Mark5C is now fully specified and has been approved by NRAO as the VLBI recorder to be used in the 4Gb/s system upgrades for both the VLBA and HSA arrays. The final system will cost approximately 1/20th of the current VLBA backend and increase bandwidth by a factor of 8. This upgrade alone, will result in a continuum sensitivity increase equivalent to enlarging the VLBA dishes from 25m diameter to 42m. An upgrade of this magnitude for less than 1% of the capital cost of the array is nothing short of remarkable. A key element of this part of the proposal is that the recording system rates exactly match the maximum bandwidth of the VLBA front ends, and, most importantly, will provide immediate expansion of VLBI capability for the general astronomy community.

The second major thrust of this proposal is to develop, in partnership with the Smithsonian Astrophysical Observatory (SAO) and UC Berkeley, new digital phased array processors that can be used to flexibly sum all the collecting area of mm and submm arrays for VLBI. The same FPGA hardware used for DBE1 and DBE2 development will be adapted to phase up to 8 apertures together and will be seamlessly integrated with DBE1 and DBE2 backends to provide an end-to-end VLBI system. The SAO has already designed a 4Gb/s phased array system for the Submillimeter Array (SMA) using DBE1 technology. Funds from this proposal will be used to replicate and integrate this system on the CARMA array, increasing the sensitivity of the CARMA-SMA 230GHz VLBI baseline by a factor of 8, and the sensitivity of baselines between a single dish and either of these arrays by nearly a factor of 3. A further factor of 2 sensitivity increase will be achieved by completing the phased array processor upgrade to

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16Gb/s capability using DBE2 hardware. In collaboration with the Max Planck Institute for Radio Astronomy (MPIfR) and IRAM, a copy of this phased array processor will also be installed (at no cost to this proposal) at the Plateau de Bure interferometer in France, where it will enable the array to be used for VLBI at frequencies from 86GHz to 345GHz (see attached letter from MPIfR).

The third main activity of this proposal is to deploy full wideband VLBI systems at high frequency (>=230GHz) facilities to form the most sensitive submm VLBI arrays yet assembled. Our group at Haystack has led the development of high frequency VLBI, pushing the technique to record angular resolutions of 34 micro arcseconds. The essential jump in sensitivity provided by wideband recording systems is critical for VLBI arrays at high frequencies (230/345GHz) where atmospheric turbulence limits the time that VLBI can increase SNR through coherent integration. This proposal thus leverages the substantial investment of the international astronomy community in sub-mm facilities (ALMA, ASTE, APEX, SMA, SMTO, CARMA), and will provide an enabling technology to unite them all into a new and uniquely high resolution instrument.

1.3. Unique Scientific Opportunities

The work herein proposed will have a long lasting and profound impact on the entire field of Radio Astronomy. On the VLBA and HSA, the wideband systems made possible through this proposal will enable new and detailed studies of Gamma Ray Burst afterglow structure, parallax of the Galactic Center, faint Gravitational Lens systems, and will open a new window on pulsar astrophysics through proper motion studies. Through the normal peer-review observing process, it is also to be expected that a wide range of exciting science will result from using these new backends on National Facility instruments. On submm VLBI arrays, which will include at least one site in Chile during the proposed work, observations of the massive black hole candidate SgrA* will probe size scales down to the Schwarzschild Radius. Of any technique, high frequency VLBI has the most promise and the highest probability of observing the environment near the Event Horizon. As will be discussed in later sections, these observations can sensitively test for the presence of strong GR effects, and provide a unique window on accretion dynamics at the edge of a black hole. In our view, the recent successful detection of SgrA* on a 230GHz VLBI baseline with 55µas resolution (Doeleman et al 2008), combined with the planned sensitivity upgrades proposed here, represents a scientific opportunity of great importance.

1.4. Building Infrastructure for Radio Astronomy and Broader Impact

This proposal builds on the strong history of VLBI instrumentation at Haystack that has resulted in the Mark3, Mark3A, Mark4, and Mark5 systems, all of which have become the worldwide VLBI standards of their generation. This proposal leverages that experience as well as recent successes in wideband VLBI observations to create new resources for astronomers and means to study a wide range of astrophysical problems with dramatically improved sensitivity. In recognition that VLBI is an international enterprise, and that success of the submm VLBI observations of SgrA* will require a broad collaborative effort, Haystack has forged formal links with three groups through sub-awards in this proposal, as well as scientific and technical collaborations with several other institutes (see attached support letters). We fully expect to involve at least one graduate student from Avi Loeb’s group at Harvard in all aspects of this project (see letter). In addition, Haystack is committed to astronomy training through long standing REU/RET programs, and will establish research topics within these programs from work on this proposal.

2. Results from prior NSF support

AST-0521233: $372,777 to date, 4/6/2006-present, PI – Whitney, Co-I’s – S. Doeleman, A. E. Rogers

“Development of a Flexible Wideband Digital Backend for Radio Interferometry – A Consortium Proposal”

This ongoing project is developing digital VLBI backends to replace the older analog systems that have been in use for ~25 years. The first DBE prototype (DBE1) is now fully designed and fabricated and has

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been used in numerous test and science experiments. It was developed in collaboration with UC Berkeley and uses sampling and FPGA hardware developed by the CASPER group (see section 4.3). The DBE1 accepts 2 IF inputs of 500MHz each and uses polyphase filter banks on FPGA chips to channelize and format the data for recording. The total aggregate output data rate of 4Gbit/sec (2 x 2Gbit/sec streams) is in standard VSI (VLBI Standard Interface) format, suitable for recording on current Mark5B+ storage systems. DBE1 systems have been field tested and were most recently used in successful 230GHz VLBI observations on a three station array. Observations show excellent agreement between data processed through the DBE1 system and through the older analog Mark4 system. The successful completion of this major milestone demonstrates that a single DBE1 will be able to replace the Mark4 system continuum VLBI functionality at just 5% of the cost. As part of this grant, an IF pre-processing stage has been designed and built, which accepts any wideband signal from 100MHz to 15GHz then mixes and filters the input to select tunable 500MHz bands for input to the DBE1. These pre-processing stages will be used to interface the IF signals at mm/submm antennas to the VLBI backends for the observations planned in this proposal.

The rapid pace of DBE1 progress – first successful tests were carried out halfway through the first year of a three year grant – will be an important factor in accelerating the scheduling of >230GHz VLBI, which will require maximum recording bandwidths. The next generation backend (DBE2), which is currently under development within AST-0521233, will use a new hardware platform based on the Virtex5 family of FPGA chips, which is being developed jointly by NRAO, UCB, Haystack, and other international groups. This DBE2 work, which will be used to upgrade the VLBA and HSA arrays, is a direct outgrowth of the DBE1 work that originated at Haystack.

AST-0352953: $949,252 to date, 8/15/2004-present, PI Doeleman, Co-I’s – C. Lonsdale, A. Whitney

“Ultra High Sensitivity VLBI: A Leap in Bandwidth”

Technical Progress: The UVLBI project deploys the highest bandwidth VLBI backends available on both cm wavelength arrays of large aperture antennas as well as high frequency VLBI arrays. The emphasis of this program is to promote access to new cm VLBI capabilities by the astronomy community and to extend VLBI to the highest frequencies by integrating VLBI systems into mm and submm sites. At cm wavelengths, Haystack personnel supported the GBT with a 1Gb/s backend during two open calls for Global VLBI proposals and partnered with NRAO and the European VLBI Network (EVN) for scheduling the observations and correlating the data. These proposal calls were specifically for an array consisting of the 5 largest cm apertures (Arecibo, GBT, Westerbork, Effelsberg, and the Lovell 76m) for which noise levels reached 6µJy/beam. In addition to these widely advertised Global opportunities, our group has also carried out 4Gb/s UVLBI observations to search for faint central images in Gravitational Lens systems on the Arecibo-GBT baseline. These Lens observations, used the new DBE1/Mark5B+ systems at each site to reach noise levels of 1-2 µJy/beam. Our group has also, in collaboration with the Harvard Center for Astrophysics and NRAO, deployed 4Gb/s DBE1 systems at 6 VLBA stations for 86GHz VLBI observations of SgrA*. The data throughput of the DBE1 is exactly matched to the available IF bandwidth of the VLBA front ends, and the eight-fold increase in recording rate over the current VLBA maximum (512Mb/s) has enabled the first in a series of astrometric measurements that will lead to a parallax of SgrA*, resulting in a precise geometric distance to the Galactic Center.

At mm/submm frequencies, the UVLBI program has ongoing collaborations with several high frequency sites including the SMTO, SMA, CARMA, JCMT, and the CSO. Preliminary 230GHz VLBI tests were carried out in April 2006 between the CSO and the SMTO, during which a Hydrogen maser frequency standard was refurbished and shipped to the Mauna Kea summit to generate stable Local Oscillators at the CSO. In April 2007, a more ambitious 230GHz experiment was carried out on a three station array including the SMTO, one CARMA dish, and the JCMT (with Local Oscillator signals provided by the SMA facility). These observations were successful in every respect and resulted in robust detections of 8 bright AGN calibrators as well as SgrA*. The UVLBI grant supported months of detailed and

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careful preparation by Haystack personnel working with engineers and scientists at all four sites. These preparations included modifications to the Hydrogen Maser to increase stability, testing the Local Oscillator stability at the JCMT, fabrication of polarizing plates for all sites to allow reception of circular polarization, determination of the JCMT location through geodetic GPS data, and verification of the new VLBI backends at CARMA by carrying out 86GHz VLBI tests with the nearby VLBA dish at the old OVRO site.

UVLBI Science Results:

The Burst of the Decade: GRB030329: GRB030329, discovered by HETE-2 (Vanderspek et al 2003), represents a unique opportunity for VLBI observations. At a redshift of z=0.1685 (Greiner et al 2003) this is the second nearest cosmological burst in the northern sky and the brightest radio afterglow detected to date. In the case of GRB 030329 we have an exceptional instance of a radio afterglow near and bright enough that it can be resolved with VLBI, enabling studies over much longer periods than has been possible before. To date, the afterglow in GRB030329 has been observed during seven VLBI campaigns; the last one with the 1Gb/s UVLBI array 806 days after the γ-ray burst itself (Pihlstrom et al 2007). Despite the continued decay in afterglow brightness, the UVLBI sensitivity allowed measurement of the afterglow size and expansion rate, and also revealed a deceleration in the expansion rate. This expansion is best modeled by a blast wave encountering an ISM of uniform external density, rather than an R-2 wind-like density profile (Figure 1). The latest observations limit the proper motion (<0.9c), which argues against the ‘cannonball’ model of GRB evolution.

Figure 1: Fits of theoretical models for the evolution of the afterglow source size to the observed image size. In model 1 there is relativistic spreading of the jet, while in model 2 there is no significant spreading until the non-relativistic phase. The density is a power law, ρext=Ar-k, where k=0 for a uniform medium, and k=2 is expected for a stellar wind (Pihlstrom et al 2007).

Structure of the Galactic Center Black Hole: While detection of AGN calibrators on all baselines during the April 2007 230GHz VLBI observations provided complete validation of the technical preparation and setup, of primary scientific interest are long baseline detections of SgrA*, the massive black hole candidate in the Galactic Center. These measurements of the correlated flux density on the ARO/SMT-JCMT baseline, combined with detections on the shorter ARO/SMT-CARMA baseline, has allowed us to determine a robust size estimate for SgrA* (See Figure 2). Lower sensitivity on the third baseline, CARMA-JCMT, prevented detection of SgrA* but produced an upper limit for that baseline. When fitted to a circular Gaussian source, the size (Full Width Half Max) of SgrA* is 43µas (+14,-8; 3σ), and when the scatter-broadening due to the ionized ISM (~22µas; Bower et al 2006) is removed, we find an intrinsic size for SgrA* of 37µas (+16,-10; 3σ). The 3σ intrinsic size upper limit at 1.3 mm, combined with a lower limit to the mass of Sgr A* of

4 !10

5M!

from measured proper motions (Reid & Brunthaler 2004) yields a lower limit for the mass density of

9.3!10

22M!pc

"3 . This density lower limit and central mass rule out most alternatives to a black hole for SgrA* because other concentrations of matter would

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have collapsed or evaporated on timescales that are short compared with the age of the Milky Way (Maoz 1998).

Figure 2: Correlated flux density of SgrA* as a function of baseline length for a three-station 1.3mm VLBI array. Squares show SMT (Mt. Graham, AZ) – CARMA baselines, triangles show SMT-JCMT baselines, and the upper limit is from JCMT-CARMA baselines. The solid curve is the fit to a Gaussian source of 43µas FWHM, and the dotted line shows the expected correlated flux for a uniform annulus of inner diameter 35µas and outer diameter 80µas (with scattering by the ISM included). The filled circle is the total flux density of SgrA* measured by the CARMA array.

While these new observations confirm the presence of structure on 4Rsch scales in SgrA*, models other than the circular Gaussian can be fit to the data. This is illustrated by the dotted line in Figure 2, which shows the expected flux density as a function of baseline length for a uniform circular annulus with inner diameter 35µas and outer diameter 80µas that has been scatter-broadened by the ISM. Future higher-sensitivity observations will distinguish between these two models by allowing detections of SgrA* on the CARMA-JCMT baseline, which is now represented in Figure 2 only as an upper limit (see Section 3.1).

Because of gravitational lensing effects due to the extreme gravity near the assumed black hole, radiation emitted from near the event horizon of a non-spinning black hole will have an apparent size of 3 3R

sch.

For SgrA*, this expected diameter is 5.2Rsch ≈ 52µas, which differs by 3σ from the circular Gaussian fit. Even if the black hole were maximally spinning (a=1), the diameter of the event horizon in the equatorial plane (45µas) would still exceed the estimated size. This suggests that SgrA* is not an optically thick emission region that symmetrically enfolds the black hole. Rather, it is likely due either to emission from a jet, or from the approaching (and therefore Doppler enhanced) side of an accretion disk that is inclined to our line of sight (Falcke & Markoff 2000, Noble et al 2007, Broderick & Loeb 2006). Either scenario results in emission that is offset from the black hole position. This marks the first time that astronomical observations of any kind have directly constrained the spatial relationship between SgrA* and the black hole.

Through the technical developments and observations in this proposal, the Arizona-Hawaii baseline will become 5 times more sensitive, and the CARMA-Hawaii baseline sensitivity will improve by nearly a factor of 9. These higher sensitivity observations are required to follow time variable structures if they exist and to reliably extract structural information from the data. There is now an excellent prospect that

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future observations including JCMT at 230 and 345GHz will be able to test strong GR effects and accretion flow dynamics right at the black hole event horizon.

3. Research Activities 3.1. Observing an Event Horizon: The Massive Black Hole at the Galactic Center

The case for linking SgrA*, the compact radio source at the center of our Galaxy, with emission from a ~4x106 solar mass black hole is now quite strong. Observations of stars orbiting SgrA* (Schoedel et al 2003, Ghez et al 2008) indicate an enclosed mass of ~4x106 solar masses within a radius of 45 AU. These results, combined with proper motion limits on Sgr A* (Backer & Sramek 1999, Reid & Brunthaler 2004) and the new intrinsic sizes measured using VLBI at 230GHz (Doeleman et al 2008a), imply central mass densities in excess of 9.3x1022 solar masses per cubic parsec. Since any conceivable aggregation of matter with this density would coalesce or evaporate on time scales well below the age of the Galaxy (Maoz 1998), this result is the best evidence to date for the existence of massive black holes (MBH).

SgrA* is only a small fraction of the compact central masses (~109 Msol) assumed to power the brightest extragalactic AGN, but the importance of SgrA* as a guide to our understanding of AGN and the extreme environments surrounding massive black holes cannot be overstated. This is due to its proximity, which makes the characteristic angular size scale of the Schwarzschild radius larger than for any other Black Hole candidate. At a distance of only ~8 kpc (Reid 1993), SgrA* can be studied with mm/submm-VLBI on sub-AU linear scales (Rsch(SgrA*)~ 10µas ~ 0.1AU). The intervening ionized ISM, however, scatter broadens images of SgrA* with a λ2 dependence, and VLBI at the highest frequencies is the only available means to observe intrinsic structures near the event horizon. VLBI at 43GHz and 86GHz has detected evidence for intrinsic structure of SgrA*, but these observations remain dominated by scattering effects, and the measured intrinsic sizes at these frequencies, set by optical depth effects, far exceed the predicted size scales associated with observed variability.

These new VLBI results from our previous NSF grant open a fundamentally new and exciting window on accretion and black hole physics. Firstly, they corroborate numerous lines of evidence that point toward the presence of small scale structures near the event horizon. Light curves during SgrA* flares from the submm to Xray (Yusef-Zadeh et al 2006, Eckart et al 2006, Marrone et al 2008) implicate structures on ~5 Rsch scales, and single dish polarimetry results (Aitken et al 2000) indicate activity at the 2 Rsch level. Secondly, mm/submm-VLBI is now a proven method for directly extracting structural information on the scales of sophisticated simulations that include accretion, emission, and radiative transfer in the strong GR environment of a black hole. Models of SgrA* flares, in which 'hot spots' orbit at the ISCO (e.g. Broderick & Loeb 2006) show detailed structural distortions due to strong gravity, and mm/submm-VLBI can now sensitively test for such time variable, and potentially periodic, structures (Doeleman et al 2008b). The ‘shadow’ or ‘silhouette’ predicted to appear in front of the black hole due to extreme GR effects (Falcke, Melia & Agol 2000) can, for the first time, reasonably be modeled with the future observations described in this proposal. And emission models of SgrA* such as Radiatively Inefficient Accretion Flows (RIAF), which explain the low luminosity of SgrA*, can be fitted to mm/submm-VLBI data to constrain black hole spin, accretion disk orientation and inclination (Fish et al 2008, Broderick et al 2008).

The technical developments in this proposal will enable 230/345GHz VLBI on arrays that will include the phased CARMA array, all Mauna Kea submm sites phased together, the SMTO, and a Chilean site (yet to be determined, but likely ASTE or APEX), and which will achieve recording rates from 4 to 16Gb/s. Even with higher sensitivities, a 4 element array will not be able to make true images via the usual route of fourier inversion and deconvolution. Instead, efforts will focus on using closure phases (the sum of interferometric phase around the triangle of baselines) and closure amplitudes (the ratio of four baseline amplitudes), which are both referred to as ‘good’ observables as they are immune to most calibration errors (Rogers, Doeleman & Moran 1995). Model fitting using closure quantity-based algorithms on

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86GHz VLBI data was first demonstrated in Doeleman et al (2001) and has been used for reduction of SgrA* VLBI data ever since.

A number of theory groups are creating sophisticated models of SgrA* structure with very fine (~2µas) angular resolution that incorporate the effects of opacity, MHD, and full General Relativity. To assess the technique of model fitting using closure quantities, synthetic data sets were generated for various MHD models based on the characteristics of the 4-element array described above. Figure 2 shows a dynamically self-consistent model based on GR numerical simulations of an accretion flow (courtesy C. Gammie) and an attempt to model the synthetic data. Since many models show the ‘shadow’ effect towards the position of the black hole, we have fit the data with a torus-like component and find, as anticipated, good agreement. When a simple circular Gaussian is used, however, the fit is quite poor, and though the torus model has more degrees of freedom, it is statistically more likely. In short, increasingly complicated models are meaningfully better fits to the simulated data than simple ‘blobs’, and ongoing work, including collaborations between Haystack and theory groups, will focus on tailoring these closure fitting algorithms to the SgrA* case.

Figure 2 Left: Self-consistent MHD model of a disk around Sgr A* with GR included assuming a = 0.5, seen at 5 deg inclination. Middle: Same model with interstellar scattering at 230 GHz. Right: Best fit model to synthetic data set generated for 4Gb/s 4-element 230GHz VLBI array assuming middle panel structure on the sky. Model consists of an elliptical Gaussian plus an elliptical Gaussian "doughnut". The hole in the disk can be detected with high statistical significance.

One of the most promising areas where VLBI can make unique contributions is in searching for time variable structures associated with in-spiraling hot spots (Broderick & Loeb 2006). Figure 3 shows the closure phase signatures over a single hot-spot orbit around the black hole for the proposed VLBI array. Asymmetry in the model appears as time variable closure phase that very nearly repeats as the hot-spot makes multiple orbits. The closure phase signature changes slightly from orbit to orbit due to Earth rotation. The intriguing result is that given realistic signal to noise on the array, deviations of closure phase for a single hot-spot orbit can be detected, and if the hot-spot persists for multiple orbits, one can search for periodicities in the closure phase to establish the period. Determining the orbital period of the ISCO provides direct estimates of the black hole spin and mass.

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Figure 3: Signature of a hot spot orbiting the SgrA* black hole. The left panel shows a quiescent Radiatively Inefficient Accretion Flow (RIAF) model for a non-spinning 4x106 solar mass black hole, and a hot spot orbiting at the Innermost Stable Circular Orbit (ISCO), at a disk inclination of 30 degrees. The raw model is shown for 3 orbital phases in the top three figures, and the bottom three show the effects of scattering by the ISM. The right panel shows closure phases every 10-seconds on the Hawaii-CARMA-SMTO triangle for an 8Gb/s recording system with the model shown as a red curve (Doeleman et al 2008b).

In addition to carrying out searches for structure and asymmetry in total intensity images, increased bandwidths will allow high sensitivity VLBI polarimetry observations of this black hole candidate. At 230GHz, the measured linear polarization fraction can be ~10% (Marrone et al 2006), and models of magnetized accretion flows show that 230GHz VLBI should be sensitive to small scale polarization structures that arise near the event horizon due to strong gravity effects (Bromley, Melia & Liu 2001, Broderick & Loeb 2006). In fact, the theoretical hot spot models of Broderick & Loeb (2006) exhibit fractional polarization across the image that can reach levels of over 30%. It is thus possible that all previous polarimetery of SgrA*, even with connected element arrays (e.g. Marrone et al 2006), has suffered from beam dilution. VLBI polarimetry is the only technique that can observe small scale polarized structures that models predict, and can provide a very powerful new diagnostic of hot spot and accretion emission. VLBI polarimetry of SgrA* at both 230GHz and 345GHz will be aggressively pursued under this proposal. In summary, Increasingly detailed and fully relativistic models in combination with rapid advances and innovation in wideband high frequency VLBI are poised to transform SgrA* into a unique laboratory for strong field GR theory and detailed mechanisms of matter inflow and energy output surrounding a massive black hole. In addition to SgrA*, this proposal will target M87 (Virgo A), which represents a much more luminous class of AGN, and at a distance of 18.7 Mpc the ~3x109Msol central black hole has an apparent Schwarzschild radius smaller than that of SgrA* by only a factor of 3.

3.2. Science on Global VLBI Arrays at 4Gb/s

Haystack development of the Mark5C recorder (through this proposal), coupled with the existing NRAO-Haystack collaboration on the DBE2 backend, represents a full path to 4Gb/s VLBI systems. Matched by parallel efforts of the European VLBI Network (EVN), this path will extend to include Global VLBI networks (see attached letter from the EVN Chair). Routine VLBI at 4Gb/s will bring a significant new capability to radio astronomy, with excellent opportunities for progress in science areas that require the highest sensitivity. Just a few of the many possibilities are mentioned below.

3.2.1. Scientific Potential of Astrometric Pulsar Measurements

By virtue of their intense gravitational and magnetic fields, their extreme densities, and their regular pulses, radio pulsars constitute unique astrophysical laboratories. Pulsars enable study of fundamental astrophysics areas including stellar evolution, condensed nuclear matter, the structure and content of the ISM, Supernovae core-collapse, generation of Gravitational waves and tests of Relativity, and forging links to inertial reference frames. Accurate pulsar distances and velocities are fundamental inputs to all of these areas, and ideally one wants these quantities for a broad population of pulsar types in order to sample the large phase space of pulsar properties. Pulsar timing can derive position information only for recycled pulsars with very stable pulse periods and profiles, such as relatively rare milli-second pulsars (MSPs). VLBI, in contrast, can use a reference frame of background extra-galactic quasars to pinpoint the position and measure the proper motion of all pulsar types.

With the developments in this proposal, the VLBA alone will be able to determine accurate parallax and proper motions for pulsars at 5GHz, where the ionospheric effects that contaminate 1.4GHz astrometry are much reduced (Chatterjee et al 2004). Currently, there are 14 pulsars for which VLBI has determined parallax and velocities. With 4Gb/s systems, these measurements can be made for over 70 pulsars from the ATNF pulsar database at 5GHz, with many more accessible at 1.4GHz. A three-station 5GHz VLBI

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array consisting of the GBT, Effelsberg and the Lovell 76m could detect over 300 pulsars from the ATNF database, cutting a broad swath through pulsar age, magnetic field and spin period. Adding Arecibo to this array boosts sensitivity in the declination range (5 to 35 degrees) where it is estimated that the new Arecibo L-Band Feed Array (ALFA) will double the number of known pulsars. Depending on the ALFA pulsar yield, the number of pulsars detectable by VLBI could climb above 700. In short, it is envisioned that 4Gb/s systems will allow measurement of hundreds of pulsar parallaxes and motions with the goal of fundamentally enhancing our understanding of pulsar astrophysics.

3.2.2. Gamma Ray Burst Afterglows

We now know that a GRB represents an energy release of about 1051 erg when corrected for jet collimation (Frail et al 2001, Bloom et al 2003). The afterglow emission is usually attributed to a strong relativistic shock that is driven into the external medium as it decelerates the ejecta (Piran 2005). This synchrotron radio afterglow is the longest surviving remnant of the original blast, and GRB models predict both luminosity and size evolution of this emission, which are observable parameters. Since the sizes are extremely compact, radio VLBI techniques are the only means by which the emission can be directly resolved.

The radio afterglow of the nearby GRB030329 (see section 2) is fading with a power law fall off, Fν~t-1.23, and 4Gb/s VLBI on large apertures will allow GRB030329 to be imaged at 10 years from the original blast. Extension of this time window will allow a unique long term study of the blast wave as it continues to propagate into the progenitor wind and ISM. This VLBI monitoring will become particularly interesting should a radio re-brightening occur, as is predicted when the counter jet becomes non-relativistic and its emission is no longer beamed away from us (Granot 2003). Such a re-brightening would be accompanied by a shift of the flux density centroid that would be measurable with VLBI. In addition to the continued observations of GRB030329, higher VLBI sensitivities will provide opportunities to study weaker GRBs to determine if the behavior of GRB030329 is typical.

3.2.3. Missing Central Images in Gravitational Lenses

Gravitational lensing theory predicts that lens galaxies should generally produce an odd number of images of a background source (Burke 1981). This expectation stands in stark contrast to the observed lens population, which consists almost exclusively of 2 and 4-image systems (e.g. the JVAS/CLASS sample of radio lenses). The missing images are those that should appear very close to the center of the lens galaxy, which is why they are referred to as “core” or “central” images. More centrally concentrated mass distributions produce fainter central images; the central image flux depends sensitively on the density structure of the lens galaxy on ~10pc scales and, if routinely detected, these images could become a powerful probe of galactic structure and the influence of central black holes (Mao, Witt & Koopmans 2001).

Detection of central odd images in even a small number of lens systems would provide important new limits on galactic density profiles at very small scales – only one central image detection has been reported so far (Winn, Rusin & Kochanek 2004). The current sensitivities of deep follow up observations to the JVAS and CLASS surveys place odd image flux density 5-σ limits at ~180-225µJy and correspond to ratios of (r = central image flux / brightest image flux) of ~0.001 (Rusin & Ma 2001, Boyce et al 2006, Zhang et al 2007). Large aperture VLBI arrays operating at 4Gb/s will achieve 1-2 µJy/beam noise, and new limits on central lensed images can be set that are factors of 25-40 lower than the best recent efforts. It should be noted that for gravitational lenses, with typical image separations of ~2 arcseconds, the proposed array will be able to self-calibrate on the bright images and coherently integrate as long as needed to search for the central image. With 4Gb/s sensitivity levels, it is estimated that central images will be detected in 50% of observed lens systems (Evans & Hunter 2002), with the potential to dramatically affect our understanding of galactic structure and gravitational lensing.

3.2.4. Astrometry: Measuring the Parallax of the Galactic Center

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Phase-referenced VLBI observations have become an indispensable tool for the study of target sources too faint to detect within the coherence times imposed by ionospheric and atmospheric turbulence. By observing a nearby calibrator and quickly slewing to the faint target, calibrator phase solutions can be used to build up many hours of integration on the target source. This technique is by no means a niche observing mode: over half of all VLBA observations rely on phase referencing (Wrobel & Ulvestad 2005). Increasing recording rates to 4Gb/s improves phase referencing in two ways. First, the duration of the calibration cycle can be significantly decreased while maintaining signal to noise ratio on both calibrator and target sources. And, second, wideband recording allows closer and fainter calibrator sources to be used, increasing the accuracy of the phase calibration transfer.

An illustrative example of phase referencing is application of the technique to astrometry of the Galactic Center. Currently, the most cited and generally accepted value for the distance to the Galactic Center is 8+/-0.5 kpc (Reid 1993). This value, Ro, is critical for many studies of Galactic objects, and affects distances determined using rotational models of the Galaxy, estimates of gravitational and luminous mass, and even extragalactic distances since these have their basis in calibration with the Galaxy. Indeed, the biggest component in the uncertainty of the mass estimate of the central black hole in the Milky Way is due to the uncertainty in Ro (Ghez et al 2005). Using 4Gbit/sec on 6 VLBA stations observing at 86GHz could improve on the current astrometric errors on the position of SgrA* (resulting from short 43GHz VLBA baselines) to a 1-σ level of 0.024 milli arc seconds (Reid & Brunthaler 2004). At this level, it would take only ~25 independent phase-referenced observations of SgrA* over 1.5 years to solve for the parallax of SgrA* (~125 microarcseconds) to the 4% level, which would be the most accurate determination of Ro to date and would allow recalibration of the extragalactic distance scale.

4. Technical Developments for Wideband High Sensitivity VLBI

4.1. Mark5C System Development

The Mark 5C is the third generation of disk-based Mark 5 high-data-rate VLBI data systems (Figure 4) designed by Haystack Observatory in collaboration with Conduant Corporation of Longmont, CO. It follows in the footsteps of the Mark 5A/5B (1 Gbps) and Mark 5B+ (2 Gbps) systems, which in less than four years have become the global de facto VLBI data systems with approximately 150 units deployed worldwide. Replacement of the old tape systems is essentially complete, at a cost of ~10% of the old tape systems, and with higher performance and reliability.

Figure 3: Mark5 Hard Disk VLBI Recorder

4.1.1. Mark5C System Description

The Mark 5C system will extend the maximum data-recording rate to 4096 Mbps and will be the first to adopt a commodity data interface – the international standard 10 Gigabit Ethernet interface. Adoption of the 10GigE interface as a VLBI data standard brings with it a rapidly expanding suite of available standard commercial support tools (e.g. FPGA IP cores implementing 10GigE) and data-management hardware (e.g. NIC cards, switches, routers, disk storage arrays, etc.) that improve flexibility and lower cost. The new digital backend system, dubbed ‘DBE2’, which is being jointly developed by Haystack, in collaboration with NRAO, UC Berkeley and South Africa [see Section 2], will support a 10 GigE data interface to feed data directly to the Mark 5C for experiments to be conducted under this proposal.

The backbone of all the Mark 5 systems is a disk interface system optimized for sustained high-data-rate real-time performance developed by Conduant Corporation. This specialized disk-interface card, called ‘StreamStor’, plugs into the PCI-X bus of a standard PC motherboard to support the 16 disks of a Mark 5

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system and provides a direct user interface to the disk subsystem. This allows target data rates to be maintained independently of any normal computer activity that would potentially impact data flowing through normal internal computer buses (PCI, PCI-X, etc).

Importantly, the Mark 5C will be compatible with the same disk modules used by the Mark 5A and Mark 5B/5B+, with the exception of some older modules with disks incapable of supporting the necessary data rates. This will allow a smooth transition from the older Mark 5 systems to the new Mark 5C.

4.1.2. Mark5C Playback and correlation

The Mark 5C is being targeted for correlation processing on a software correlator, a major departure from the large synchronous hardware correlators that have dominated VLBI for most of its existence. The shift to software correlators is made possible by the extraordinary processing power now available in a PC for modest cost, plus the ability to aggregate the computing power of many such processors into clusters connected by high-speed networks. Such a system is inherently asynchronous, demanding input data only as fast as it is able to process it.

In order to match the capabilities of software correlator systems, playback of Mark 5C data will also be asynchronous, and will utilize standard PC data buses and 10GigE NIC cards at whatever rates they can support. Since the VLBI data recorded on the Mark 5C disk modules will be made to appear as standard Linux files to the host OS, management of playback of that data is straightforward.

There will be one exception to the asynchronous playback mode of the Mark 5C. In order to maintain backwards compatibility with the large Mark 4 correlator systems in place at Haystack, MPI (Germany) and JIVE (The Netherlands), the Mark 5C will support a recording mode that creates disk modules with the same data format as the Mark 5B+. These modules can then be processed (at rates slower than real-time recording) by the Mark 4 correlators.

4.1.3. Mark 5C Development Tasks

Though the Mark 5C will build on the experience of the earlier Mark 5 systems, a number of tasks will need to be undertaken to realize an operational system, and these will be supported under this proposal. In Year 1 of this proposal, 15.5 person-months are allocated to completion of these tasks: − A new hardware 10GigE I/O daughterboard for the StreamStor card must be designed, built and

tested. This work will be done by Conduant Corp in consultation with Haystack Observatory. − Host-computer software support for the Mark 5C must be designed, written and tested to operate

the Mark 5C in accordance with the operational requirements; this will be undertaken by Haystack Observatory.

− The control and management of the Mark 5C must be integrated into the standard Field System that is used by much of the radio-astronomy; this will be done by Haystack Observatory in collaboration with NASA/GSFC.

4.2. Burst Mode Recorder Development

Under a separately funded program (AST-0705062) that has just commenced, Haystack is designing a new type of VLBI recorder that is capable of 16Gb/s recording rates during short (~1min) intervals. This recorder will be designed using completely Commercial Off The Shelf (COTS) components and is focused on high frequency and phase referenced VLBI applications where achieving improved signal to noise in a limited integration time is essential. For high frequency VLBI this limit is set by the atmospheric coherence time, and for phase referencing the limit is the need to switch quickly between a target source and a nearby calibrator. The Burst Mode Recorder program will produce three prototype burst mode recorders that will be deployed in the final year of this proposal. The burst mode design uses commodity 10GbEthernet Network Interface Cards (NIC) as interfaces to the DBE2 backends. The data are then sent over a fast (PC Express) bus to volatile memory where it is stored until recording is finished.

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The data are then piped to a commodity external RAID array for long term storage and transport to the correlator.

4.3. Phased Array Processor Development

To make optimal use of a mm/submm array facility for VLBI, the collecting area of all antennas in the array must be coherently summed prior to recording on a VLBI system. The resulting sensitivity multiplier is typically quite large – in the case of CARMA, phasing up 8 of the 10m diameter dishes gives a collecting area equivalent to a 28m dish, a sensitivity increase for all baselines to that station of x3. The Smithsonian Astrophysical Observatory (SAO) is designing a flexible phased array processor that will be capable of coherently summing analog IF signals from up to 8 antennas. The first application of this system will be to phase together all the submm apertures at the Mauna Kea summit: CSO, JCMT and up to 6 SMA dishes for an aggregate collecting area equivalent to a 23m antenna. This phasing operation will make use of the existing eSMA infrastructure, which brings IF signals from the CSO and JCMT back to the SMA facility. The use of fast ADC and FPGA hardware has allowed rapid design and testing of this phasing system, and it is due to be installed at the SMA site in early 2008 for field tests. Of primary importance is that the low replication cost of this system and the flexible processing algorithms within the FPGA firmware allow relatively straightforward adaptation of the phasing hardware to other mm/submm arrays. Through this proposal, the current (4Gb/s) SAO phased processor design will be replicated and integrated in the CARMA system in the first year, a second generation (16Gb/s) system will be designed by SAO in the second year, and in the third year this 16Gb/s phasing system will be installed at both CARMA and on Mauna Kea. The resulting increase in bandwidth and collecting area will lead to a factor of x9 increase in sensitivity on the CARMA – Mauna Kea VLBI baseline. This order of magnitude increase will enable much of the SgrA* science discussed in section 2.

The current SMA phased array VLBI processor (Nagpal 2006) uses fast processing and sampling hardware which has been developed by the UC Berkeley Wireless Research Center (BWRC) and Center for Astronomy Signal Processing and Electronics Research (CASPER) group. The Berkeley hardware consists of iADC boards based on Atmel Corporation analog to digital converters for sampling and iBOB boards based on Xilinx Virtex II Pro FPGAs for data processing. CASPER has developed a library of Radioastronomy-specific functions that are available for quick application to particular instrumentation needs (Parsons et al., 2005). The unit processes data from 8 antennas, computes their phased sum in real time and spools it to a Mark5b+ data storage unit. The bandwidth of a quantum of hardware is 512 MHz. Scaling to the 1024MHz bandwidth system (4Gb/s with 2-bit sampling) is achieved by replicating the 512MHz hardware.

Phased array summing to a delay precision of 0.1 ns has been demonstrated in the SAO laboratories in Cambridge, and the full phased array processor has been tested at a sample rate of 1024MSamp/s. A calibration correlator running on a separate iBOB board (see Figure 5) dynamically measures the delay of each channel with respect to a reference channel. The correlator runs in the background during operation enabling adaptive delay calibration. To improve correlator dynamic range, Walsh switching will be enabled on the array during VLBI observations, and then removed in the phased array processor before the data is stored on the VLBI recorder. Figure 4: Architecture for a single 1GHz channel of the phased array processor to be developed under this proposal. Each FPGA processor (second generation iBOB boards) supports two dual channel 2GSamp/sec ADCs; so two FPGA processors support eight 1GHz blocks of antenna bandwidth. A third FPGA processor forms the composite sum of the two 4-channel sums, and formats the data for VLBI recording. A calibration FPGA processor runs a cross-correlator, and feeds back delay and phase

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calibration data to the system. Fast 10Gb/s lines link the FPGA processors. This design will be replicated x4 to reach 2GHz of bandwidth for two polarizations: a total data rate of 16Gb/s.

Integration of the DBE interface with the phased array processor will be supported under this proposal with testing carried out by Haystack personnel in collaboration with SAO engineers. In Year 1, a sub-award to UC Berkeley will support replication and integration of the 4Gb/s phased array processor at CARMA, with collaboration by Haystack personnel, including VLBI tests on the CARMA – VLBA(OVRO) baseline that will be correlated at Haystack. In Year 2, SAO will receive a sub-award to extend the phased array processor to 16Gb/s using the next generation of FPGA hardware from the CASPER group that will use more capable Virtex5 chips. This new iBOB2 board being jointly designed and fabricated by a collaboration including Haystack, NRAO and UC Berkeley with prototype boards scheduled for release by the end of 2007. Year 3 of this proposal supports a second sub-award to UC Berkeley for fabrication of the new 16Gb/s phased array system (re-using as much as possible of the first system). These new systems are modest investments that will have a great impact on mm/submm array infrastructure by delivering an order of magnitude increase in VLBI sensitivity.

4.4. Frequency Standards for mm/submm VLBI

As VLBI moves to higher frequencies, the stability of frequency references required to maintain phase coherence between VLBI sites becomes critical. Though the troposphere limits high frequency VLBI coherence times, high altitude sites often experience very good weather conditions. At the ALMA site in Chile, the measured coherence time of the atmosphere is > 10 seconds 60% of the time at 230GHz and 45% of the time at 345GHz (Holdaway 1997). To match this excellent ‘seeing’, a VLBI frequency standard has to have a fractional stability (Allan Standard Deviation) of σy(10s)<2x10-14 for 10 second integrations. Only the very best Hydrogen masers can achieve this, but other frequency references, notably Cryogenic Sapphire Oscillators, have stabilities at least an order of magnitude better than Hydrogen masers over 1-200 second time scales. Haystack is collaborating with the Frequency and Metrology Group at the University of Western Australia to explore adaptation of the Sapphire Oscillators produced by that group for VLBI use. Through this NSF-funded work (AST-0722168), we expect that one Cryogenic Sapphire Oscillator with a GPS-conditioned, phase locked 10MHz reference output will be available for high frequency VLBI work at a Chilean site in the third year of this proposal.

At the SMTO, recent VLBI has made use of a vintage Hydrogen Maser first assembled at the Smithsonian Astrophysical Observatory over 35 years ago. While it performed admirably during the April 2007 experiment at 230GHz, it has developed multiple symptoms including an intermittent vacuum leak, which have now compromised its ability to maintain sufficient stability for high frequency VLBI. This is, in part, due to the need to run Hydrogen masers at a high level of hydrogen flux during observations to increase stability on short (~10sec) time scales. The maser at SMTO cannot now reliably be used at such a remote site, and technicians qualified to work on this unit are not available for travel to Arizona. For this reason, and because SMTO is an important submm VLBI site, this proposal has funds for procuring a modern Hydrogen maser with quoted stability at the σy(10s)=2x10-14 level. This represents a long lasting and important infrastructure improvement, and should a new site be deemed more important in the future, the maser could be moved.

4.5. Creating High Sensitivity mm/submm VLBI Arrays

Much of the work involved in this proposal is aimed squarely at deploying state of the art wideband VLBI instrumentation at mm/submm VLBI sites to form arrays that have unprecedented angular resolution and sensitivity. While these modern systems are in many ways easier to install and operate than older (and

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much bulkier) equipment, it is still necessary for knowledgeable practitioners to travel to each site in advance of scheduled mm/submm VLBI experiments to prepare the site for observations. In large measure, this results from the fact that VLBI is an infrequent observing mode for all the sites in Table 1, and VLBI-specific systems, including the Hydrogen Maser, are only exercised perhaps once per year. Though a number of participating facilities intend to purchase new Mark5C recorders and DBE backends (see letters of support), historically all of the VLBI equipment has been made available on short term loans from other Haystack programs and shipped to the sites. Thus, observing campaign logistics necessarily include a re-installation of VLBI equipment each time with the consequent need for careful testing of not only the VLBI backends, but also the stability of the telescope Local Oscillator and Hydrogen Maser. With the combined simultaneous telescope time allocated for VLBI observations, and the added requirement for good weather at all sites, the importance placed on thoroughly vetting the technical setup at each site is high.

In addition, the introduction of new, more capable instrumentation inevitably requires hands-on integration and testing at multiple sites. The enhancement of capability planned for each year of this proposal will entail a level of sustained effort throughout the year leading up to the observations. There is a particularly intense effort required to bring up a high frequency VLBI station for the first time, as is proposed for the site in Chile. Though our Chilean colleagues are very supportive of this proposal (see attached letter from Prof. Nagar), it is envisaged that at least two trips by Haystack personnel to the site (most probably ASTE) will be needed before the first VLBI session with the new site can be scheduled.

Testing the stability of Hydrogen masers in the field has long been a challenge for VLBI, and it is often said that a second maser is needed to test the first – an impractical arrangement. As part of the Sapphire Oscillator program (Section 4.4), Haystack has assembled a system capable of accurately testing the stability of Hydrogen Masers in situ, which compares the output of the maser to an extremely low noise Quartz Crystal Oscillator. This specialized oscillator (Oscilloquartz BVA 8607) is more stable than a maser at 1 second integration times. By using this system to measure the maser Allan Standard Deviation at 1 second, the maser performance at 10 seconds can be estimated using

!

y"( ) ! " #1 , which holds in the

1-10second regime. A second critical test at each VLBI site is to measure the coherence of the receiver Local Oscillator system. An effective method is to create a test tone near the observing frequency by multiplying up a low noise ~10GHz oscillator that is phase-locked to the Hydrogen maser. When injected directly into the receiver feed, the phase noise of this tone can be measured as it descends the LO chain, through the IF to the VLBI backend. Such a test provides a direct measure of the coherence loss in the signal path, and also provides an important check that the frequency setup is correct. There are funds in this proposal to assemble two portable test tone systems that can be used in the field.

The experience of the Haystack group, after having mounted successful 230GHz VLBI observations, is that it is easy and dangerous to underestimate the resources required to carry out 3 and 4 station VLBI at high frequencies. The arrays proposed in this project will record at the highest bandwidths and observe at the highest frequencies ever attempted, with new instrumentation that will push VLBI into a regime where fundamentally important science can be addressed. The personnel support requested to assemble these arrays is approximately 1.5 FTE/year and covers:

All bench preparations including: assembly and testing of full VLBI systems for each site, assembly and testing of all systems to be used for tests at the sites, assembly and testing of all hard disk modules for recording, planning and implementing downconversion chains for each telescope IF.

All site testing including: maser and Local Oscillator tests, installation of VLBI systems, VLBI tests at lower frequencies where possible (e.g. CARMA), determination of VLBI precision antenna positions, installation of Sapphire Oscillator system at Chilean site.

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Logistics: Scheduling telescope time, creating control schedules for observations, creating station specific software to acquire calibration information, all shipping and tracking of equipment, observing proposal preparation.

Data Processing: Correlating calibrator sources to determine delays and rates for each VLBI site, production correlation of all data (8 weeks of Mark4 correlator time for an 8Gb/s, 40 hour, 4 station observation), post correlation processing, and analysis.

The table (Table 1) below summarizes the experiments and required equipment for the observations to be pursued in this proposal.

Year 2 (4Gb/s) 230GHz

Year 3 (8Gb/s) 230/345GHz

Year 4 (8-16Gb/s) 230/345GHz

Mauna Kea (Phased) CSO/JCMT/SMA

2 – Mark5B+ 1 – DBE1 Phased Array Proc.

2 – Mark5C 1 – DBE2 Phased Array Proc.

2 – Mark5C 1 – Burst Recorder 2 – DBE2Phased Array Proc.

SMTO

2 – Mark5B+ 1 – DBE1

2 – Mark5C 1 – DBE2 New H-Maser

2 – Mark5C 1 – Burst Recorder 2 – DBE2

CARMA (Phased)

2 – Mark5B+ 1 – DBE1 Phased Array Proc.

2 – Mark5C 1 – DBE2 Phased Array Proc.

2 – Mark5C 1 – Burst Recorder 2 – DBE2 Phased Array Proc.

Chilean Site (ASTE)

Not Participating.

2 – Mark5C 1 – DBE2 Sapphire Oscill. Ref.

2 – Mark5C 1 – Burst Recorder 2 – DBE2

Table 1: Equipment required at mm/submm sites to be supported in this proposal for planned VLBI observations.

5. Implementation Plan

Tasks/Lead Personnel

1. Mark5C Development: Dr. Alan Whitney will lead the Mark5C development outlined in section 4.1 above. Dr. Whitney designed the Mark4 and Mark5A/B systems and will work with Conduant Corp to specify the daughter card required to interface the recorder to the DBE2 with 10GbEthernet. Control software will be written by Haystack software engineers led by Dr. Whitney.

2. Phased Array Processor Development: Dr. Jonathan Weintroub at the SAO will expand his current design to 16Gb/s capability using new iBOB2 hardware from the CASPER group at UC Berkeley. Dr. Weintroub developed and built the SMA correlator and is leading the effort to combine the submm apertures on Mauna Kea for VLBI. Dr. Alan Rogers and Haystack digital engineers will work with Dr. Weintroub on integrating the DBE2 algorithms into the phased array processor. Dr. Shep Doeleman will lead integration and testing of the final system. Prof. Geoff Bower will lead the UCB team in replicating and installing the phased array processors on CARMA.

3. mm/submm VLBI Arrays: Dr. Shep Doeleman will lead the logistics, bench and field testing, experiment planning, correlation and post processing analysis for the mm/submm observations. Dr. Doeleman has led the current UVLBI program at Haystack and carried out successful 1.3 and 2mm VLBI observations. Dr. Doeleman will be assisted by Haystack technical staff who have extensive experience in all aspects of high frequency VLBI observations.

Risk Factors: This is an ambitious proposal that will push the technique of VLBI to the point where it is feasible and, indeed, strongly compelling to form submm VLBI arrays in pursuit of fundamental questions at the intersection of astronomy and physics. All activities of this project, as described in the technical sections above present relatively low risk factors, and it is not expected that insurmountable technical challenges will arise. Engineers at Haystack Observatory have considerable experience in carrying out the observations and tasks in this proposal. The greatest risk is one of spreading Observatory staff too thin and not being able to ensure that proper preparations are made. The attached budget description details a level of support that is well matched to the challenging work to be done.