T. E. Clarke

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (127)260.38 Total impact

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    ABSTRACT: We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalogue as a sky model to correct the ionospheric distortions in the data and create a new set of sky maps and corresponding catalogue at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map rms noise level of σ ̃ 0.1 Jy beam-1. The clean bias is 0.66 × σ and the theoretical largest angular size is 36 arcmin. Six previously unimaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalogue includes 92 964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing centre in the VLSS.
    04/2014; 440(1).
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    ABSTRACT: A community meeting on the topic of "Radio Astronomy in the LSST Era" was hosted by the National Radio Astronomy Observatory in Charlottesville, VA (2013 May 6--8). The focus of the workshop was on time domain radio astronomy and sky surveys. For the time domain, the extent to which radio and visible wavelength observations are required to understand several classes of transients was stressed, but there are also classes of radio transients for which no visible wavelength counterpart is yet known, providing an opportunity for discovery. From the LSST perspective, the LSST is expected to generate as many as 1 million alerts nightly, which will require even more selective specification and identification of the classes and characteristics of transients that can warrant follow up, at radio or any wavelength. The LSST will also conduct a deep survey of the sky, producing a catalog expected to contain over 38 billion objects in it. Deep radio wavelength sky surveys will also be conducted on a comparable time scale, and radio and visible wavelength observations are part of the multi-wavelength approach needed to classify and understand these objects. Radio wavelengths are valuable because they are unaffected by dust obscuration and, for galaxies, contain contributions both from star formation and from active galactic nuclei. The workshop touched on several other topics, on which there was consensus including the placement of other LSST "Deep Drilling Fields," inter-operability of software tools, and the challenge of filtering and exploiting the LSST data stream. There were also topics for which there was insufficient time for full discussion or for which no consensus was reached, which included the procedures for following up on LSST observations and the nature for future support of researchers desiring to use LSST data products.
    01/2014; 126.
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    ABSTRACT: LOBO is a proposed, dedicated, radio synoptic, high-z spectroscopy, and real-time transient and ionosphere monitoring capability of the Karl G. Jansky VLA. It will make use of the primary focus feeds to observe in parallel with the higher-frequency, Cassegrain feeds. LOBO will have dedicated samplers, fiber transmission, and backend processing systems, the latter to include correlator and pipelined calibration, imaging, and archive systems. With a ≥ 5 deg^2 field-of-view at meter wavelengths and longer (< 500 MHz), LOBO will perform efficient, blind searches for non-thermal transients and high-redshift spectral lines, e.g. by surveying 64 Mpc^2 at 4 at 330 MHz in each pointing. LOBO will provide synoptic, wide-field continuum images in a publicly available archive of all targeted VLA fields, annually surveying for ~6000 hours or over 25% of the available sky. We explore the potential for leveraging the scientific potential of this “Radio LSST” capability in the LSST era. A 10-antenna pilot project called the VLA Low Frequency Ionosphere and Transient Experiment (VLITE) is currently funded by NRL and under development with NRAO to explore the LOBO concept.
    01/2014;
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    ABSTRACT: Diffuse radio emission permeating the cluster gravitational potential reveals the widespread presence of relativistic particles and magnetic fields in the intracluster medium (ICM). This emission is observed in numerous clusters which are dynamically complex. The radio emission is observationally classified as halo or relic. Additional low frequency ICM emission is detected from cluster-center radio galaxies which are important for energy feedack into the ICM. I will present recent low frequency radio (VLA and GMRT) data and Chandra X-ray results on several cluster systems. The recently discovered ultra-steep spectrum source in Abell 2443 may be a member of the relatively rare class of adiabatically compressed radio relics. Chandra observations reveal the presence of two surface brightness edges in the ICM and new GMRT observations provide additional details of the spectral index distribution in the ICM. Upcoming improvements in radio instruments will be crucial for expanding our understanding of the relativistic particle and magnetic field content of the ICM. I will briefly discuss a new exploratory concept (LOBO or LOw Band Observatory) which could enable low band (<500 MHz) observing using the new NRL/NRAO Low Band receivers in parallel with all high frequency VLA observing programs.
    01/2014;
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    ABSTRACT: We outline the science case for extended radio emission and polarization in galaxy clusters which would be a scientifically important area of research for an upcoming Jansky Very Large Array Sky Survey. The survey would provide a major contribution in three key areas of the physics of clusters: 1) the active galactic nucleus population and the impact of feedback on the evolution of the intra-cluster medium, 2) the origin and evolution of diffuse cluster radio sources to probe the physics of mergers with implications for cosmology, and 3) the origin and role of magnetic fields in the ICM and in large scale structures. Considering all three areas, a survey must have sufficient spatial resolution to study the tailed galaxies which trace the cluster weather as well as the radio lobes driving energy into the cluster from the central AGN. The survey must also have sensitivity to low surface brightness emission and large angular scales to probe radio halos and relics as well as the WHIM residing in the large scale structure filaments. Finally, we note that full polarization information would be a highly valuable tool to probe a number of cluster-related issues. Due to the general steep spectral index of the emission we consider the survey is best suited to this science when conducted in P, L, or S bands. We conclude that the choices of S Band + D Configuration, L Band + C Configuration, and P Band + B Configuration offer optimal resolutions for constraining galactic interactions and feedback in cluster environments, while still probing large scale structure and the bulk cluster environment itself. While the push to probe higher redshifts and lower mass limits strongly favors a narrow and deep (or even targeted) survey strategy, we note that a wide survey covering roughly 1/4-2/3 of the sky will have significant scientific return, discovery potential, and archival value.
    01/2014;
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    ABSTRACT: Cool cores of some galaxy clusters exhibit faint radio "minihalos." Their origin is unclear; their study has been limited by their small number. We undertook a systematic search for minihalos in a large sample of X-ray luminous clusters with high-quality radio data. In this paper, we report four new minihalos (A 478, ZwCl 3146, RXJ 1532.9+3021 and A 2204), and five candidates, found in the reanalyzed archival Very Large Array observations. The radio luminosities of our minihalos and candidates are in the range $10^{23-25}$ W Hz$^{-1}$ at 1.4 GHz, consistent with this type of radio sources. Their sizes (40-160 kpc in radius) are somewhat smaller than those of the previously known minihalos. We combine our new detections with previously known minihalos, obtaining a total sample of 21 objects, and briefly compare the cluster radio properties to the average X-ray temperature and the total masses estimated from Planck. We find that nearly all clusters hosting minihalos are hot and massive. Beyond that, there is no clear correlation between the minihalo radio power and cluster temperature or mass (in contrast with the giant radio halos found in cluster mergers, whose radio luminosity correlates with the cluster mass). Chandra X-ray images indicate gas sloshing in the cool cores of most of our clusters, with minihalos contained within the sloshing regions in many of them. This supports the hypothesis that radio-emitting electrons are reaccelerated by sloshing. Advection of relativistic electrons by the sloshing gas may also play a role in the formation of the less-extended minihalos.
    The Astrophysical Journal 11/2013; 781(1). · 6.73 Impact Factor
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    ABSTRACT: We present flux density measurements and pulse profiles for the millisecond pulsar PSR J2145-0750 spanning 37 to 81 MHz using data obtained from the first station of the Long Wavelength Array. These measurements represent the lowest frequency detection of pulsed emission from a millisecond pulsar to date. We find that the pulse profile is similar to that observed at 102 MHz. We also find that the flux density spectrum between ~40 MHz to 5 GHz is suggestive of a break and may be better fit by a model that includes spectral curvature with a rollover around 730 MHz rather than a single power law.
    The Astrophysical Journal Letters 09/2013; 775(1). · 6.35 Impact Factor
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    ABSTRACT: [1] We present a new passive, bistatic high-frequency (HF) radar system consisting of the transmitters for the radio station WWV and the dipole antenna array that comprises the first station of the Long Wavelength Array (LWA) or “LWA1.” We demonstrate that these two existing facilities, which are operated for separate purposes, can be used together as a unique HF radar imager, capable of monitoring the entire visible sky. In this paper, we describe in detail the techniques used to develop all-sky radar capability at 10, 15, and 20 MHz. We show that this radar system can be a useful tool for probing ionospheric structure and its effect on over-the-horizon (OTH) geolocation. The LWA1+WWV radar system appears to be especially adept at detecting and characterizing structures associated with sporadic-E. In addition, we also demonstrate how this system may be used for long-distance, OTH mapping of terrain/ocean HF reflectivity. Finally, we discuss the potential improvements in the utility of these applications as more LWA stations are added.
    Radio Science. 09/2013; 48(5).
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    ABSTRACT: At the request of the conference attendees, we have compiled a classication of ex- tended radio sources in clusters. These range from scales of tens of parsecs to over a megaparsec in scale, and include both sources associated with AGN and sources thought to derive from the electron population in the ionized ICM. We pay special attention to distinguishing between the types of AGN in the cores of cooling o w clusters and between the multiple classes of objects referred to over the years as \ra- dio relics." We suggest new names based on physical arguments for some of these classes of objects where their commonly used names are inappropriate or confusing. This conference note was inspired by the frequent lamentation during discussions of radio halos and relics that the nomenclature for these sources is confusing. \We need a new name for relics" has become a common refrain, since three physically distinct sources are all re- ferred to in the literature as \radio relics," and at least one of them is not a relic of anything. The phenomeno- logical approach used to classify these diuse sources has produced this confusion, whereas a classication scheme based on physical properties of the sources would suer no such drawback. To make matters worse, the radio galaxies at the centers of clusters are often referred to by a Fanaro-Reilly type when in fact neither an FR I nor FR II classication is appropriate. This short paper, then, is intended to clear up this confusion to the extent possible. The confusion will only go away, however, if the classications described herein are adopted by the com- munity at large and, of course, if nature kindly agrees to follow our theoretical pictures of these phenomena. We propose several new names for sources previously classi- ed as \radio relics" and suggest, with no small hint of hubris, that all who read this article start using them. We have attempted to classify the sources discussed herein by their current physical interpretations rather than by their phenomenological properties, as this promises to establish a rmer basis for our classications. While some of these physical interpretations may need to be modied or re-worked entirely as the quality of the data improves, we view this as a natural component of any scientic endeavor. It is even possible that some of the sources we identify here will ultimately require new, as yet unimagined physical interpretations. If some of the sources we mention need to be re-classied in future, so be it. This article lays the groundwork for a rational- ized classication scheme for cluster radio sources, and we expect (and hope!) that this scheme will be built upon in the future. 2. Classications The numerous types of large-scale radio sources in clus- ters of galaxies span approximately 5 orders of magni- tude in linear size and possess a wide range of radio properties. They can be broken down into two basic classes: those associated with AGN, and those associ- ated with the ICM. The rst of these two classes is the larger of the two, comprising all but three of the source types we will discuss here. This rst class can be bro- ken down further into sources associated with currently active AGN and those associated with extinct or dying AGN. Of the three types not associated with AGN, two appear to be associated with cluster mergers while one, the \mini-halo," has an even less certain origin and may not even be a distinct class of source. Here, then, are the 9 classes of extended cluster radio sources with their fun- damental observational properties and generally agreed upon (though occasionally uncertain) physical origins. We have arranged them roughly in order of increasing linear size. The basic properties of these sources are sum- marized in Table 1.
    06/2013;
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    ABSTRACT: We present a new Chandra X-ray observation of the intracluster medium in the galaxy cluster Abell 2443, hosting an ultra-steep spectrum radio source. The data reveal that the intracluster medium is highly disturbed. The thermal gas in the core is elongated along a northwest to southeast axis and there is a cool tail to the north. We also detect two X-ray surface brightness edges near the cluster core. The edges appear to be consistent with an inner cold front to the northeast of the core and an outer shock front to the southeast of the core. The southeastern edge is coincident with the location of the radio relic as expected for shock (re)acceleration or adiabatic compression of fossil relativistic electrons.
    The Astrophysical Journal 06/2013; 772(2). · 6.73 Impact Factor
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    ABSTRACT: Recent X-ray observations of galaxy clusters have shown that there is substructure present in the intracluster medium (ICM), even in clusters that are seemingly relaxed. This substructure is sometimes a result of sloshing of the ICM, which occurs in cool core clusters that have been disturbed by an off-axis merger with a sub-cluster or group. We present deep Chandra observations of the cool core cluster Abell 2029, which has a sloshing spiral extending radially outward from the center of the cluster to approximately 400 kpc at its fullest extent---the largest continuous spiral observed to date. We find a surface brightness excess, a temperature decrement, a density enhancement, an elemental abundance enhancement, and a smooth pressure profile in the area of the spiral. The sloshing gas seems to be interacting with the southern lobe of the central radio galaxy, causing it to bend and giving the radio source a wide-angle tail (WAT) morphology. This shows that WATs can be produced in clusters that are relatively relaxed on large scales. We explore the interaction between heating and cooling in the central region of the cluster. Energy injection from the active galactic nucleus (AGN) is likely insufficient to offset the cooling, and sloshing may be an important additional mechanism in preventing large amounts of gas from cooling to very low temperatures.
    The Astrophysical Journal 06/2013; 773(2). · 6.73 Impact Factor
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    ABSTRACT: We present VLA radio and Chandra X-ray observations of the merging galaxy cluster Abell 3411. For the cluster, we find an overall temperature of 6.4 keV and an X-ray luminosity of 2.8 x 10^{44} erg s^{-1} between 0.5 and 2.0 keV. The Chandra observation reveals the cluster to be undergoing a merger event. The VLA observations show the presence of large-scale diffuse emission in the central region of the cluster, which we classify as a 0.9 Mpc size radio halo. In addition, a complex region of diffuse, polarized emission is found in the southeastern outskirts of the cluster, along the projected merger axis of the system. We classify this region of diffuse emission as a radio relic. The total extent of this radio relic is 1.9 Mpc. For the combined emission in the cluster region, we find a radio spectral index of -1.0 \pm 0.1 between 74 MHz and 1.4 GHz. The morphology of the radio relic is peculiar, as the relic is broken up into five fragments. This suggests that the shock responsible for the relic has been broken up due to interaction with a large-scale galaxy filament connected to the cluster or other substructures in the ICM. Alternatively, the complex morphology reflects the presence of electrons in fossil radio bubbles that are re-accelerated by a shock.
    The Astrophysical Journal 05/2013; 769(2). · 6.73 Impact Factor
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    ABSTRACT: We report the results of a multi-wavelength study of the nearby galaxy group, Abell 3581 (z=0.0218). This system hosts the most luminous cool core of any nearby group and exhibits active radio mode feedback from the super-massive black hole in its brightest group galaxy, IC 4374. The brightest galaxy has suffered multiple active galactic nucleus outbursts, blowing bubbles into the surrounding hot gas, which have resulted in the uplift of cool ionised gas into the surrounding hot intragroup medium. High velocities, indicative of an outflow, are observed close to the nucleus and coincident with the radio jet. Thin dusty filaments accompany the uplifted, ionised gas. No extended star formation is observed, however, a young cluster is detected just north of the nucleus. The direction of rise of the bubbles has changed between outbursts. This directional change is likely due to sloshing motions of the intragroup medium. These sloshing motions also appear to be actively stripping the X-ray cool core, as indicated by a spiraling cold front of high metallicity, low temperature, low entropy gas.
    Monthly Notices of the Royal Astronomical Society 04/2013; 435(2). · 5.52 Impact Factor
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    ABSTRACT: We report the detection and observed characteristics of giant pulses from the Crab Nebula pulsar (B0531+21) in four frequency bands covering 20-84 MHz using the recently-completed Long Wavelength Array Station 1 (LWA1) radio telescope. In 10 hours of observations distributed over a 72-day period in Fall of 2012, 33 giant pulses having peak flux densities between 400 Jy and 2000 Jy were detected. Twenty-two of these pulses were detected simultaneously in channels of 16 MHz bandwidth centered at 44 MHz, 60 MHz, and 76 MHz, including one pulse which was also detected in a channel centered at 28 MHz. We quantify statistics of pulse amplitude and pulse shape characteristics, including pulse broadening. Amplitude statistics are consistent with expectations based on extrapolations from previous work at higher and lower frequencies. Pulse broadening is found to be relatively high, but not significantly greater than expected. We present procedures that have been found to be effective for observing giant pulses in this frequency range.
    The Astrophysical Journal 04/2013; 768(2). · 6.73 Impact Factor
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    ABSTRACT: We observed Sgr A* with the GMRT in late January and early February 2013 in search of enhanced meter-wavelength emission resulting from the interaction of the cloud G2 with the accretion disk of Sgr A* (Gillessen et al. 2012, Nature, 481, 51; Narayan et al. 2012, ApJ, 757, L20). Recent models suggest that the bow shock of G2 has already crossed pericenter and that peak radio synchrotron emission should occur in February or March 2013 (Sadowski et al.
    The Astronomer's Telegram. 03/2013;
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    ABSTRACT: Recent observations of galaxy clusters have shown that there is substructure present in the intracluster medium (ICM), even in clusters that are seemingly relaxed. This substructure is sometimes a result of sloshing of the ICM, which occurs in cool core clusters that have been disturbed by an off-axis merger with a sub-cluster or group. Evidence of sloshing can be seen using X-ray observations with Chandra. We present a deep Chandra observation of the cool core cluster Abell 2029, which has a sloshing spiral extending to approximately 400 kpc from the core—the largest one observed to date. We examine the surface brightness, temperature, density, and pressure of the feature. The sloshing gas that causes these features can also distort radio lobes associated with active galactic nuclei (AGN) located in cluster centers. These distorted AGN can act as signposts for new clusters at high redshift. We present Spitzer observations of candidate clusters found using this technique.
    01/2013;
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    ABSTRACT: We present an update on the Jansky Very Large Array Low Band (VLA-LB) project, currently undergoing scientific commissioning and expected to be fully available in 2013. VLA-LB is a joint NRL and NRAO initiative to equip the VLA with broadband low frequency receivers that cover the spectrum between 66 and 470 MHz. The current system can already access the 66 to 86 MHz and 230 to 436 MHz sub-bands by working with existing 74 and 330 MHz feeds, respectively. The bandwidth at 74 MHz will increase by more than an order of magnitude while the 330 MHz bandwidth increases by approximately a factor of 6. The improved bandwidth and system temperature, coupled with the power of the WIDAR correlator, promise significantly enhanced performance compared to past VLA capabilities. Early commissioning results at “P band” (330 MHz) with a handful of antennas accessing the larger bandwidth indicate sensitivity rivaling that of the legacy 27-antenna, narrow-band old VLA capability. New feeds that can exploit a larger fraction of the available receiver bandwidth are being explored. While VLA-LB is useful as a conventional system, we are looking to enhance its power by leveraging the VLA’s capability to detect radiation at its prime and Cassegrain foci simultaneously. The ability to observe with more than one band in parallel is a powerful multiplier of a telescope’s function, and many instruments (e.g. the GMRT, WSRT and VLA) offer this. A variant is being explored for VLA-LB: observing from the prime focus during all normal Cassegrain observations. This proposed VLA-LB commensal system would piggyback normal VLA observing time to survey at low frequencies with relatively large field of views. Shared fields with other multi-beaming, dipole-based arrays that view the same sky with the VLA, e.g. the first station of the Long Wavelength Array (LWA1), would be possible. The collected data will be assembled into a database of spectra and wide-field images, suitable for studies of individual objects as well as searches for transients and high redshift spectral features (eg. HI absorption or OH mega-masers). We describe how the VLA-LB commensal system might be implemented, and explore early ideas for its scientific promise.
    01/2013;
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    ABSTRACT: We describe an "active" antenna system for HF/VHF (long wavelength) radio astronomy that has been successfully deployed 256-fold as the first station (LWA1) of the planned Long Wavelength Array. The antenna system, consisting of crossed dipoles, an active balun/preamp, a support structure, and a ground screen has been shown to successfully operate over at least the band from 20 MHz (15 m wavelength) to 80 MHz (3.75 m wavelength) with a noise figure that is at least 6 dB better than the Galactic background emission noise temperature over that band. Thus, the goal to design and construct a compact, inexpensive, rugged, and easily assembled antenna system that can be deployed many-fold to form numerous large individual "stations" for the purpose of building a large, long wavelength synthesis array telescope for radio astronomical and ionospheric observations was met.
    Publications of the Astronomical Society of the Pacific 10/2012; · 3.69 Impact Factor
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    ABSTRACT: The Long Wavelength Array Software Library (LSL) is a Python module that provides a collection of utilities to analyze and export data collected at the first station of the Long Wavelength Array, LWA1. Due to the nature of the data format and large-N ($\gtrsim$100 inputs) challenges faced by the LWA, currently available software packages are not suited to process the data. Using tools provided by LSL, observers can read in the raw LWA1 data, synthesize a filter bank, and apply incoherent de-dispersion to the data. The extensible nature of LSL also makes it an ideal tool for building data analysis pipelines and applying the methods to other low frequency arrays.
    Journal of Astronomical Instrumentation. 09/2012; 1(1).
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    ABSTRACT: In a number of nearby clusters of galaxies, recent X-ray observations have revealed the detailed interaction of AGN radio jets and lobes with the intracluster medium. These AGN provide feedback to their host galaxies and surrounding clusters, and significantly affect their evolution. Sloshing of cluster cores, related to off-axis cluster or group mergers, may distort the AGN's radio lobes, sometimes resulting in a "bent" shape. I will discuss detailed, nearby AGN/cluster interactions as well as describe a high-redshift cluster survey being conducted in the optical and IR using distorted radio sources as signposts. This sample will yield hundreds of high-z clusters with central, active galaxies allowing us to study feedback in distant systems.
    09/2012;

Publication Stats

595 Citations
260.38 Total Impact Points

Institutions

  • 2012–2013
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2003–2013
    • University of Virginia
      • Department of Astronomy
      Charlottesville, Virginia, United States
  • 2011
    • SpecTIR™ Remote Sensing Division
      Reno, Nevada, United States
    • Boston University
      • Institute for Astrophysical Research
      Boston, Massachusetts, United States
  • 2001
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States