E. Fomalont

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

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Publications (84)202.29 Total impact

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    ABSTRACT: Deep radio observations at 1.4 GHz for the Extended Chandra Deep Field South were performed in 2007 June through September and presented in a first data release. The survey was made using six separate pointings of the Very Large Array with over 40 hr of observation per pointing. In the current paper, we improve on the data reduction to produce a second data release (DR2) mosaic image. This DR2 image covers an area of about a third of a square degree, reaches a best rms sensitivity of 6 μJy, and has a typical sensitivity of 7.4 μJy per 2.''8 by 1.''6 beam. We also present a more comprehensive catalog, including sources down to peak flux densities of five or more times the local rms noise along with information on source sizes and relevant pointing data. We discuss in some detail the consideration of whether sources are resolved under the complication of a radio image created as a mosaic of separate pointings each suffering some degree of bandwidth smearing, and the accurate evaluation of the flux densities of such sources. Finally, the radio morphologies and optical/near-IR counterpart identifications are used to identify 17 likely multiple-component sources and arrive at a catalog of 883 radio sources, which is roughly double the number of sources contained in the first data release.
    The Astrophysical Journal Supplement Series 03/2013; 205(2):13. · 16.24 Impact Factor
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    ABSTRACT: The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.
    Astronomy Reports 03/2013; 57(3):153-194. · 0.76 Impact Factor
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    ABSTRACT: This paper presents accurate absolute positions from a 24 GHz Very Long Baseline Array (VLBA) search for compact extragalactic sources in an area where the density of known calibrators with precise coordinates is low. The goals were to identify additional sources suitable for use as phase calibrators for galactic sources, determine their precise positions, and produce radio images. In order to achieve these goals, we developed a new software package, PIMA, for determining group delays from wide-band data with much lower detection limits. With the use of PIMA, we have detected 327 sources out of 487 targets observed in three 24 hr VLBA experiments. Among the 327 detected objects, 176 are within 10° of the Galactic plane. This VGaPS catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots, as well as weighted CLEAN images, and calibrated visibility data are available on the Web in FITS format. Approximately one-half of objects from the 24 GHz catalog were observed at dual-band 8.6 GHz and 2.3 GHz experiments. Position differences at 24 GHz versus 8.6/2.3 GHz for all but two objects on average are strictly within reported uncertainties. We found that for two objects with complex structures, positions at different frequencies correspond to different components of a source.
    The Astronomical Journal 06/2011; 142(2):35. · 4.97 Impact Factor
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    ABSTRACT: We have carried our a series of Very Long Baseline Array (VLBA) astrometric observation of the Cassini spacecraft, which is currently orbiting Saturn, to determine the position of Saturn's center of mass in the International Celestial Reference Frame (ICRF). Our positional accuracies are typically 0.2-0.3 milli-arcseconds, corresponding to about 2 km at the distance of Saturn. The goal of the project is to improve the Saturn ephemeris. This paper reports the results of our first eight epochs of observing, from October 2006 through April 2009. A new planetary ephemeris (DE 422) has been fit to these data, plus two earlier VLBA observations of Cassini by others and a Cassini-based VLBA gravitational deflection experiment by Fomalont et al. in February 2009. Post-fit residuals for DE 422 have a mean offset less than 0.2 mas in both coordinates, but not all of our phase reference sources have ICRF position with this accuracy yet. Future observations will improve reference source positions, and will continue to follow Saturn through more than 1/4 of its orbital period. (The Cassini mission is currently funded to operate until 2017.) We are grateful to Larry Teitelbaum for support of this project through the Advanced Tracking and Observational Techniques of JPL's Interplanetary Network Directorate, and to John Benson and the VLBA operations team at NRAO for their excellent support of these observations. The VLBA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
    01/2011;
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    ABSTRACT: The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemeredes), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric VLBI observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5 nrad), or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009 to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn with respect to the VLBA data are generally 0.2 mas, but additional observations are needed to improve the positions of all of our phase reference sources to this level. Over time we expect to be able to improve the accuracy of all three coordinates in the Saturn ephemeris (latitude, longitude, and range) by a factor of at least three. This will represent a significant improvement not just in the Saturn ephemeris but also in the link between the inner and outer solar system ephemeredes and in the link to the inertial ICRF. Comment: Accepted for publication in the Astronomical Journal
    12/2010;
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    Edward Fomalont, Sergei Kopeikin
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    ABSTRACT: Since VLBI techniques give microarcsecond position accuracy of celestial objects, tests of GR using radio sources as probes of a gravitational field have been made. We present the results from two recent tests using the VLBA: In 2005, the measurement of the classical solar deflection; and in 2002, the measurement of the retarded gravitational deflection associated with Jupiter. The deflection experiment measured γ to an accuracy of 3 × 10 −4 ; the Jupiter experiment measured the retarded term to 20% accuracy. The controversy over the interpretation of the retarded term is summarized.
    Proceedings of the International Astronomical Union 01/2010; 248.
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    01/2010;
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    ABSTRACT: We have used the VLBA at eight epochs since 2006 to make astrometric observations of the Cassini spacecraft in orbit about Saturn. Phase-referenced VLBA measurements of the position of Cassini with respect to the IRCF, combined with accurate Cassini orbit solutions by JPL, allow us to determine the absolute position of Saturn at each epoch. These accurate positions of Saturn will improve the tie between the outer and inner planet ephemerides. One of the limiting sources of error is the uncertainty of our phase reference source positions in the ICRF grid, and we are using special VLBA observations to tie these sources closer to the ICRF grid. We plan to continue this program through 2012, when we will have covered more than 1/4 of Saturn's orbital period. By that point the uncertainly in latitude (the inclination of Saturn's orbit) will be reduced by at least a factor of two compared to the current planetary ephemeris. Future observations of this type during the Juno mission will enable Jupiter's ephemeris to be improved as well. Part of this research has been carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The VLBA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation.
    12/2009; 42:456.
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    E. Fomalont, S. Kopeikin
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    ABSTRACT: Since VLBI techniques give microarcsecond position accuracy of celestial objects, tests of GR using radio sources as probes of a gravitational field have been made. We present the results from two recent tests using the VLBA: In 2005, the measurement of the classical solar deflection; and in 2002, the measurement of the retarded gravitational deflection associated with Jupiter. The deflection experiment measured PPN-gamma to an accuracy of 0.0003; the Jupiter experiment measured the retarded term to 20% accuracy. The controversy over the interpretation of the retarded term is summarized. Comment: 4 pages: IAU240
    12/2009;
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    ABSTRACT: We report on some preliminary results on the evolution and luminosity functions of submillijansky (sub-mJy) radio sources, based on the VLA Chandra Deep Field South (CDFS) sample.
    Proceedings of the International Astronomical Union 08/2009; 5(S267).
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    ABSTRACT: VSOP-2 phase referencing, needed to image faint sources and to determine accurate positions, will require an orbit accuracy of about 2 cm at 23 GHz. This accuracy, however, may not be obtainable by direct orbital measurements. We propose an observation scheme, similar to that used at the VLBA, to be included during a phase referencing observation of about one orbit in order to determine a more accurate orbit determination. We show the effects of orbit errors, explain the suggested observations and reduction methods, and discuss the potential problems that might impede the use of this technique.
    07/2009; 402:310.
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    S. Likhachev, L. Kogan, E. Fomalont, F. Owen
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    ABSTRACT: New Multi-Frequency Synthesis (MFS) algorithms were developed and implemented in the Astro Space Locator (ASL) operating under MS Windows system. In November 2005 multi-frequency VLA observations of the radio source M87 were carried out at the following frequencies: 14.7, 15.2 21.3, 22.2, 23.0, and 23.4 GHz. We used the new MFS algorithms to determine the structure of M87 at the central frequency (19 GHz) and obtained both the image and spectral index map of the source. Comparison with more straight-forward imaging techniques (with single frequency images) shows that the new MFS algorithms increase the fidelity of the image by at least a factor of two and provides accurate spectral indices across the emission. Application to simulated Radioastron data is also shown.
    07/2009; 402:444.
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    ABSTRACT: We discuss the X-ray properties of the radio sources detected in a deep 1.4 and 5 GHz VLA Radio survey of the Extended Chandra Deep Field-South (E-CDFS). Among the 266 radio sources detected, we find 89 sources (1/3 of the total) with X-ray counterparts in the catalog of the 1 Ms exposure of the central 0.08 deg2 or in the catalog of the 250 ks exposure of the 0.3 deg2 E-CDFS field. For 76 (85%) of these sources, we have spectroscopic or photometric redshifts, and therefore we are able to derive their intrinsic properties from X-ray spectral analysis, namely intrinsic absorption and total X-ray luminosities. We find that the population of submillijansky radio sources with X-ray counterparts is composed of a mix of roughly 1/3 star-forming galaxies and 2/3 active galactic nuclei (AGNs). The distribution of intrinsic absorption among X-ray-detected radio sources is different from that of the X-ray-selected sample. Namely, the fraction of low-absorption sources is at least 2 times larger than that of X-ray selected sources in the CDFS. This is mostly due to the larger fraction of star-forming galaxies present among the X-ray-detected radio sources. If we investigate the distribution of intrinsic absorption among sources with L X > 1042 erg s–1 in the hard 2-10 keV band (therefore in the AGN luminosity regime), we find agreement between the X-ray population with and without radio emission. In general, radio-detected X-ray AGNs are not more heavily obscured than the non-radio-detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGNs. For the radio sources without cataloged X-ray counterparts, we measure their average photometric properties in the X-ray bands with stacking techniques. We detect emission with very high confidence level in the soft band and marginally in the hard band. Given their redshift distribution, the average X-ray luminosity of these sources is consistent with being powered by star formation. We note that on average, the spectral shape of our radio sources is soft with HR ~ –0.5 and constant in different bins of radio flux. This result shows that the statistics do not indicate a significant trend in the average X-ray spectral properties, but it is consistent with the radio source population being dominated by star-forming galaxies below 100 μJy, as shown by our morphological and multiwavelength analysis presented in Mainieri et al. and Padovani et al..
    The Astrophysical Journal 05/2009; 698(1):740. · 6.73 Impact Factor
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    ABSTRACT: During the past three years we have carried out a series of astrometric VLBA observations of the Cassini spacecraft. At each epoch, we used phase referencing to obtain high precision relative positions between Cassini and an angularly nearby calibration source. The calibration sources were separately tied to nearby defining sources of the International Celestial Reference Frame (ICRF) through additional phase-referenced VLBA experiments. By combining our position measurements of Cassini with a model of Cassini's orbit around Saturn (from Doppler measurements by the Deep Space Network), we are able to determine the ICRF position of Saturn at each epoch to about 0.3 mas. This is about 2 km at the average distance of Saturn. These results will improve the Saturn ephemeris, particularly in ecliptic latitude (the plane of Saturn's orbit). The error in latitude decreases dramatically as the total time span of VLBA data approaches 1/4 of Saturn's orbital period in 2011-2012. Saturn is the first outer planet whose ephemeris can be improved and more closely tied to the ICRF and the inner solar system through long-term observations of an orbiter. The planned Juno mission to Jupiter will allow this technique to be applied there also. The planetary ephemeris is an essential tool for studies of solar system dynamics, interplanetary spacecraft navigation, and test of general relativity. It requires continuous maintenance and improvement. This research has been carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and has relied on observations by the Very Long Baseline Array, a facility of the National Science Foundation operated by the National Radio Astronomy Observatory under a cooperative agreement with Associated Universities, Inc.
    05/2009;
  • Edward B. Fomalont, S. Kopeikin, O. Titov, M. Honma
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    ABSTRACT: We report on VLBA/VERA/geodetic observational tests of General Relativity. First, we will summarize the results from recent VLBA experiments which measured gamma from the bending of radio waves of quasars by the solar gravitational field, and the experiment that measured the the aberration of gravity using the Jovian gravitational field. We will then describe tentative results from several recent experiments that continue the measurements of the aberration of gravity from Jupiter and Saturn using the VLBA, VERA and the geodetic network. Finally, we discuss the limits of precision that can be obtained with VLBI now and in the future.
    04/2009; 261:1503.
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    ABSTRACT: We present astrometric and imaging results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24-hour observing sessions made over an approximately 5-year period. These observations were motivated by the need to extend the International Celestial Reference Frame to higher radio frequencies to enable improved deep space navigation after 2015 and to improve state-of-the-art astrometry. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted for systematic errors such as residual tropospheric and ionospheric refraction limit the overall accuracy of the catalogs. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved celestial reference frame and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.
    04/2009; 214.
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    E. Fomalont, S. Kopeikin, G. Lanyi, J. Benson
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    ABSTRACT: We have used the Very Long Baseline Array (VLBA) at 43, 23 and 15 GHz to measure the solar gravitational deflection of radio waves among four radio sources during an 18-day period in October 2005. Using phase-referenced radio interferometry to fit the measured phase delay to the propagation equation of the parameterized post-Newtonian (PPN) formalism, we have determined the deflection parameter gamma = 0.9998 +/- 0.0003$ (68% confidence level), in agreement with General Relativity. The results come mainly from 43 GHz observations where the refraction effects of the solar corona were negligible beyond 3 degrees from the sun. The purpose of this experiment is three-fold: to improve on the previous results in the gravitational bending experiments near the solar limb; to examine and evaluate the accuracy limits of terrestrial VLBI techniques; and to determine the prospects and outcomes of future experiments. Our conclusion is that a series of improved designed experiments with the VLBA could increase the presented accuracy by at least a factor of 4. Comment: 22 pages, 3 figures
    The Astrophysical Journal 04/2009; · 6.73 Impact Factor
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    ABSTRACT: We present the multiwavelength properties of 266 cataloged radio sources identified with 20 and 6 cm VLA deep observations of the CDFS at a flux density limit of 42 \mu Jy at the field centre at 1.4 GHz. These new observations probe the faint end of both the star formation and radio galaxy/AGN population. X-ray data, including upper limits, turn out to be a key factor in establishing the nature of faint radio sources. We find that, while the well-known flattening of the radio number counts below 1 mJy is mostly due to star forming galaxies, these sources and AGN make up an approximately equal fraction of the sub--millijansky sky, contrary to some previous results. We have also uncovered a population of distant AGN systematically missing from many previous studies of sub-millijansky radio source identifications. The AGN include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a significant radio-quiet component, which amounts to approximately one fifth of the total sample. We also find that radio detected, X-ray AGN are not more heavily obscured than the X-ray detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGN.
    03/2009;
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    R. Dodson, E. Fomalont, K. Wiik
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    ABSTRACT: In February 1997, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Programme (VSOP) mission. A significant fraction of the mission time was to be dedicated to the VSOP Survey of bright compact Active Galactic Nuclei (AGN) at 5 GHz, which was lead by ISAS. The VSOP Survey Sources are an unbiased dataset of 294 targets, of which 82% were successfully observed. These are now undergoing statistical analysis to tease out the characteristics of typical AGN sources. We present here the summary of the imaging and conclusions we have reached.
    02/2009;
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    ABSTRACT: The H2O maser connected with the young stellar object in the globule IC 1396N has been mapped with the VLBA during its highest state of activity in 1996 June. The spectrum of the H2O maser consisted of a dense group of strong low-velocity features near the LSR velocity of the globule, and two high-velocity features: one redshifted to 9.3 km s-1 and the other blueshifted to -14.1 km s-1. The map of low-velocity features displays a remarkable chain of at least eight maser spots located very close to a straight line about 15 AU in extent, with LSR velocities varying linearly along the line. The two high-velocity features are offset from the low-velocity group by 410 and 10,000 AU for the blue and red features, respectively. We discuss three models that can describe the observed distribution of maser spots: a Keplerian disk, a shock front, and a molecular outflow. The final model that we propose incorporates all three of these models: the low-velocity features arise in the Keplerian disk with maser emission excited by shock waves traveling in the disk, while the high-velocity features arise at the root of the molecular outflow originating from the central 4 M☉ young star or a protostar. The mass of the disk and its angular momentum are similar to those of the solar system planets. It is suggested that it is a circumstellar accretion disk accumulating the excess angular momentum of the collapsing molecular core, which may give rise to the formation of a planetary system. This model can be tested by measurements of the proper motion and radial velocity variations of the maser spots.
    The Astrophysical Journal 01/2009; 526(1):236. · 6.73 Impact Factor

Publication Stats

923 Citations
202.29 Total Impact Points

Institutions

  • 1999–2013
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2002–2010
    • University of Missouri
      • Department of Physics and Astronomy
      Columbia, Missouri, United States
  • 2005–2009
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 1997
    • University of New Hampshire at Manchester
      Manchester, New Hampshire, United States
  • 1984
    • University of Houston – Victoria
      Houston, Texas, United States