Recent Publications


Last updated 30 June 2007


Low Frequency Radio Astronomy from the Moon: Cosmic Reionization and More

Christopher L. Carilli, Jacqueline N. Hewitt, and Abraham Loeb

To appear in "Astrophysics Enabled by the Return to the Moon," Cambridge University Press, ed. M. Livio.

We discuss low frequency radio astronomy from the moon, predominantly in the context of studying the neutral intergalactic medium during cosmic reionization using the HI 21cm line of neutral hydrogen. The epoch of reionization is the next frontier in observational cosmology, and HI 21cm studies are recognized as the most direct probe of this key epoch in cosmic structure formation. Current constraints on reionization indicate that the redshifted 21cm signals will likely be in the range of 100 MHz to 180 MHz, with the pre-reionization signal going to as low as 10 MHz. The primary observational challenges to these studies are: (1) ionospheric phase fluctuations, (2) terrestrial radio frequency interference, and (3) Galactic and extragalactic foreground radiation. Going to the far side of the moon removes the first two of these challenges. Moreover, a low frequency telescope will be relatively easy to deploy and maintain on the moon, at least compared to other, high frequency, telescopes. We discuss the potential 21cm signals from reionization, and beyond, and the telescope specifications needed to measure these signals. The near-term ground-based projects will act as path-finders for a potential future low frequency radio telescope on the moon. If it is found that the terrestrial interference environment, or ionospheric phase fluctuations, preclude ground-based studies of reionization, then it becomes imperative to local future telescopes on the far side of the moon. Besides pursuing these path-finder reionization telescopes, we recommend a number of near-term studies that could help pave the way for low frequency astronomy on the moon.

Electronic Preprint


Constraints on Fundamental Cosmological Parameters with Upcoming Redshifted 21cm Observations

Judd D. Bowman, Miguel F. Morales, and Jacqueline N. Hewitt

2007, The Astrophysical Journal, 661, 1.

Constraints on cosmological parameters from upcoming measurements with the Mileura Wide-field Array Low Frequency Demonstrator (MWA-LFD) of the redshifted 21cm power spectrum are forecasted assuming a flat Lambda-CDM and assuming that the reionization of neutral hydrogen in the intergalactic medium occurs before a redshift of z=8. We find that observations with the MWA LFD cannot constrain the underlying cosmology in this scenario. In principle, a similar experiment with a 10-fold increase in collecting area could provide useful constraints on the slope of the inflationary power spectrum, n_s, and the running of the spectral index, alpha_s, but these constraints are subject to the caveat that even a small reionization controbution could be confused with the cosmological signal. In addition to the redshifted 21cm signal, we include two nuisance components in our analysis related to the systematics and astrophysical foregrounds present in low-frequency radio observations. These components are found to be well separated from the signal and contribute little uncertainty (<30%) to the measured values of the cosmological model parameters.

Electronic Preprint


Field Deployment of Prototype Antenna Tiles for the Mileura Widefield Array Low Frequency Demonstrator

Judd D. Bowman, David G. Barnes, Frank H. Briggs, Brian E. Corey, Merv J. Lynch, N. D. Ramesh Bhat, Roger J. Cappallo, Sheperd S. Doeleman, Brian J. Fanous, David Herne, Jacqueline N. Hewitt, Chris Johnston, Justin C. Kasper, Jonathon Kocz, Eric Kratzenberg, Colin J. Lonsdale, Miguel F. Morales, Divya Oberoi, Joseph E. Salah, Bruce Stansby, Jamie Stevens, Glen Torr, Randall Wayth, Rachel L. Webster, J. Stuart B. Wyithe

2007, The Astronomical Journal, 133, 150.

Experiments were performed with prototype antenna tiles for the Mileura Widefield Array Low Frequency Demonstrator (MWA-LFD) to better understand the widefield, wideband properties of their design and to characterize the radio frequency interference (RFI) between 80 and 300 MHz at the site in Western Australia. Observations acquired during the six month deployment confirmed the redicted sensitivity of the antennas, sky-noise dominated system temperatures, and phase-coherent interferometric measurements. The radio spectrum is remarkably free of strong terrestrial signals, with the exception of two narrow frequency bands allocated to satellite downlinks and rare bursts due to found-based transmissions being scattered from aircraft and meteor trails. Results indicate the potential of the MWA-LFD to make significant achievements in its three key science objectives: epoch of reionization science, heliospheric science, and radio transient detection.

Electronic Preprint


The Extragalactic Lens VLBI Imaging Survey (ELVIS) I. A Search for the Central Image in the Gravitational Lens PMN J1838-3427

Edward R. Boyce, Joshua N. Winn, Jacqueline N. Hewitt, and Steven T. Myers

2006, The Astrophysical Journal, 648, 73.

The Extragalactic Lens VLBI Imaging Survey (ELVIS) searches for central images of lensed radio quasars, in order to measure the central density profiles of distant galaxies. Here we present sensitive multi-epoch Very Long Baseline Array (VLBA) observations of PMN J1838-3427 at 8 GHz, with a 1-sigma noise level of 38 microJy/beam. Based on the absence of a central image of the background source at this level, we explore the possibilities for the central matter distribution in the lens galaxy. A power-law density profile, rho~r^(-gamma), must have gamma > 1.93. Thus the density profile is close to an isothermal profile (gamma=2) or steeper. The upper limit on any constant-density core in an otherwise isothermal profile is ~< 5 parsecs. We also derive the constraints on models in which the density profile is isothermal on kiloparsec scales, but is allowed to have a different power law in the central ~100 parsecs. If the lens galaxy harbors a supermassive black hole, the galaxy profile is allowed to be shallower, but for the expected black hole mass the galaxy profile must still be close to isothermal or steeper.

Electronic Preprint


Statistical EOR Detection and the Mileura Widefield Array

Miguel F. Morales, Judd D. Bowman, Roger J. Cappallo, Jacqueline N. Hewitt, and Colin J. Lonsdale

2006, New Astronomy Reviews, 50, 173.

Statistical epoch of reionization (EOR) observations are one of the emerging fields of experimental cosmology and could provide unique constraints on the formation of structure and the emergence of the first luminous objects. The primary challenge in observing the 21cm EOR signature is subtracting the strong astrophysical and instrumental foregrounds. In this paper, we review the three-dimensional statistical EOR analysis, present a multi-stage foreground removal framework, and review the design and early field testing for the Mileura Widefield Array.

PDF


Improving Foreground Subtraction in Statistical Observations of 21cm Emission from the Epoch of Reionization

Miguel F. Morales, Judd D. Bowman, and Jacqueline N. Hewitt

2006, The Astrophysical Journal, 648, 767.

Statistical observations of the Epoch of Reionization using the 21cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground removal techniques have been proposed in the literature, and it has been assumed that these would be used in combintation to reveal the Epoch of Reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground removal techniques, we identify an additional subtraction stage that can further reduce the EOR foreground contamination, and study the interactions between the foreground removal algorithms. This enables us to outline a comprehensive foreground removal strategy that incorporates all previously proposed subtraction techniques. Using this foreground removal framework and the characteristic subtractions errors, we discuss the complementarity of different foreground removal techniques and the implications for array design and the analysis of EOR data.

Electronic Preprint


A Search for Radio Gravitational Lenses Using the Sloan Digital Sky Survey and the Very Large Array

Edward R. Boyce, Judd D. Bowman, Adam S. Bolton, Jacqueline N. Hewitt, and Scott Burles.

2006, The Astrophysical Journal, 640, 62.

We report on a novel search for radio gravitational lenses. Using the Very Large Array, we imaged ten candidates with both dual redshifts in Sloan Digital Sky Survey spectra and 1.4 GHz radio flux >2 mJy in the FIRST survey. The VLA maps show that in each case the radio emission is associated with the foreground galaxy rather than being lensed emission from the background galaxy, although at least four of our targets are strong lenses at optical wavelengths. These SDSS dual-redshift systems do not have lensed radio emission at the sensitivity of current radio surveys.

Electronic Preprint


The GRB All-sky Spectrometer Experiment (GASE)

Miguel F. Morales, Jacqueline N. Hewitt, Justin C. Kasper, Benjamin Lane, Judd D. Bowman, Paul S. Ray, and Roger J. Cappallo

2006, ASP Conference Series, 345, 512

Recent theoretical developments concerning the plasma physics near the GRB blast wave have renewed interest in the properties of impulsive low frequency radio emission from GRBs. Prompt coherent low frequency radio emission would provide a unique probe of the plasma conditions near the blast wave during the period of peak particle acceleration and gamma ray emission. In this paper we introduce the GRB All-sky Spectrometer Experiment (GASE). This experiment will make use of advances in digital technology to record 4 MHz of bandwidth in eight 30 MHz dipole antennas for two hours after a Swift or HETE-2 trigger is received. Due to the long dispersion delay at low frequencies, this allows contemporaneous measurements of the gamma ray and radio emission. The high spectral resolution of GASE will allow the predicted prompt 30 MHz signal from GRBs to be fully de-dispersed and identified. The combination of high spectral resolution, full-sky imaging, and (if required) geographically separated detectors will also provide stringent rejection of radio inteference.


The Sensitivity of First Generation Epoch of Reionization Observatories and Their Potential for Differentiating Theoretical Power Spectra

Judd D. Bowman, Miguel F. Morales, and Jacqueline N. Hewitt

2006, The Astrophysical Journal, 638, 20

Statistical observations of the epoch of reionization (EOR) power spectrum provide a rich data set for understanding the transition from the cosmic "dark ages" to the ionized universe we see today. EOR observations have become an active area of experimental cosmology, and three first generation observatories--MWA, PAST, and LOFAR--are currently under development. In this paper we provide the first quantitative calculation of the three dimensional power spectrum sensitivity, incorporating the design parameters of a planned array. This calculation is then used to explore the constraints these first generation observations can place on the EOR power spectrum. The results demonstrate the potential of upcoming power spectrum observations to constrain theories of structure formation and reionization.

Electronic Preprint


Gravitational Lensing Signatures of Supermassive Black Holes in Future Radio Surveys

Judd D. Bowman, Jacqueline N. Hewitt, and James R. Kiger

2004, The Astrophysical Journal, 617, 81.

Observational measurements of the relationship between supermassive black holes (SMBHs) and the properties of their host galaxies are an important method for probing theoretical hierarchical growth models. Gravitational lensing is a unique mechanism for acquiring this information in systems at cosmologically significant redshifts. We review the calculations required to include SMBHs in two standard galactic lens models, a cored isothermal sphere and a broken power law. The presence of the SMBH produces two primary effects depending on the lens configuration, either blocking the core image that is usually predicted to form from a softened lens model, or adding an extra, highly demagnified, image to the predictions of the unaltered lens model. The magnitudes of these effects are very sensitive to galaxy core sizes and SMBH masses. Therefore, observations of these lenses would probe the properties of the inner regions of galaxies, including their SMBHs. Lensing cross-sections and optical depth calculations indicate that in order to readily observe these characteristic signatures, flux ratios of order 10^6 or more between the brightest and faintest images of the lens must be detectable, and thus the next generation of radio telescope technology offers the first opportunity for a serious observational campaign.

Electronic Preprint


Toward Epoch of Re-ionization Measurements with Wide-Field Radio Observations

Miguel F. Morales and Jacqueline N. Hewitt

2004, Astrophysical Journal, 617, 81

This paper explores the potential for statistical epoch of reionization (EOR) measurements using wide field radio observations. New developments in low frequency radio instrumentation and signal processing allow very sensitive EOR measurements, and the analysis techniques enabled by these advances offer natural ways of separating the EOR signal from the residual foreground emission. This paper introduces the enabling technologies and proposes an analysis technique designed to make optimal use of the capabilities of next generation low frequency radio arrays. The observations we propose can directly observe the power spectrum of the EOR using relatively short observations, and are significantly more sensitive than other techniques which have been discussed in the literature. For example, in the absence of foreground contamination the measurements we propose would produce five 3-sigma power spectrum points in 100 hours of observation with only 4 MHz bandwidth with LOFAR for simple models of the high redshift 21cm emission. The challenge of residual foreground removal may be addressed by the symmetries in the three-dimensional (two spatial frequencies and radiofrequency) radio interferometric data. These symmetries naturally separate the EOR signal from most classes of residual un-subtracted foreground contamination, including all foreground continuum sources and radio line emission from the Milky Way.

Electronic Preprint


The Design of Radio Telescope Array Configurations using Multiobjective Optimization: Imaging Performance versus Cable Length

B. E. Cohanim, J. N. Hewitt, and O. de Weck

2004, Astrophysical Journal Supplement Series, 154, 705.

The next generation of radio telescope interferometric arrays requires careful design of the array configuration to optimize the performance of the overall system. We have developed a framework, based on a genetic algorithm, for rapid exploration and optimization of the objective space pertaining to multiple objectives. We have evaluated a large space of possible designs for 27-, 60-, 100-, and 160-station arrays. The 27-station optimizations can be compared to the well-known VLA case, and the larger array designs apply to arrays currently under design such as LOFAR, ATA, and the SKA. In the initial implementation of our framework we evaluate designs with respect to two metrics, array imaging performance and the length of cable necessary to connect the stations. Imaging performance is measured by the degree to which the sampling of the uv plane is uniform. For the larger arrays we find that well-known geometric designs perform well and occupy the Pareto front of optimum solutions. For the 27-element case we find designs, combining features of the well-known designs, that are more optimal as measured by these two metrics. The results obtained by the multiobjective genetic optimization are corroborated by simulated annealing, which also reveals the role of entropy in array optimization. Our framework is general, and may be applied to other design goals and issues, such as particular schemes for sampling the uv plane, array robustness, and phased deployment of arrays.

Electronic Preprint


A Survey for Transient Astronomical Radio Emission at 611 MHz

C. A. Katz, J. N. Hewitt, B. E. Corey, and C. B. Moore

2003, Publications of the Astronomical Society of the Pacific, 115, 675.

We have constructed and operated the Survey for Transient Astronomical Radio Emission (STARE) to detect transient astronomical radio emission at 611 MHz originating from 1.4 sr of the sky over the northeastern United States. The system is sensitive to transient events on timescales of 0.125 sec to a few minutes, with a typical zenith flux density detection threshhold of approximately 27 kJy. During 18 months of round-the-clock observing with three geographically separated instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these events were rejected as locally generated interference, determined by requiring the simultaneous observations of an event at all three sites for it to be identified as having an astronomical origin. The remaining events have been found to be associated with 99 solar radio bursts. These results demonstrate the remarkably effective RFI rejection achieved by a coincidence technique using precision timing (such as GPS clocks) at geographically separated sites.

The non-detection of extra-solar bursting or flaring radio sources has improved the flux density sensitivity and timescale sensitivity imits set by several similar experiments in the 1970s. We discuss the consequences of these limits for the immediate solar neighborhood and the discovery of previously unknown classes of sources. We also discuss other possible uses for the large collection of 611 MHz monitoring data assembled by STARE.

Postscript


Time Delay Monitoring of Gravitational Lens MG2016+112

D. B. Haarsma, K. J. Hoekema, J. N. Hewitt, and G. I. Langston

Presented at the 198th meeting of the AAS, Pasadena, June 2001

The time delay of a gravitational lens, when combined with a model of the lens mass distribution and other information, yields the angular diameter distance to the lens. The gravitational lens system 2016+112 has the highest redshift lens of any system discovered so far, and thus its distance would yield important information not only about the Hubble parameter, but also about the mass density of the universe and the cosmological constant. We have made monthly observations of 2016+112 at the VLA (during favorable arrays) since June 1999 at 3.6 and 6cm, which we will combine with archive data from 1987-1993. This poster gives a status report on the effort to detect variability and determine the time delay of the lens.

PDF


PMN J0134-0931: A Gravitationally Lensed Quasar with an Unusual Radio Morphology

J. N. Winn, J. E. J. Lovell, H.-W. Chen, A. B. Fletcher, J. N. Hewitt, A. R. Patnaik, and P. L. Schechter

2002, The Astrophysical Journal, 564, 10

The radio-loud quasar PMN J0134-0931 was discovered to have an unusual morphology during our search for gravitational lenses. In VLA and MERLIN images, there are 5 compact components with a maximum separation 681 milliarcseconds. All of these components have the same spectral index from 5 GHz to 43 GHz. In a VLBA image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134-0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart.

Electronic preprint


PMN J1632-0033: A Strong Gravitational Lens Candidate

J. N. Winn, J. N. Hewitt, J. E. J. Lovell, N. D. Morgan, A. R. Patnaik, B. Pindor, P. L. Schechter, R. A. Schommer

2002, Astronomical Journal, 124, 3485

We report the discovery of a double radio source that is probably a two-image gravitational lens. The object PMN J1632-0033 is composed of two compact, flat-spectrum components with separation 1.47" and flux density ratio 12.4. The spectral indices of the components are the same within 0.05, as determined by measurements ranging from 1.4 GHz to 43 GHz using the VLA, ATCA, MERLIN and VLBA. The optical counterpart is double, with roughly the same separation and position angle as the radio double. An optical spectrum of the bright component, obtained with the first Magellan telescope, reveals quasar emission lines at redshift 3.42. Although J1632-0033 could be a binary quasar, the small separation and similarity of radio continuum spectra of the components suggest that they are lensed images of a single quasar.

Electronic preprint


A Nearly Symmetric Double-Image Gravitational Lens

J. N. Winn, J. N. Hewitt, A. R. Patnaik, P. L. Schechter, R. A. Schommer, S. Lopez, J. Maza, and S. Wachte

2001, Astronomical Journal, 121, 1223.

We report the discovery of a new double-image quasar resulting from our search for gravitational lenses in the southern sky. Radio source PMN J2004-1349 is composed of two flat-spectrum components of approximately equal brightness separated by 1.13 arcseconds in VLA, MERLIN, and VLBA images. The I-band optical counterpart is also double, with roughly the same separation, position angle, and flux ratio as the radio double. Upon subtraction of the quasars from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information (such as the redshifts of the quasar and lens galaxy) will be neeeded before many interesting astrophysical quantities (such as the mass of the galaxy, or the predicted time delay between lensed images) can be inferred.

Electronic preprint


Exploring the Performance of Large-N Radio Astronomical Arrays

C. J. Lonsdale, S. S. Doeleman, R. J. Cappallo, J. N. Hewitt, and A. R. Whitney

Proc. SPIE (2000), volume 4015, page 126.

New radio telescope arrays are currently being contemplated which may be built using hundreds, or even thousands, of relatively small antennas. These include the One Hectare Telescope of the SETI Institute and UC Berkeley, the LOFAR telescope planned for the New Mexico desert surrounding the VLA, and possibly the ambitious international Square Kilometer Array (SKA) project. Recent and continuing advances in signal transmission and processing technology make it realistic to consider full cross-correlation of signals from such a large number of antennas, permitting the synthesis of an aperture with much greater fidelity than in the past. In principle, many advantages in instrumental performance are gained by this "large-N" approach to the design, most of which require the development of new algorithms. Because new instruments of this type are expected to outstrip the performance of current instruments by wide margins, much of their scientific productivity is likely to come from the study of objects which are currently unknown. For this reason, instrumental flexibility is of special importance in design studies. A research effort has begun at Haystack Observatory to explore large-N performance benefits, and to determine what array design properties and data reduction algorithms are required to achieve them. The approach to these problems, involving a sophisticated data simulator, algorithm development, and exploration of array configuration parameter space, will be described, and progress to date will be summarized.


LOFAR: A Low Frequency Radio Array

J. N. Hewitt, representing the LOFAR collaboration

Viewgraphs for a presentation made to the LIGO Scientific Collaboration (LSC) at the August 2000 meeting of the LSC in Hanford, Washington. This presentation served to introduce LOFAR to the LSC members and to initiate discussion on a possible exchange of data between the two collaborations.

Postscript version -- PDF version


PMN J1838-3427: A New Gravitationally Lensed Quasar

J. N. Winn, J. N. Hewitt, P. L. Schechter, A. Dressler, E. E. Falco, C. D. Impey, C. S. Kochanek, J. Lehar, J. E. J. Lovell, B. A. McLeod, N. D. Morgan, J. A. Munoz, H.-W. Rix, M. T. Ruiz

The Astronomical Journal (2000), volume 120, page 2868.

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components, separated by 1 arcsec in VLA and VLBA images, with matching spectral indices. An HST/WFPC2 image reveals the lens galaxy between the components. The background object is a z=2.78 quasar, but the redshift of the lens galaxy has not been measured. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest that the time delay between the flux variations of the components is measurable, and could thus provide an independent measurement of H_0.

Electronic preprint


Further Investigation of the Time Delay, Magnification Ratios, and Variability in the Gravitational Lens 0218+357

A. S. Cohen, J. N. Hewitt, C. B. Moore, D. B. Haarsma

2000, Astrophysical Journal, 545, 578

High precision VLA flux density measurements for the lensed images of 0218+357 yield a time delay of 10.1+1.5-1.6 days (95% confidence). This is consistent with independent measurements carried out at the same epoch (Biggs et al. 1999), lending confidence in the robustness of the time delay measurement. However, since both measurements make use of the same features in the light curves, it is possible that the effects of unmodelled processes, such as scintillation or microlensing, are biasing both time delay measurements in the same way. Our time delay estimates result in a confidence intervals that are somewhat larger than those of Biggs et al., probably because we adopt a more general model of the source variability, allowing for constant and variable components. When considered in relation to the lens mass model of Biggs et al., our best-fit time delay implies a Hubble constant of Ho = 71+17-23 km/sMpc for Omega_o = 1 and lambda_o=0 (95% confidence; filled beam). This confidence interval for Ho does not reflect systematic error, which may be substantial, due to uncertainty in the position of the lens galaxy. We also measure the flux ratio of the {\it variable} components of 0218+357, a measurement of a small region that should more closely represent the true lens magnification ratio. We find ratios of 3.2+0.3-0.4 (95% confidence; 8 GHz) and 4.3+0.5-0.8 (15 GHz). Unlike the reported flux ratios on scales of 0.1'', these ratios are not strongly significantly different. We investigate the significance of apparent differences in the variability properties of the two images of the background active galactic nucleus. We conclude that the differences are not significant, and that time series much longer than our 100-day time series will be required to investigate propagation effects in this way.

Postscript version of this paper -- PDF version -- 8 GHz light curve data -- 15 GHz light curve data


A Multipole-Taylor Expansion for the Potential of Gravitational Lens MG J0414+0534

C. S. Trotter, J. N. Winn, and J. N. Hewitt

The Astrophysical Journal (2000), Volume 535, page 671.

We employ a multipole-Taylor expansion to investigate how tightly the gravitational potential of the quadruple-image lens MG J0414+0534 is constrained by recent VLBI observations. These observations revealed that each of the four images of the background radio source contains four distinct components, thereby providing more numerous and more precise constraints on the lens potential than were previously available. We expand the two-dimensional lens potential using multipoles for the angular coordinate and a modified Taylor series for the radial coordinate. After discussing the physical significance of each term, we compute models of MG J0414+0534 using only VLBI positions as constraints. The best-fit model has both interior and exterior quadrupole moments as well as exterior m=3 and m=4 multipole moments. The deflector centroid in the models matches the optical galaxy position, and the quadrupoles are aligned with the optical isophotes. The radial distribution of mass could not be well constrained. We discuss the implications of these models for the deflector mass distribution and for the predicted time delays between lensed components.

Electronic preprint


"The Radio Wavelength Time Delay of Gravitational Lens 0957+561"

Deborah B. Haarsma (Haverford), Jacqueline N. Hewitt (MIT),
Joseph Lehár (CfA), Bernard F. Burke (MIT)

The Astrophysical Journal, 1 January 1999, Vol. 510, pp 64-70.

Abstract

The gravitational lens 0957+561 was monitored with the Very Large Array from 1979 to 1997. The 6 cm light curve data from 1995-1997 and the 4 cm data from 1990-1997 are reported here. At 4 cm, the intrinsic source variations occur earlier and are twice as large as the corresponding variations at 6 cm. The VLBI core and jet components have different magnification factors, leading to different flux ratios for the varying and non-varying portions of the VLA light curves. Using both the PRHQ and Dispersion statistical techniques, we determined the time delay, core flux ratio, and excess non-varying B image flux density. The fits were performed for the 4 cm and 6 cm light curves, both individually and jointly, and we used Gaussian Monte Carlo data to estimate 68% statistical confidence levels. The delay estimates from each individual wavelength were inconsistent given the formal uncertainties, suggesting that there are unmodeled systematic errors in the analysis. We roughly estimate the systematic uncertainty in the joint result from the difference between the 6 cm and 4 cm results, giving 409+-30 days for the PRHQ statistic and 397+-20 days for the Dispersion statistic. These results are consistent with the current optical time delay of 417+-3 days, reconciling the long-standing difference between the optical and radio light curves and between different statistical analyses. The unmodeled systematic effects may also corrupt light curves for other lenses, and we caution that multiple events at multiple wavelengths may be necessary to determine an accurate delay in any lens system. Now that consensus has been reached regarding the time delay in the 0957+561 system, the most pressing issue remaining for determining H_0 is a full understanding of the mass distribution in the lens.

Postscript version of this paper -- 6cm light curve data -- 4cm light curve data