T. E. Clarke

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

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Publications (135)263.89 Total impact

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    ABSTRACT: Giant Radio Halos (RH) are diffuse, Mpc-sized, synchrotron radio sources observed in a fraction of merging galaxy clusters. The current scenario for the origin of RHs assumes that turbulence generated during cluster mergers re-accelerates pre-existing fossil and/or secondary electrons in the intra-cluster-medium (ICM) to the energies necessary to produce the observed radio emission. Moreover, more relaxed clusters could host diffuse "off state" halos produced by secondary electrons. In this Chapter we use Monte Carlo simulations, that combine turbulent-acceleration physics and the generation of secondaries in the ICM, to calculate the occurrence of RHs in the Universe, their spectral properties and connection with properties of the hosting clusters. Predictions for SKA1 surveys are presented at low (100-300 MHz) and mid (1-2 GHz) frequencies assuming the expected sensitivities and spatial resolutions of SKA1. SKA1 will step into an unexplored territory allowing us to study the formation and evolution of RHs in a totally new range of cluster masses and redshift, allowing firm tests of the current theoretical hypothesis. In particular, the combination of SKA1-LOW and SUR will allow the discovery of ~1000 ultrasteep- spectrum halos and to detect for the very first time "off state" RHs. We expect that at least ~2500 giant RHs will be discovered by SKA1-LOW surveys up to z~0.6. Remarkably these surveys will be sensitive to RHs in a cluster mass range (down to ~10^14 solar masses) and redshifts (up to ~1) that are unexplored by current observations. SKA1 surveys will be highly competitive with present and future SZ-surveys in the detection of high-redshift massive objects.
    12/2014;
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    ABSTRACT: We are conducting a large survey of distant clusters of galaxies using radio sources with bent jets and lobes as tracers. These radio sources are driven by AGN and achieve their bent morphologies through interaction with the surrounding gas found in clusters of galaxies. Based on low-redshift studies, these types of sources can be used to identify clusters very efficiently. We present initial results from our survey of 653 bent-double radio sources with optical hosts too faint to appear in the SDSS. The sample was observed in the infrared with Spitzer, and it has revealed $\sim$200 distant clusters or proto-clusters in the redshift range $z\sim0.7 - 3.0$. The sample of bent-doubles contains both quasars and radio galaxies enabling us to study both radiative and kinetic mode feedback in cluster and group environments at a wide range of redshifts.
    11/2014;
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    ABSTRACT: We present high resolution (9$^{\prime \prime}$) imaging of the Sunyaev-Zel'dovich Effect (SZE) toward two massive galaxy clusters, MACS J0647.7+7015 ($z=0.591$) and MACS J1206.2-0847 ($z=0.439$). We compare these 90 GHz measurements, taken with the MUSTANG receiver on the Green Bank Telescope, with generalized Navarro-Frenk-White (gNFW) models derived from Bolocam 140 GHz SZE data as well as maps of the thermal gas derived from {\it Chandra} X-ray observations. For MACS J0647.7+7015, we find a gNFW profile with core slope parameter $\gamma= 0.9$ fits the MUSTANG image with $\chi^{2}_{red}=1.005$ and probability to exceed (PTE) = 0.34. For MACS J1206.2-0847, we find $\gamma=0.7$, $\chi^{2}_{red}=0.993$, and PTE = 0.70. In addition, we find a significant ($>$3-$\sigma$) residual SZE feature in MACS J1206.2-0847 coincident with a group of galaxies identified in VLT data and filamentary structure found in a weak-lensing mass reconstruction. We suggest the detected sub-structure may be the SZE decrement from a low mass foreground group or an infalling group. GMRT measurements at 610 MHz reveal diffuse extended radio emission to the west, which we posit is either an AGN-driven radio lobe, a bubble expanding away from disturbed gas associated with the SZE signal, or a bubble detached and perhaps re-accelerated by sloshing within the cluster. Using the spectroscopic redshifts available, we find evidence for a foreground ($z=0.423$) or infalling group, coincident with the residual SZE feature.
    11/2014;
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    ABSTRACT: New observations of Jupiter's decametric radio emissions have been made with the Long Wavelength Array Station 1 (LWA1) which is capable of making high quality observations as low as 11 MHz. Full Stokes parameters were determined for bandwidths of 16 MHz. Here we present the first LWA1 results for the study of six Io-related events at temporal resolutions as fine as 0.25 ms. LWA1 data show excellent spectral detail in Jovian DAM such as simultaneous left hand circular (LHC) and right hand circular (RHC) polarized Io-related arcs and source envelopes, modulation lane features, S-bursts structures, narrow band N-events, and interactions between S-bursts and N-events. The sensitivity of the LWA1 combined with the low radio frequency interference environment allow us to trace the start of the LHC Io-C source region to much earlier CML III than typically found in the literature. We find the Io-C starts as early as CML III =230° at frequencies near 11 MHz. This early start of the Io-C emission may be valuable for refining models of the emission mechanism. We also detect modulation lane structures that appear continuous across LHC and RHC emissions, suggesting that both polarizations may originate from the same hemisphere of Jupiter. We present a study of rare S-bursts detected during an Io-D event and show drift rates are consistent with those from other Io-related sources. Finally, S-N burst events are seen in high spectral and temporal resolution and our data strongly support the co-spatial origins of these events.
    Journal of Geophysical Research: Space Physics 10/2014; · 3.44 Impact Factor
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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: The galactic continuum sources W51D and W51e1e2 have been long recognized as remarkable centers of ammonia maser phenomena in the centimeter wavelength range. Henkel et al. (2013 A&A 549, A90) have measured 19 masers, of which 13 are newly found for W51-IRS2, otherwise known as W51D. These arise from inversion-rotation transitions. The single dish data were taken with the Effelsberg 100-m radio telescope of the MPIfR with an angular resolution of 43 arc seconds. The conclusion that these lines were caused by maser action is based on: (1) time variability, and (2) narrow linewidths. In addition, some lines showed systematic velocity variations. High brightness temperatures and compact sizes are needed to conclusively prove maser action. We have measured a sub-set of these ammonia lines with the C array of the Jansky-Very Large Array of the National Radio Astronomy Observatory in June 2013 with an angular resolution of better than 1 arc second. Source sizes, positions, excitation models and reasons why W51 shows such a plethora of masers will be presented.
    01/2014;
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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;

Publication Stats

634 Citations
263.89 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
  • 2009
    • Virginia Polytechnic Institute and State University
      • Department of Electrical and Computer Engineering
      Blacksburg, VA, United States
  • 2008
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2001
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States