Science at Low Frequencies II
Albuquerque, NM - Wednesday December 2 - Friday December 4, 2015


Polarization leakage in epoch of reionization windows: accuracy of the prediction and correction
Khan Muhammad bin Asad (Kapteyn Astronomical Institute)

One of the final obstacles in detecting the 21-cm signal coming from the epoch of reionization (EoR) is the leakage of polarized emission (Stokes Q, U) into total intensity (Stokes I) which can possibly mimic the EoR signal. Polarization leakage is caused mainly by the polarized primary beams of the antennae of an array. Therefore, if we know the primary beams to a high accuracy, the amount of leakage from Stokes Q, U to I can be predicted and possibly corrected in the observed data. We have compared the leakages predicted by the LOFAR nominal model beam with the leakages found in the LOFAR observations of two EoR observing windows: 3C196 and 3C295. We have found that the observed leakages are less than 1.6% within a field of view of 8 degrees, and they differ from the predicted leakages by a factor of 1.5 to 2 on average. We have also calculated the accuracy of direction dependent (DD) calibration in correcting the leakages from Stokes I to Q, U, and we have found that DD calibration with an unpolarized sky model can remove 50% to 60% of the leakage on average. As a further test, we aim to quantify the improvement of DD calibration in the presence of a polarized sky model.

Tuning in the Low-Frequency Transient Radio Sky with AARTFAAC
Yvette Cendes (University of Amsterdam)

In this talk I will present the first science results from the Amsterdam-ASTRON Radio Transients Facility and Analysis Centre (AARTFAAC) Project, a commensal LOFAR 24 hour all-sky radio imaging facility designed to detect transient phenomena. The talk will focus both on our automatic system designed to observe transients and our science results from the first 24 hour survey. I will also discuss the transient rate limit set by AARTFAAC, as well as plans for future survey capabilities.

GMRT Update
Yashwant Gupta (National Centre for Radio Astrophysics)


The GLEAM survey - Images to Catalogs
Paul Hancock (Curtin University)

The first year observations for the GLEAM survey are complete and calibration and imaging has been completed. Ionospheric and observation effects mean that further flux and astrometry calibration is needed for individual snapshot images before they are formed into week-long mosaics. A final catalog is produced from these week-long mosaics and includes spectral and morphological information of all sources at 20 frequencies spanning 70-230MHz. In this talk I'll outline our post-imaging calibration process, and the new techniques that we have developed to create our final catalog.

VLA measurements of Faraday rotation through a coronal mass ejection
Jason Kooi (University of Iowa)

Coronal mass ejections (CMEs), large scale eruptions of plasma from the Sun, play an important role in space weather. Although CMEs have been an active field of research since their discovery, there is no consensus on the effective trigger that initiates a CME. Faraday rotation (FR) is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma. FR observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch and shed light on the initiation process. We made Very Large Array (VLA) full-polarization observations at 1 - 2 GHz in August, 2012, of a 'constellation' of radio sources through the solar corona at heliocentric distances that ranged from 6 - 15 solar radii. Of the nine sources observed, two were occulted by a CME. The radio source 0843+1547 has a strongly polarized central component and a weaker extended component, allowing for FR measurements along two closely spaced lines of sight. The data show a FR transient for 0843+1547; the Faraday rotation measure changes from ~ 0 before CME occultation to a value of about -12 rad/m^2 before declining after CME passage. The background coronal FR is -2 rad/m^2, suggesting that the CME enhanced the measured FR by a factor > 5. In this paper, we discuss these results and their implications in terms of models for the plasma structure of CMEs. This work was supported at the University of Iowa by grant ATM09-56901.

Spectrum protection for Low Frequency Radio Astronomy
Glen Langston (NSF)

The National Science Foundation seeks to protect the radio spectrum for science services, particularly for radio astronomy at all frequencies. We discuss the outlook for spectrum protection and solicit suggestions for strategies to maintain projection for radio astronomy.

The NSF also seeks input on possible citizen science projects that include active participation by students and groups operating their own telescope and data acquisition systems. These projects might include aspects of astronomy, scientific research methods, engineering, transient detection of astronomical and cosmic ray sources and spectrum monitoring.

Studying the diffuse galactic synchrotron emission around 1.4 GHz with GALFACTS
Indy Leclercq (University of Manchester)

The study of the diffuse, polarised synchrotron emission from the galaxy can provide useful information pertaining to two fields at the forefront of current astrophysics: CMB foreground analysis, and studies of the galactic magnetic field. The CMB approach relies on angular power spectrum (APS) analysis, a statistical technique which describes overall scale-dependent properties of the sky. Studies of the magnetic field using radio data rely on Faraday Rotation Measure synthesis (RM synthesis), which sheds light on the properties of the galactic ISM. I have combined these two techniques to analyse early science data from GALFACTS, a 1.4 GHz spectro-polarimetric survey of the sky visible from Arecibo. I will describe the GALFACTS survey, and detail the analysis techniques mentioned above. I will show results from the RM analysis and APS analysis of the data, focusing on the spatial variations of the slope of the power spectrum. I will also discuss how the results can help us constrain the behaviour of the synchrotron foreground at small scales, useful for the possible detection of B-modes in CMB experiments.

Low frequency transients with the MWA
Christene Lynch (CAASTRO/University of Sydney)

Historically, low-frequency radio variability studies have focused on quantifying the properties of discrete samples of bright extragalactic radio sources. The advent of SKA pathfinder telescopes open up the possibility of doing sensitive multi-epoch blind surveys for radio transients and variables for the first time. We are currently conducting the Murchison Widefield Array Transients Survey (MWATS) to do a blind search for slow transient events. The MWATS is a survey of the southern sky at 154 MHz, exploring variability timescales of minutes, hours, months and, in the future years. In this presentation I will discuss the detectability of a range of phenomena including exoplanets, stellar flares, AGN variability and atmospheric effects. Additionally, I will present the results from this survey and discuss the data and processing challenges that are relevant for current and future radio telescopes.

OzPipe: An Australian Pipeline for the MWA EOR Experiment
Bart Pindor (University of Melbourne)

The MWA EOR experiment is attempting to detect 21cm radiation emitted by neutral hydrogen present during the reionization of the universe. I will discuss the progress of the Australian EOR pipeline whose goal is to make a statistical detection of the cosmological signal.

DRAGNET: A Real-Time Pulsar and Fast Transient Backend for LOFAR
Sotiris Sanidas (University of Amsterdam)

The characterization of the relatively unexplored transient radio sky is one of the hottest topics in modern astronomy due to its possible link with the most energetic and exotic astrophysical phenomena in the Universe. The short duration of some such events and their non-repeating nature, however, demands large areas of the sky to be observed for long periods of time. Therefore, the current and future planned low-frequency arrays, with their vast sky coverage capabilities, are powerful observational tools for such investigations.

In this talk, we will present DRAGNET, a recently-deployed GPU-powered observing system for LOFAR, which aims to significantly expand LOFAR's observing capabilities, turning it into an all-sky synoptic radio camera that will detect such elusive radio signals. We will give a brief description of DRAGNET, how it is currently implemented in LOFAR, and how it is expected to significantly speed up the data processing of the ongoing LOTAAS pulsar survey. Finally, we will elaborate on the near-future planned modes of operation for DRAGNET, which will not only boost the pulsar related research with LOFAR, but also dramatically increase its sky coverage and dwell times for transients' searching.

Designing EoR experiments to tackle foregrounds in wide-field measurements
Nithyanandan Thyagarajan (Arizona State University)

Detection of 21 cm emission of HI from the epoch of reionization, at redshifts z>6, is limited primarily by foregrounds. We studied the signatures of wide-field measurements and an all-sky foreground model using the delay spectrum technique that maps the foreground locations through signal delays between antenna pairs. We recently confirmed with high significance that interferometric measurements are inherently sensitive to all scales, including the largest angular scales, due to the nature of wide-field measurements. They are characterized by enhanced power from foregrounds far away from the primary field of view in Fourier modes adjacent to those considered the most sensitive to the cosmological signal. We identify diffuse emission near the horizon as a significant factor, even on wide antenna spacings that usually denote structures on small scales. For signals entering via the primary field of view, compact emission dominates the foreground contamination. These two mechanisms imprint a characteristic "pitchfork" signature on the foreground wedge in Fourier space. These wide-field effects are generic to all observations but antenna shapes impact their amplitudes substantially. A dish antenna yields the most desirable features from a foreground contamination viewpoint, than a dipole or a phased array. Selective downweighting of data based on antenna spacing and LST can mitigate foreground contamination substantially by a factor of ~100 with negligible loss of sensitivity.


Searching Past the Confusion: Stokes V Imaging of the Transient Sky
Marin M Anderson (Caltech)

The Owens Valley Long Wavelength Array (OVRO-LWA), a new, low frequency, 288-element dipole array operating below 100 MHz, will continuously monitor the entire viewable sky at high-cadence to search for transient phenomena, particularly the low frequency radio bursts due to extrasolar space weather events such as stellar coronal mass ejections (CMEs) and planetary aurorae.

It is predicted that the pulsed radio emission from hot Jupiters close in to their parent stars could have flux densities as high as 100s of mJy below 100 MHz. Unlike previous deep low frequency searches for decametric radio bursts from extrasolar planets, we are able to monitor a large enough sample of hot Jupiters over an extended period of time by continually imaging the entire viewable sky. This allows us to overcome geometrical selection effects caused by narrow beaming of emission, as well as detect those rare events in which the radio emission brightens substantially due to the associated increase in the velocity of the host star's stellar wind following CME events.

I will describe the status of OVRO-LWA Stokes V imaging, which enables us to probe below the noise set by classical confusion in the bright, total intensity sky, and detect the polarized emission from CMEs and exo-planetary aurorae in the virtually empty circularly polarized sky. I will also describe our pilot transient survey monitoring thousands of stellar and low-mass objects out to 25 pc with the OVRO-LWA.

Murchison Widefield Array 21cm Year One Results
Adam Beardsley (ASU)

The Murchison Widefield Array (MWA) has collected hundreds of hours of EoR data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. I will present the first attempt at a deep integration from the MWA, incorporating new techniques such as diffuse foreground model subtraction, cable reflection calibration, and quality control methods. I will conclude with EoR power spectrum upper limits at redshifts 6.5 < z < 7.25, and offer strategies for continuing to integrate deeper.

All-sky Calibration and Imaging with the OVRO-LWA
Stephen Bourke (Caltech)

The Owens Valley Long Wavelength Array is a 288 element interferometer with baselines up to 1.5 km and a correlated bandwidth of 58 MHz. An overview of the calibration and imaging system will be given as well as other aspects of the data processing involved in the project.

The LOFAR EoR project: challenges and progress
Ger de Bruyn (ASTRON)

The LOFAR EoR project has recently started its fourth season of observations. The project concentrates on two fields which are centered on the NCP and the bright source 3C196. I will briefly highlight some of the issues that we are working on: the construction of high-resolution sky models, the modelling of station beams and the effects of the ionosphere. The comparison between the Stokes I and Stokes V images cubes and the resulting 2-D and 3-D power spectra indicate that we are still a factor of a few away from thermal noise limited performance. I will end with our current limits on the 21cm power spectra for the redshift range from z=7 to 10 based on 140 h of data on the NCP

Precision Measurements of Cosmic Rays with LOFAR
Stijn Buitink (Vrije Universteit Brussel)

The origin of cosmic rays remains one of the large mysteries in astrophysics. Radio measurements of air showers provide a new way of searching for the answers. In particular, with LOFAR, the mass composition of cosmic rays can be studied in the energy regime of 1017 eV to 1018 eV. This range is of particular interest as it may harbor the transition from a Galactic to an extragalactic origin.

LOFAR measures the radio emission from air showers with unprecedented precision. In the dense core individual air showers are detected by hundreds of dipole antennas. The complicated radio pattern on the ground is accurately reproduced by modern radio simulation codes and contains information about the longitudinal shower development. With a hybrid reconstruction technique, we measure the depth of the shower maximum with an accuracy of less than 20 g/cm2.

We will present the latest LOFAR results of cosmic-ray analysis and discuss how the technique can be used with the SKA in the future to achieve even higher accuracy.

The Dark Ages Radio Explorer (DARE) Space Mission Concept
Jack Burns (University of Colorado Boulder)

DARE will probe the epoch of formation of the first stars, black holes, and galaxies using the redshifted 21-cm transition from HI. These first objects to illuminate the Universe at 35>z>11 will be studied via their heating and ionization of the intergalactic medium (IGM). The global 21-cm spectrum is a powerful tool to investigate the effects on the IGM medium of UV ionizing radiation from the first generation of hot stars and heating via X-rays from black hole accretion disks. These properties are measured via the frequencies and amplitudes of distinct "Turning Points" that are expected in the spectrum. Over its lifetime of two years, DARE will observe at frequencies of 40-120 MHz in a 125 km altitude lunar orbit. The Moon occults both the Earth and the Sun as DARE makes observations on the lunar farside, shielding it from the corrupting effects of radio interference, Earth's ionosphere, and solar emissions. Bi-conical dipole antennas, pseudo-correlation receivers, and a digital spectrometer achieve the sensitivity required to observe the cosmic signal. The frequency structure of the 21-cm signal and its uniformity over large angular scales are unlike the spectrally featureless, spatially varying characteristics of the Galactic foreground, allowing the signal to be separated from the foreground using an MCMC statistical framework.

A MWA Study of Gigahertz-Peaked Spectrum and Compact Steep-Spectrum Sources
Joseph Callingham (Sydney Institute for Astronomy)

Gigahertz-peaked spectrum (GPS) and compact steep spectrum (CSS) sources sources have been hypothesised to represent an early stage of radio galaxy evolution. However, such an interpretation is contentious as it is possible that these sources are not young but are confined to a small spatial scales due to a high density medium. One of the reasons there has not been resolution between these two competing hypotheses is because the absorption mechanism responsible for the turnover in their radio spectra still ambiguous since the spectra of these sources below the turnover has not been well enough sampled to date.

The Murchison Widefield Array (MWA) has conducted an all-sky survey at low radio frequencies (72 to 231 MHz). This survey provides an unparalleled number of GPS and CSS sources with broad spectral coverage below the turnover. In this talk I will present results of spectral modelling of these sources and discuss the impact such a frequency domain has on our understanding of the absorption mechanism. I will demonstrate that the MWA all-sky survey has identified a large population of GPS sources that have ceased activity, and will show that a portion of the ultra-steep spectrum source population will be composed of GPS sources in a relic phase.

The Ooty Wide Field Array
Jayaram Chengalur (NCRA-TIFR)

The Ooty Radio Telescope (ORT) is being upgraded to work as a wide field array, the Ooty Wide Field Array (OWFA). One of the primary drivers behind this is to enable observations of HI power spectrum at a redshift of 3.3. The OWFA is well suited for such observations as it has a large total collecting area (~ 9000 square meters) and provides excellent sampling of the angular scales at which the HI signal is expected to be strongest. The telescope is also equitorially mounted, which allows one to make coherent measurements of the Fourier components as the sky is tracked. In this talk I will describe the changes that need to be made to the ORT to enable it to function as a wide field interferometer. The signal processing is based on a novel architecture, the Networked Signal Processing System, (NSPS), which I will briefly describe. I will also present predictions on what we expect the observations to yield and give a brief update on the current status of the upgrade.

VLA Low Band and the Commensal VLITE System: Current Status and Future Plans
Tracy Clarke (NRL)

The Jansky Very Large Array (VLA) has undergone an upgrade to a broadband system operating in the frequency range of 1-50 GHz. The Naval Research Laboratory (NRL) has collaborated with NRAO to develop a new broadband low frequency receiver operating at 56-86 MHz (4-band) and 240-470 MHz (P-band).

The low frequency feeds on the VLA are located at the prime focus while the 1-50 GHz feeds are at the Cassegrain focus. NRL has taken advantage of this difference in feed focus location to expand the VLA low frequency capabilities. We have developed, installed, and commissioned a new commensal system called the VLA Low Band Ionospheric and Transient Experiment (VLITE). Dedicated samplers and fibers tap the signal from 10 VLA P-band receivers and correlate those through a real-time DiFX correlator. The operation of VLITE requires no additional resources from the VLA system running the primary science. VLITE commensal operations run in parallel during nearly all Cassegrain programs.

The commensal system greatly expands the science capabilities of the VLA through value-added PI science, opening a new window on transient searches, and stand-alone astrophysics. In the first ten months of operation we have obtained more than 5300 hours in 9100 unique positions. I will discuss details of the low band system, VLITE, and present some of the astrophysical results and on-going programs that are engaging VLITE data.

Abell 1682: An ultra steep spectrum radio halo
Alex Clarke (Manchester University)

Abell 1682 is a merging galaxy cluster with strong radio synchrotron emission from several components. Seeing such emission reveals the presence of micro-gauss magnetic fields throughout the intra cluster medium, and also implies a method for accelerating electrons to the required relativistic speeds.

I will present LOFAR data from the LBA (30-90MHz) and HBA (110-170MHz), revealing the diffuse radio emission (radio halo) associated with A1682. In particular, these results indicate it has a very steep spectrum (spectral index of -2.5), which has implications on the methods required to accelerate the electrons. I will show how this favours the model that turbulence re-accelerates electrons over large scales to form radio halos, and how this helps understand the dynamics of clusters and their associated magnetic fields.

Transient Searches with the Murchison Widefield Array
Steve Croft (UC Berkeley)

The Murchison Widefield Array is undertaking a number of programs in search of low frequency radio transients. In addition to dedicated surveys of the Galactic Plane (including coordinated observations in the optical with the Intermediate Palomar Transient Factory) and of extragalactic fields, the MWA has been used to search for counterparts to transients seen by other instruments. This has included observations in rapid response to gamma ray burst triggers, as well as archival searches for radio counterparts to events seen by the ANTARES neutrino telescope. I will describe initial results from these surveys, and our plans for transient searches with the proposed expanded MWA. These include ongoing collaborations with the ANTARES and IceCube neutrino observatories, and the LIGO gravitational wave observatory.

Pulsars at Low Frequencies
Paul Demorest (NRAO)

Due to their steep spectra and information-rich time and frequency domain signals, pulsars are ideal targets for most low-frequency radio telescopes. I will present a (probably biased) overview of current and future pulsar science at sub-GHz radio frequencies. An important overall theme in this topic is the effect of the ionized interstellar medium on the propagation of the radio signal, and the unique effects that can only be clearly seen with time-variable point sources such as pulsars. Near 500 MHz, low frequency observations provide one end of the "lever" needed to precisely estimate and model time-variable dispersion measure effects in high-precicion millisecond pulsar timing. Moving down in frequency, the effects of multi-path ISM propagation become more important. This results in both challenges and opportunities for new discovery -- in particular, combining low frequency and VLBI techniques may provide unprecedented views of both the ISM and the pulsars themselves.

The Long Wavelength Array
Jayce Dowell (University of New Mexico)

The Long Wavelength Array will be a general purpose instrument operating between 10 and 88 MHz in the southwestern United States. Once completed the telescope will have arcsecond resolution and will explore pulsars, extrasolar planets, the structure of the ionosphere, and space weather. Currently there are two stations: LWA1 located near the VLA and the recently completed LWA-SV located on the Sevilleta National Wildlife Refuge about 70 km northeast of LWA1. I will present a summary of the status of both stations and highlight some of the science that is currently being done. I will also present the early testing results and a description of the eLWA concept, which combines the two LWA stations with the new VLA low band system.

Construction of the LWA has been supported by the Office of Naval Research under Contract N00014-07-C-0147 and by the Air Force Office of Scientific Research DURIP program. Support for operations and continuing development of the LWA1 is provided by the National Science Foundation under grants AST-1139963 and AST-1139974 of the University Radio Observatory program.

Detecting Fluctuations of the Cosmic Dawn with the Owens Valley Long Wavelength Array
Michael Eastwood (Caltech)

The Owens Valley Long Wavelength Array (OVRO LWA) is a 288-antenna interferometer covering 30 to 80 MHz located at the Owens Valley Radio Observatory (OVRO) near Big Pine, California. I am leading the effort to detect spatial fluctuations of the 21 cm transition from the cosmic dawn (z~20) with the OVRO LWA. These spatial fluctuations are primarily sourced by inhomogeneous X-ray heating from early star formation. The spectral hardness of early X-ray sources, stellar feedback mechanisms, and baryon streaming therefore all play a role in shaping the power spectrum.

I will present the application of m-mode analysis (Shaw et al. 2014, Shaw et al. 2015) to OVRO LWA data to 1. compress the data set, 2. create maps of the northern sky that can be fed back into the calibration pipeline, and 3. filter foreground emission. Finally I will present the current status and future prospects of the OVRO LWA for detecting the 21 cm power spectrum at z~20. This will include our current upper limits in a non-science band at 45 MHz.

Faraday Tomography with LOFAR
Cameron Van Eck (Radboud University)

With LOFAR, we now have the ability to perform Faraday tomography with unprecedented sensitivity and Faraday depth resolution. This will allow us to detect and characterize previously unseen structures in Faraday depth. Since diffuse synchrotron emission should be present in every field observed with LOFAR, we now have an abundance of data suitable for Faraday tomography.

In my talk, I will report on my work producing rotation measure cubes from fields observed as part of LOFAR's Nearby Galaxies and Tier 1 Surveys. I will show some of the RM cubes I have produced and discuss the diffuse structures visible in them.

Carbon radio recombination lines in extragalactic sources with LOFAR
Kimberly Emig (Leiden Observatory)

Cold, yet still diffuse gas in the interstellar medium marks the transition into and out of the molecular gas phase. Although this diffuse gas is a key component in the lifecycle of gas in galaxies, and in the regions surrounding AGN, it has yet remained elusive due to our inability to constrain its fundamental characteristics such as temperature and density. Carbon radio recombination lines (CRRLs) can provide a new probe of this gas. In particular at low radio frequencies an enhancement of CRRLs occur due to the dielectronic capture of electrons and the strong radiation fields. With the advent of sensitive low-frequency telescopes with high spatial and spectral resolution, such as the Low Frequency Array (LOFAR), CRRLs can be traced in extragalactic objects for the first time. With the first detections now made, a campaign to search for CRRLs from extragalactic objects is underway. In this talk, I will highlight the first results of the survey and the steps we are taking to map out CRRLs in these sources.

Galaxy clusters at the lowest frequencies
Francesco de Gasperin (Leiden University)

I will introduce the LOFAR LBA (low band antenna) system which is capable of observing at 30-70 MHz. Observing at these frequencies is extremely challenging and several new techniques are being developed to deal with ionospheric induced systematic errors. Some preliminary LOFAR results on galaxy clusters will be shown. In the second part of my talk I will present studies merging galaxy clusters made with GMRT and VLA, focusing on the phenomena of radio relics and radio phoenixes and on their interplay.

A Bayesian analysis of redshifted 21-cm HI signal and foregrounds: Simulations for LOFAR
Abhik Ghosh (Kapteyn Astronomical Institute)

Observations of the EoR with the 21-cm hyperfine emission of neutral hydrogen (HI) promise to open an entirely new window onto the formation of the first stars, galaxies and accreting black holes. In order to characterize the weak 21-cm signal, we need to develop imaging techniques which can reconstruct the extended emission very precisely. Here, we present an inversion technique for LOFAR baselines at NCP, based on a Bayesian formalism with optimal spatial regularization, which is used to reconstruct the diffuse foreground map directly from the simulated visibility data. We notice the spatial regularization de-noises the images to a large extent, allowing one to recover the 21-cm power-spectrum over a considerable k-k|| space in the range of 0.03Mpc-1<k<0.19Mpc-1 and 0.14 Mpc-1<k||< 0.35 Mpc-1 without subtracting the noise power-spectrum. We find that, in combination with using the GMCA, a non-parametric foreground removal technique, we can mostly recover the spherically average power-spectrum within 2σ statistical fluctuations for an input Gaussian random rms noise level of 60mK in the maps after 600 hrs of integration over a 10MHz bandwidth.

Diffuse non-thermal emission in high and low thermal gas density environments
Gabriele Giovannini (Bologna University)

The number of known diffuse radio sources in clusters (halos and relics) has grown in recent years owing to available survey data. The study of these sources is important to derive information on the non-thermal components in the Intracluster Medium (particles and large scale magnetic fields) and therefore to understand the physical processes at work in clusters and the formation of large scale structures in the Universe. I will use data published up to now to discuss the implications of the strong correlation present between the halo and relic radio power and the X-ray Luminosity, putting in evidence a few clusters with a clear discrepancy, whose understanding is still problematic. Moreover, I will present new observational evidences on the presence of some non-thermal diffuse radio sources in regions of very low thermal gas density, i.e. low or missing X-ray emission, and I will discuss the importance and necessary characteristics of the new generation of radio surveys to properly investigate this class of sources. The presence of radio emission in low thermal gas density environments allows to expand our knowledge of magnetic fields from clusters to filaments. This will be a fundamental step to understand the origin and properties of cosmological magnetic fields.

Dark Age Telescope (DAT)
Lincoln Greenhill (Harvard / Smithsonian)

Prior to widespread star and galaxy formation during the Epoch of Reionization (EOR; z~6-15), the λ21 cm transition of neutral Hydrogen (HI) is the only available ubiquitous tracer of the early Universe. HI brightness fluctuations from Cosmic Dawn (z~15-30) encode information about (i) the first stars and black holes, tracing the inhomogeneous distribution of ultraviolet and X-ray radiation emitted by them, (ii) the initial conditions for reionization, and (ii) the end-state conditions for the Dark Age, from which inferences about the nature of dark matter may be inferred via extrapolation. The proposed Dark Age Telescope (DAT) is an extensible interferometric aperture designed specifically to target (initially) z~15-22. It comprises 127 dual polarization antennas, placed on a redundant 150m-diameter coplanar hexagonal grid (~2000 sq-m), and a proximate beamformed LWA station that is cross-correlated with elements in the DAT hexagon to obtain calibration cross -checks. All power-spectral information is derived along the frequency axis, and there is no differential loss at long spacings as occurs due to the delay horizon. The DAT concept leverages experience developed with the Large Aperture Experiment to Detect the Dark Age (LEDA) and Long Wavelength Array (LWA), including large-N array construction, RF electronics, algorithms, and High Performance Computing (HPC) applied to stream signal processing.

GLEAM science from the first year of observations
Paul Hancock (Curtin University)

The Galactic and Extragalactic All-sky MWA Survey (GLEAM) has completed it's first year of observations and data reduction, and will shortly be releasing the GLEAM point source catalog. In this talk I will describe the science that is being done with the first year GLEAM data, including both published results, and work that is in progress.

The OVRO-LWA: Status and Future Plans
Gregg Hallinan (Caltech)

The Owens Valley Long Wavelength Array (OVRO-LWA) is conceived to be a 352-antenna array, spanning a 2.5 km diameter area at the Owens Valley Radio Observatory (OVRO) in California. It is a powerful instrument designed to target science that is uniquely accessible through all-sky imaging at frequencies < 100 MHz, as well as serving as a survey instrument that can stimulate science with the eventual multi-station high spatial resolution LWA. To date, the 200 m diameter 256-antenna core of the OVRO-LWA has been completed; the LEDA correlator has been installed and the necessary pipelines have been put in place to carry out all-sky imaging at ~1.5 degree resolution. Most recently, 32-outlier antennas have been constructed, making use of a new low-cost optical fiber link that has been developed to signal transport over many km, allowing much higher resolution imaging. Future plans will extend the number of outliers to 96 antennas delivering the 2.5 km baselines, as well as uv-coverage, required to image the radio sky with the fidelity and resolution necessary to realize the project's key science goals. I will summarize the science motivation for the OVRO-LWA and discuss the current status and outstanding challenges faced by the project. I will describe the first surveys that have carried out with the telescope to deliver early science and will outline future plans for the project.

LOFAR Imaging Surveys: MSSS and beyond
George Heald (ASTRON)

The Low Frequency Array (LOFAR) is a powerful radio telescope operating between 10-250 MHz. One of LOFAR's unique strengths is the capability to perform deep, high resolution imaging surveys in a little-explored region of the radio spectrum. In this talk I will outline a comprehensive survey effort that is already underway. I will begin by focusing on a shallow commissioning survey, the Multifrequency Snapshot Sky Survey (MSSS), that has already generated a broadband source catalog of over 100,000 sources in the northern sky. After reviewing the survey's image and catalog data products, I will highlight the first discoveries and science results, and describe plans for followup efforts at higher angular resolution and in full polarization. I will then highlight the capability for substantially deeper LOFAR survey products and present early results of that ongoing effort.

Ionospheric and Plasmaspheric Science with Low-frequency Radio Interferometers
Joseph Helmboldt (NRL)

For decades, the biggest limiting factor to the angular resolution and image fidelity achievable with low-frequency (<500 MHz) interferometers was the ionosphere. Thanks to calibration techniques developed in the 1980s and 90s, the low-frequency sky was opened up to a new class of high-angular resolution low frequency telescopes. With new telescopes striving to push to even better resolution and/or wider fields of view, the development of ionospheric calibration methods continues to be a very active area of research. However, while the sensitivity of these telescopes to fine-scale fluctuations in electron density is a great nuisance to many, it can also be harnessed for cutting edge ionospheric, and even plasmaspheric, science. When observing relatively bright low-frequency cosmic sources, such interferometers can be used to measure the horizontal gradient in the total electron content (TEC) with virtually unmatched precision, as good as 0.0002 TECU/km. This talk will describe how low-frequency interferometic observations can be used to sense fluctuations in the TEC gradient and how such telescopes have been and are being used for space science. This will include discussions of the types of ionospheric and plasmaspheric disturbances for which low-frequency interferometers are uniquely suited. The application of synthesis imaging techniques to more traditional, radar-based HF/VHF ionospheric remote sensing methods and the unique science it enables will also be considered.

LWA1 Observations of Jupiter
Chuck Higgins (Middle Tennessee State University)

From 2012 ~ 2015 the Long Wavelength Array station 1 (LWA1) was used to observe Jupiter from 10~40 MHz at high spectral and temporal resolution, 5 kHz and 0.21 ms, respectively. Jupiter has four strong sources in the magnetosphere that are influenced by the satellite Io. According to current models, sources Io-A and Io-B emit from the northern hemisphere with right-hand (RH) circularly polarized signatures, and Io-C and Io-D emit from the southern hemisphere with left-hand (LH) polarization. Previous data have shown new details of known features in Jupiter's spectrum, such as millisecond S-bursts, narrowband N-events, and modulation lanes (Clarke et al., 2014). Work by K. Imai shows both LH and RH polarized emissions with modulation lane structures that will allow for a new approach to understand the emission beam structure and source locations. M. Imai has made correlations of Jupiter emissions using the LWA1, the Nancay Decameter Array (NDA) in France, and the URAN2 telescope in Ukraine. He is using these observations to measure the Jupiter radio beaming thickness and to constrain beaming models. C. Higgins has found that the Io-D radio source has some previously unknown morphology: a double-envelope and narrow-banded structures, and a higher peak frequency and a larger Io influence than previously known. Studies may lead to a better understanding of the Jupiter magnetic field in the southern hemisphere. These and other LWA1 observations of Jupiter will be highlighted.

An External Calibrator for Hydrogen Observatories
Daniel Jacobs (ASU)

A new generation of radio arrays is being developed that use large numbers of low-cost elements, such as phased tiles of dipole antennas, to map Hydrogen at very high redshift. These maps are three dimensional tracers of matter and energy when the universe was only 400,000 years old and covers scales much larger than previously possible. Calibration of the primary beams of phased dipole arrays has been found to be crucial to analysis of observations from the the Murchison Widefield Array (MWA), the Precision Array for Probing the Epoch of Reionization (PAPER), and the upcoming Hydrogen Epoch of Reionization Array (HERA). The goal of the Experimental Calibrator for Hydrogen Observatories (ECHO) is to map the primary beam response using a drone-mounted transmitter which is flown along a pre-programmed flight path designed to cover the entire sky. As a test of this method, beams of calibrated reference dipoles were mapped and compared with measurements by an altern ate method. Here we present the results of these tests.

Filamentary structures in LOFAR observations of the interstellar medium
Vibor Jelic (University of Groningen/ASTRON)

The low radio frequency observations are very sensitive to small column densities of the interstellar medium that are difficult to detect at higher radio frequencies. The wide frequency coverage and good angular resolution of the Low Frequency Array (LOFAR), make LOFAR an excellent instrument for studying Galactic polarized emission. In combination with the rotation measure (RM) synthesis, LOFAR observations allow us to study the relative distribution of synchrotron-emitting and Faraday-rotating regions at an exquisite resolution of 1 rad/m2 in Faraday depth. Recent observations with LOFAR revealed diffuse polarization in several fields at high Galactic latitudes with interesting morphological features: i) a very long and straight filament, showing excess in RM comparing to the background emission and (ii) an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. During my talk I will present these observat ions, discuss general properties of detected structures and of underlying magnetic fields and show puzzling correlation with the Planck dust polarization data.

Polarized foregrounds for EoR measurements: lessons from PAPER-64
Saul Aryeh Kohn (University of Pennsylvania)

Faraday-rotated polarized emission is capable of contaminating the power spectrum of unpolarized EoR emission. As present-generation EoR experiments near the sensitivity required for EoR detection, constraining the power spectrum of polarized emission inside and outside of the EoR window becomes pertinent. I present a summary of the current state of the reduction of PAPER 64-element array data in Stokes I, Q, U and V, including known systematic effects, and the implications for mitigating polarized leakage into EoR measurements by PAPER and its succcessor array, HERA.

LOFAR Census of Millisecond Pulsars
Vladislav Kondratiev (ASTRON)

We report the detection of 48 millisecond pulsars (MSPs) out of 75 observed thus far using the LOw-Frequency ARray (LOFAR) in the frequency range 110-188 MHz. We have also detected three MSPs out of nine observed in the frequency range 38-77 MHz. This is the largest sample of MSPs ever observed at these low frequencies, and half of the detected MSPs were observed for the first time at frequencies below 200 MHz. We present the average pulse profiles of the detected MSPs and flux densities, and compare these with published data from higher observing frequencies to examine their spectral behavior. We discuss MSP detectability, their spectra, profile evolution and dispersion measure variations.

The effects of positional precision in foreground subtraction
Jack Line (University of Melbourne)

Any statistical detection of the 21cm EoR signal involves overcoming the bright foregrounds created by astrophysical sources. The current MWA Australian EoR pipeline approach is to remove the brightest sources. The level of precision when removing point sources is still to be determined. Using catalogue data and new software (PUMA), I have created a positionally precise sky model. I will discuss the effects on the 2D power spectrum when using a precise and imprecise foreground model, and the possibilities of testing these models using the SKA visibility simulator, OSKAR.

Targeted pulsar searches with LOFAR
Joeri van Leeuwen (ASTRON / U. Amsterdam)

Many seminal pulsar discoveries were based on targeted deep searches toward objects suspected of having an evolutionary relation with neutron stars (e.g., the Crab Pulsar in the Crab Nebula). For several newly discovered types of neutron stars (magnetars, X-ray dim isolated neutron stars), and for pulsars in high stellar-density environments, the picture of their formation remains incomplete. LOFAR is extremely well suited for deep searches for all these sources, owing to its immense collecting area, and fractional bandwidth orders of magnitude larger than used before at these frequencies. I will present our current LOFAR findings and limits in these targeted searches.

Improved Astrophysical and Cosmological Constraints from Combined 21cm-CMB Analyses
Adrian Liu (University of California Berkeley)

Crucial in an interpretation of the soon-to-be detected 21cm signal from reionization is cosmological context. Other probes such as the cosmic microwave background (CMB) provide priors on cosmological parameters, though these still come with uncertainties. In this talk, I will show that such uncertainties have a non-negligible impact on the interpretation of the 21cm signal, even if one is only interested in the astrophysics of reionization and not in cosmology. I will further show how a detailed understanding of reionization from 21cm cosmology allows nuisance parameters to be removed from CMB studies. This reduces error bars on CMB-derived cosmological parameters. For example, constraints on the amplitude of primordial density fluctuations are reduced by a factor of 4. We also show that 21cm cosmology provides an "insurance policy" for future CMB experiments, enabling measurements of the sum of the neutrino masses to within 0.012eV even if CMB experiments cannot perform to specifications on the largest angular scales.

RAPID: A portable and reconfigurable imaging interferometer array
Colin Lonsdale (MIT Haystack)

The Radio Array of Portable Interferometric Detectors (RAPID) is an advanced radio array specifically designed for multi-role applications. The system implements a spatially diverse sparse array technology and can be deployed and reconfigured easily. Data are captured at the raw voltage level using the system in the field and processed post-experiment. Signal processing for the system is software defined and uses a scalable Cloud computing architecture. The system builds upon the Square Kilometer Array Low Frequency Aperture antenna (SKALA) in combination with custom hardware for data acquisition on a per antenna basis. The instrument uses physically disconnected elements, a high performance direct digitization receiver, swappable solid state storage, solar and battery power, and wireless control and interconnection. Schedule based operation can also be used in radio quiet locations or to enable minimally attended operation. RAPID is intended for application as both an astronomical radio telescope and geospace imaging radar system. The high degree of mobility afforded by the system enables a wide variety of interferometric configurations and allows deployment of the instrument at locations which are optimal for specific scientific goals. We will provide details of the RAPID design, describe prototype hardware, discuss early test results, and explore the science opportunities enabled by the instrument.

Chasing Low Frequency Radio Bursts from Magnetically Active Stars
Christene Lynch (CAASTRO/University of Sydney)

Flaring activity is a common characteristic of magnetically active stellar systems. These events produce emission throughout the electromagnetic spectrum, implying a range of physical processes. A growing number of objects are observed to exhibit short-duration, narrow band, and highly circularly polarised (reaching 100%) radio bursts. The observed polarisation and frequency-time structure of these bursts points to a coherent emission mechanism such as the electron cyclotron maser. Due to the stochastic nature of these bursts and the sensitivity of current instruments, the number of stars where coherent emission has been detected is few, with numbers limited to a few tens of objects. Observations of a wider sample of active stellar systems are necessary in order to establish the percentage that exhibit coherent radio bursts and to relate occurrence of these bursts to basic physical parameters of these stellar objects. New wide-field, low frequency radio telescopes will probe a frequency regime that is mostly unexplored for many magnetically active star systems and where coherent radio emissions are expected to be more numerous. I will present results of a recent search for coherent radio flares from nearby ( <60 pc ) known magnetically active stellar systems at 154 MHz using observations from the first year of the Murchison Widefield Array Transients Survey. I will also discuss future prospects of detecting these stellar systems at low frequencies and techniques.

First Batch of Pulsar Discoveries from the LOFAR High-Time-Resolution LOTAAS Survey
Daniele Michilli (University of Amsterdam)

LOTAAS (LOFAR Tied-Array All-sky Survey) is an ongoing survey for pulsars and fast radio transients. LOTAAS takes advantage of the unique characteristics of the LOFAR telescope to investigate a still unexplored parameter space in time domain radio astronomy. LOTAAS uses 222 simultaneous fields-of-view to cover 10 square degrees of sky per 1-hr hour pointing. With only a small fraction of the total survey processed, 26 pulsars have been discovered thus far - several of which are among the nearest and lowest-luminosity pulsars known. Among these pulsars, 2 were discovered based on their individual, bright pulses. While the total number of regular, "steady" pulsars expected to be detected with the survey is about 200, the number of intermittently active pulsars is unknown, and may teach us about the total Galactic population of radio-emitting neutron stars. Also, the wide field-of-view and long dwell time of the survey give the possibility to discovery more exot ic sources, in particular the mysterious "fast radio bursts", which could have revolutionary applications in astrophysics.

GPS Ionospheric calibration for the LWA
Ryan Monroe (Caltech)

Earth-based low frequency radioastronomy measurments are corrupted by the ionosphere, a layer of plasma ~300 km above Earth's surface. This layer refracts, scintillates, absorbs and emits electromagnetic radiation at levels which are disruptive for astronomy needs. Fortunately, the Long Wavelength Array (LWA) is uniquely positioned within an array of 1355 Global Positioning System (GPS) receivers, which are used for siesmology purposes. Since GPS is designed with the ability to make line-of-sight measurments of the ionosphere to correct for position errors, this GPS array provides a unique oppurtunity to apply this freely-available data as a calibration tool for the LWA. There are systematics of an unknown source in this data, which we are beginning to successfully fit and remove on a satellite pass-by-pass basis, resulting in a 20-fold reduction in data noise. Once data correction is completed, we will fit the data to a 3-d + time model of the ionosphere. We will then raytrace through this to construct correction products (similar to a "phase screen" used in adaptive optics") which allow us to deconvolve the ionosphere from LWA data products. This talk will discuss challenges in using the GPS data, the methods by which we intend to use it in correcting our data products, as well as alternative ionospheric calibration techniques possible using a high-spatial-resolution low frequency array such as the LWA.

Characterizing Cosmic Dawn with the Low-Band EDGES Instrument
Raul Monsalve (Arizona State University)

Until 2015 EDGES has been measuring the global sky signal between 100 and 200 MHz attempting to detect the spectral feature due to the redshifted 21-cm radiation from the Epoch of Reionization. Starting in late 2015, a second instrument is being deployed at the Murchison Radio-astronomy Observatory in Western Australia to measure the global signal in the 50-100 MHz range with the objective of detecting the predicted absorption spectral feature produced by UV radiation from the first galaxies and X-ray radiation from the first black holes in the redshift range between z ~ 30 and 14 (known as the Cosmic Dawn period). Difficulties of this measurement include strong ionospheric effects, radio-frequency interference, and foregrounds with intensity about 105 higher than the signal expected from Cosmic Dawn. The talk will describe the instrumental highlights and characterization of EDGES low-band. It will also present preliminary data and discuss the current constraints on the cosmological signal.

Measurements of Interplanetary Scintillation with the Murchison Widefield Array
John Morgan (ICRAR / Curtin)

Interplanetary Scintillation (IPS) can be used as a probe both of the solar wind, and of the structure of radio sources on arcsecond scales. Previous studies have generally relied on single concentrated collecting areas (phased arrays and dishes). The Murchison Widefield Array (MWA) by contrast is a 128-element interferometer with an extremely wide field of view, as well as a very wide bandwidth which can be flexibly deployed. These defining characteristics make it an outstanding instrument for IPS studies. Not only can it detect and localise many hundreds of sources simultaneously with sufficient signal to noise for IPS studies, but the exceptional instantaneous imaging capability of the MWA means that the variability of each source can reliably be resolved, and the abundance of non-scintillating sources can be used to simultaneously characterise instrumental and ionospheric effects. I will report on a test IPS observation where a full field has been imaged at a 0.5s cadence at 80MHz and 160MHz. With just 4 minutes of observing time the sky density of sources detected and characterised as scintillating or non-scintillating exceeds existing catalogues of IPS sources by a factor of two or more. I will also discuss plans for more extensive IPS surveys with the MWA (including higher time resolution), as well as possible science results on both the astrophysics of compact sources and the heliosphere.

HF and VHF Radio Emission from Meteor Trails
Kenneth Obenberger (AFRL)

Using the LWA1 telescope, we recently discovered that trails left by some bright, high velocity meteor emit a long duration (10 s of seconds) radio afterglow in the HF and VHF frequency bands. This emission occurs near the plasma frequency of the meteor trails and is thought to be the signature of radiated electron plasma waves from the turbulent trail. However, the electron/neutral collision frequency is high enough that the plasma waves would be aggressively damped, requiring a continuous source of energy lasting up to minutes after ablation of the meteor. Persistent optical trains occasionally follow bright, high velocity meteors and can last up to several hours. Chemical reactions are thought to be the energy source of these poorly understood trains. I will discuss a possible connection between optical trains and radio afterglows, and present recent results of coincident optical/radio observations.

Low Frequency Recombination Lines as a Probe of the Cold Neutral Medium
J.B.R. Oonk (Leiden University)

The interstellar medium (ISM) is the repository of stellar ejecta and the birthsite of new stars and, hence, a key factor in the evolution of galaxies over cosmic time. Cold, diffuse, atomic clouds are a key component of the ISM, but so far this phase has been difficult to study, because its main tracer, the HI 21 cm line, does not constrain the basic physical information of the gas (e.g., temperature, density) well. New low frequency telescopes open up the opportunity to study this component of the ISM through a complementary tracer in the form of low-frequency carbon radio recombination lines (CRRL). These CRRLs provide a sensitive probe of the physical conditions in cold, diffuse clouds.

In this talk I will focus on three aspects. First of all I will focus on a new suite of low frequency CRRL optical depth models that we have completed this year. These new models differ significantly from previous investigations done in the 1980s. Our new models include the latest rates and cross-sections, a full treatment of the carbon atom, as well as amplification and radiative transfer effects. Secondly, I will show our new LOFAR measurements of CRRLs towards the archetypical CRRL source, Cassiopeia A, and how these in combination with our new models lead to a change in our understanding of the CRRLs at the lowest frequencies. Finally, I will provide an update on our ongoing LOFAR CRRL survey of the Galactic plane and look forward to observing CRRLs with the Square Kilometer Array.

LOFAR Status and Updates
Emanuela Orru (ASTRON)

The LOw Frequency ARray is a phased-array radio interferometer constructed in the north of the Netherlands and across Europe. The telescope covers a frequency range from 10 up to 240 MHz, operating with a maximum bandwidth of about 90 MHz at the time. Datastreams from the stations are sent via high-speed fiber network to COBALT, the software correlator located in Groningen (in the north of the Netherlands). LOFAR can be used in several observing modes, such as: classical interferometer, tied array, transient buffer board and cosmic ray detector. Combinations of these operations can also be run in parallel. From the correlator the data are sent to the central processing cluster (CEP) for further processing. Processed data are then stored in the LOFAR long-term archive (LTA) for cataloging and dissemination to the community. In this talk a summary status of the LOFAR array and of the related infrastructure will be presented. The algorithms developed to process LOFAR data will be briefly described. Finally a panoramic overview will be given to introduce the scientific goals and achievements of such a versatile telescope as LOFAR.

The VLA at Low Frequencies
Frazer Owen (NRAO)


The Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) and Hydrogen Epoch of Reionization (HERA): A status update
Nipanjana Patra (UC Berkeley)

With the science case for redshifted 21 cm cosmology well established, the technical path toward detecting the redshifted 21cm line emission from the high redshifts is being explored by multiple instruments. While experiments such as EDGES, SARAS, BIGHORNS, DARE and SCIHI attempting to measure the sky averaged redshifted 21cm monopole signal, instruments like LOFAR, MWA, GMRT, PAPER and HERA are aiming at measuring the redshifted 21cm power spectrum. The Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) is a low frequency radio interferometer with 128 elements, designed with the aim of detection of neutral hydrogen power spectrum at the redshift range z= 7-12. While, PAPER-64, deployed in the Karoo desert in South Africa, has set the upper limit on the 21cm power spectrum at the redshift of 8.4, PAPER-128 is currently operational with increased total collecting area. The Hydrogen Epoch of Reionization Array (HERA) which is the successor of PAPER-128 is currently under construction of its phase1 with larger collecting area per element. In this talk, I will present the design and science goals of both the instruments including the results of PAPER-64 and strategic design improvements of HERA over PAPER.

An Accurate, All-Sky, Absolute, Low Frequency Flux Density Scale
Rick Perley (NRAO)

The flux density scale below 2 GHz has never been accurately established. Proposed scales based on measurements in the 1960s and 1970s differ by 5 to 10%, or more. The scale proposed by Scaife and Heald (2012) is a reconciliation of older scales, but does not include new observations, nor does it include southern sources. The major cause of these discrepancies is in the limited resolution and linearity of the telescopes utilized. The Jansky VLA is a high resolution, highly linear array, capable of measuring exquisitely accurate flux density ratios between sources. Combined with an accepted absolute standard, the Jansky VLA can measure spectral flux densities in the 50 -- 2000 MHz range accurately to ~1%.

I report on work from two parallel programs, undertaken with the Jansky VLA: (a) Establishment of the absolute flux densities of 18 sources (including 7 southern sources) from 75 MHz to 48 GHz, using Cygnus A as the reference (with Bryan Butler) (b) Extension of this scale to ~30 compact southern sources proposed as calibrators (with Joe Callingham).

Cosmic ray tomography with LOFAR
Irene Polderman (Radboud University Nijmegen)

In the low frequency radio regime the Milky way emission is dominated by synchrotron radiation from low energy cosmic rays. By mapping this emission we will gain further knowledge of these cosmic rays and may better constrain existing theories. Most of the studies till now have integrated the synchrotron emission over the entire line of sight. Other works have made use of the HII regions in the galaxy. These HII regions are opaque in the low radio frequency regime, and can only be seen against the background of the galactic synchrotron emission. Effectively these HII regions divide the line of sight into multiple parts hereby gaining dimensional information. We plan to continue that work and in future endeavours expand the lines of sight extensively with help of the newly finished LOFAR telescope. More lines of sight translate to a larger covered area of the Milky Way from which we can extract the synchrotron emission. Known distances for different HII regions convert synchrotron emission into synchrotron emissivity. Combining all lines of sight gives us a map with the synchrotron emissivity in a large part of the Milky Way. From this map we can draw conclusions about the cosmic ray density by comparing it to different cosmic ray models.

The Large-Aperture Experiment to Detect the Dark Age: current status and future outlook
Danny Price (Harvard-Smithsonian CfA)

The Large-Aperture Experiment to Detect the Dark Age (LEDA) seeks to detect absorption of neutral Hydrogen in the intergalactic medium (IGM) at the end of the Universe's Dark Age, during the "Cosmic Dawn" when the first stars were born. LEDA is deployed at the Long Wavelength Array station in Owens Valley (LWA-OV), and at the LWA station in Socorro, NM (LWA-NM). The signal of interest is a ~100 mK trough in the all-sky averaged power as a function of frequency, over the 30-88 MHz band (15

VLITE-FAST: GPU-based real-time searches for fast transients with the VLA
Paul Ray (NRL)

VLITE is a commensal system on the VLA that performs imaging searches for radio transients on time scale of 2 seconds (the correlator dump time) and longer. The VLITE correlator nodes are general purpose computers, which can have GPUs installed in them, and the sampled voltage data (in VDIF format) are delivered to the nodes via 10 Gbps ethernet links. As a result, it is possible to implement GPU-based searches for fast, dispersed transients in a parallel with the normal VLITE operation at low cost and in a non-interfering basis. We describe our development effort and plans for such a system. This will allow for studies of fast timescale phenomena such as FRBs, pulsar giant pulses, RRATs, and more, with thousands of on-sky hours per year. If an FRB can be detected in real time and a dump of the raw data can be triggered, it will be possible to image and localize the FRB to high accuracy (depending on the current VLA configuration).

This research at NRL was supported by the Chief of Naval Research.

Wide, deep and low: a low-frequency study of the Super-CLASS super-cluster with LOFAR and the GMRT.
Christopher Riseley (Jodrell Bank Centre for Astrophysics, University of Manchester)

The Super-CLASS super-cluster is a region of sky known to contain 5 Abell galaxy clusters at redshifts z ~ 0.2. In this talk I will present the results of low-frequency radio surveys of the Super-CLASS field with the Giant Metrewave Radio Telescope (GMRT) at 325 MHz (the deepest ever survey at this frequency) and the Low-Frequency Array (LOFAR) at 150 MHz. The data cover an area of around 6.5 square degrees, large enough to probe sources from a variety of environments within the individual clusters, the supercluster and the field.

Combining LOFAR and GMRT data on the same field allows us to better constrain the spectral behaviour of sources in these environments at low frequencies, and our high sensitivity allows us to probe the population of low-luminosity active galactic nuclei (AGN) and star-forming galaxies (SFG) which are believed to make up the dominant population at sub-mJy flux densities. Polarisation information from LOFAR enables us to both investigate the polarisation properties of AGN and SFG and also characterise the magnetic field strength and structure within the cluster and supercluster environments.

In this talk I will discuss the various source populations recovered from the field, and what we can learn about the source counts at low frequencies. Furthermore, I will present an initial rotation measure study of the super-cluster environment based on LOFAR observations of the polarised source population within the Super-CLASS field.

Radiative age mapping of the B2 0924+30 relic using LOFAR
Aleksandar Shulevski (ASTRON)

We present the results of our LOFAR observations of the steep spectrum radio source B2 0924+30. Hosted by a z=0.026 elliptical galaxy, it has a relatively large angular size of 30' and a morphology reminiscent of a relic FRII radio galaxy. Studying AGN radio relics can give us insight into the time-scales involved into the episodic gas accretion by AGNs and potentially their dependence on the AGN host environment. We have derived low frequency spectral index maps and used synchrotron aging models to derive radiative age maps, a first for this source at these low frequencies. Our age maps reveal the presence of "relic hot spots" in the lobes of B2 0924+30. We also showcase the potential of wide-field LOFAR imaging for serendipitous discovery.

Precise Faraday rotation measures from LOFAR pulsar observations, towards reconstructing the 3-D Galactic magnetic field
Charlotte Sobey (ASTRON)

Observations of pulsars using next generation low-frequency radio telescopes, for example LOFAR (the Low-Frequency Array), provide powerful probes of astrophysical plasmas. LOFAR's large fractional bandwidth and collecting area combine to produce the highest-quality polarisation profiles of pulsars below 200 MHz to date. These data are well suited for using the powerful technique of RM-synthesis to achieve unprecedented precision on Faraday rotation measures (RMs). The time- and position-dependent ionospheric Faraday rotation is calculated and subtracted using a code that has been verified using long-track LOFAR pulsar observations. I will present a growing catalogue of over 150 precise RMs obtained using LOFAR polarimetric observations centred at 150 MHz. Combining the accurate RMs and dispersion measures from LOFAR pulsar observations provides an efficient method to accurately reconstruct (and monitor) the 3-D Galactic magnetic field.

BIGHORNS status and results
Marcin Sokolowski (Curtin University)

The BIGHORNS is a total power radiometer designed and build at Curtin University to detect signature of the global Epoch of Reionisation in the sky averaged spectrum. In the years 2012-2014 it was deployed and tested in several remote locations in Western Australia. In October 2014, the system with a conical log spiral antenna was deployed at the Murchison Radio-astronomy Observatory (MRO) and has been collecting data since then. I will present current status of the project and analysis of the 2014/2015 data assessing the significance of ionospheric effects (such as absorption, emission and refraction) on the ground-based detection of the global EoR.

Transients at Low Frequencies
Sarah Burke Spolaor (NRAO)


Solar Physics with Low Frequency Pulsar Observations
Kevin Stovall (University of New Mexico)

Low frequency pulsar observations enable precise measurements of the effects of the solar wind. Over the past year, we have performed targeted observations of pulsars as they have passed near the sun with the first station of the Long Wavelength Array. I will present results on the increased dispersion observed. I will also talk about Faraday rotation measurements of PSR B0950+08 near the sun and initial results from an attempt to observe the effects of a CME on B0950+08's signal.

The Expanded Long Wavelength Array
Frank Schinzel (University of New Mexico)

The Expanded Long Wavelength Array will be an interferometric array combining LWA stations with VLA antennas to achieve baseline lengths up to 90 km in the frequency range 50-88 MHz. This array would provide an angular resolution of up to 10 arcseconds with 10s of mJy sensitivity. Early results using between 4 and 6 VLA antennas and the LWA1 will be presented. This effort has been made possible by the recent addition of 4-band feeds to the VLA, and the creation of a software correlator in LSL capable of handling data from both instruments. In the future the eLWA could be implemented as a commensul system similar to VLITE. Science targets include the sun, planets, pulsars, active Galaxies, and galaxy clusters to name just a few.

E-field Parallel Imaging Correlator: A Novel Imager for Next-generation Radio Telescopes
Nithyanandan Thyagarajan (Arizona State University)

Modern radio telescopes favor densely packed array layouts consisting of large numbers of antennas (N> 1000). Since the complexity of traditional correlators scales as O(N^2), there will be a steep cost for realizing the full imaging potential of these powerful instruments. Using our generic and efficient E-field Parallel Imaging Correlator (EPIC), we present the first demonstration of a generalized direct imaging algorithm, namely, Modular Optimal Frequency Fourier (MOFF) imager. It takes advantage of the multiplication-convolution theorem of Fourier transforms. It brings down the cost for dense layouts to O(N log N) and can image from irregularly arranged heterogeneous antenna arrays. EPIC is highly modular and parallelizable, implemented in object oriented Python, and publicly available. The images are verified to be equivalent to those from traditional methods. We have also validated EPIC using data from the Long Wavelength Array (LWA). Antenna systems with a dense filling factor consisting of a large number of antennas such as LWA, the Square Kilometre Array, Hydrogen Epoch of Reionization Array, and Canadian Hydrogen Intensity Mapping Experiment will gain significant advantage by deploying EPIC. Inherent availability of calibrated time-domain images on the writeout timescale of the digitizer and vastly lower bandwidth relative to FX/XF systems will make it a prime transient search candidate for Fast Radio Bursts (FRB) and planetary and exoplanetary phenomena.

A Search for Molecules at Low Frequency with the MWA
Chenoa Tremblay (Curtin University-ICRAR)

The study of molecular spectroscopy in interstellar clouds and stars has been critical in understanding star and galaxy formation. As a new approach, I am using the Murchison Widefield Array (MWA) to study molecules in astrophysical objects. This offers two unique advantages over published work; a larger field of view and a new observing range of frequencies. In this talk I will present two tentative detections of molecules within the Galactic Centre made with the MWA and new work using the transient mode of the instrument to obtain better spectral resolution.

Pulsars and Fast Transients with the MWA
Steven Tremblay (Curtin University/CAASTRO)

Low frequency instruments, such as the MWA, provide a powerful combination of their huge field of view and new spectral insight. The case for applying these advantages to short duration radio emission (e.g. pulsars, FRBs, etc.) is clear-cut, especially since the nature of these emission mechanisms are poorly understood. The MWA has been observing with a high-time resolution mode, the so-called voltage capture system (VCS), for almost a year now. I will discuss a few of the highlights from the MWA VCS results, covering topics from single pulse astrophysics to millisecond pulsars.

Observations of Giant Pulses from Pulsars using LWA1
Jr-Wei Tsai (Virginia Tech)

We report the detection of giant pulse emission from pulsars at 40 MHz and 70 MHz using the first station of the Long Wavelength Array - LWA1. Compared with the normal pulse, Giant pulses of pulsars have shorter temporal durations, higher intensities, and their cumulative distributions of pulse strength follow a power law, which is much less steep than would be expected if we were observing the tail of a Gaussian distribution of normal pulses. No detection of other transient pulses was found in a large dispersion measure range from 1.0 to 5000 pc cm^-3. We analyzed giant pulses' randomness, emission altitudes, and the scattering effect. The result of scattering time constants' spectral index shows an off Kolmogorov spectrum, if scattering is not minimal.

Lunar science at low frequencies
Harish Vedantham (ASTRON/Caltech)

A global (monopole) signal can be measured by an interferometer by observing its occultation by the Moon- a technique we recently demonstrated with LOFAR commissioning data. An interferometer is sensitive to and measures the Moon-background brightness contrast, which may be exploited to measure the brightness of the highly redshifted (30>z>8) 21-cm global signal. I will present results from more recent follow-up LOFAR observations, with a focus on how in the near term, such data can shed light on the thermal and electric properties of the lunar regolith at thus far unexplored depths (upto few 100s of m).

In addition (time permitting), I will also present results from a new pilot study with LOFAR to constrain the density of circumlunar plasma by accurately timing the epoch of occultation of background quasars. These results may be of consequence not only to lunar exospheric studies, but also to future Moon-based low frequency telescopes.

The expanding International LOFAR Telescope
Rene Vermeulen (ASTRON)

The International LOFAR Telescope has been producing science since its opening in 2010 (highlights see Orru et al). This talk focuses on the European collaboration, which is leading to steady expansion of the facilities, and plans for "LOFAR2.0", aimed at major new capabilities for LOFAR in the next decade.

MWA Update
Randall Wayth (Curtin University/ICRAR)


Merging Galaxy Clusters at Low Frequencies
Reinout van Weeren (Harvard-Smithsonian Center for Astrophysics)

In a growing number of galaxy clusters diffuse extended radio emission has been found. The existence of this diffuse radio emission implies the presence of relativistic particles and magnetic fields in the intracluster medium. An important question is how these relativistic particles are accelerated and the role of cluster-wide magnetic fields. Since diffuse cluster radio sources have steep synchrotron spectra they are best studied at low frequencies (<300 MHz). Recently, significant progress has been made in studying clusters at these frequencies. In this talk I will present an overview of some of the latest results, in particular focussing on LOFAR observations of Abell 2256 and the Toothbrush galaxy cluster.

Solar Physics at Low Frequencies
Stephen White (AFRL)

The availability of LOFAR, MWA and LWA has revitalized the tradition of low-frequency studies of the Sun. This talk will discuss the impact of these new capabilities and show how they are being applied with examples of ongoing science studies.

Exoplanets with HERA
Peter Williams (Harvard-Smithsonian Center for Astrophysics)

Motivated by the bright and phenomenologically-rich auroral radio bursts observed in the Solar System, astronomers have been attempting to detect exoplanets at radio wavelengths since before the discovery of 51 Peg b. While the first efforts were admittedly optimistic, long-wavelength radio arrays are finally achieving sensitivities comparable to the expected signal. I describe a planned search for exoplanetary radio emission with the under-construction Hydrogen Epoch of Reionization Array, HERA, arguing that it is extremely well-suited to this task even though its design is motivated by a completely different science case. Indeed, HERA has the potential to deliver breakthrough results from not only the distant universe, but also our Galactic back yard: radio observations can yield measurements of otherwise inaccessible quantities such as rotation periods and magnetic moments, the latter being particularly relevant to habitability and star/planet interactions.

The NCP region observed with 21CMA
Cathie Zheng (Victoria University of Wellington)

The 21 CentiMeter Array (21CMA)is a low-frequency radio interferometric array designed to probe statistically the 21cm radiation of neutral hydrogen from the cosmic dawn and the epoch of reionization (EoR) at z=6-27. A field of 10-100 square degrees centered on the North Celestial Pole (NCP) has been imaged for about 6 years so far in the frequency range from 50 MHz to 200 MHz with a resolution of 24 kHz. In this talk I will mainly focus on the calibration errors due to ionospheric effect and the radio sources in the NCP field observed with the 21CMA. Indeed, removals of these bright radio sources and the ionospheric influence constitute a technical challenge for doing such low-frequency radio experiment toward the detection of the cosmic dawn and EoR.