N. E. Kassim

West Virginia University, Morgantown, West Virginia, United States

Are you N. E. Kassim?

Claim your profile

Publications (313)610.31 Total impact

  • J. F. Helmboldt · N. E. Kassim · S. W. Teare ·
    [Show abstract] [Hide abstract]
    ABSTRACT: The ionospheric impact of a series of M-class solar flares that occurred on 12 March 2015 is described. A combination of data-sets from GPS receivers and two different co-located radio telescopes, the Very Large Array (VLA) and the Long Wavelength Array (LWA), were used to detect and characterize flare-induced irregularities. The data demonstrate that each flare causes a rapid step-like increase in TEC of about 0.2 TECU. The rise times of these steps are on the order of 1–3 minutes. The GPS data show signs of traveling ionospheric disturbances (TIDs) likely associated with gravity waves whose magnitudes were temporarily enhanced by the impact of two of the flares. Increased activity within the horizontal TEC gradients observed with the VLA were apparent for several minutes following the flare X-ray peak. The properties of these VLA-detected disturbances are strongly indicative of field-aligned irregularities (FAIs) within the plasmasphere. They initial form at two altitudes, approximately 2600 and 5600 km. Those that form higher up quickly disappear, whereas those that form lower appear to descend over a period of ∼7 minutes to an altitude of roughly 1450 km before disappearing. LWA observations of ionospheric reflections of the HF radio station WWV show a significant and brief (∼few minutes) Doppler-frequency disturbance near the onset of TEC enhancement, followed by a ∼5-minute drop in received power, indicating increased ionization in the D region.
    09/2015; DOI:10.1002/2015EA000116
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the detection of giant pulse emission from PSR B0950+08 in 24 hours of observations made at 39.4 MHz, with a bandwidth of 16 MHz, using the first station of the Long Wavelength Array, LWA1. We detected 119 giant pulses from PSR B0950+08 (at its dispersion measure), which we define as having SNRs at least 10 times larger than for the mean pulse in our data set. These 119 pulses are 0.035\% of the total number of pulse periods in the 24 hours of observations. The rate of giant pulses is about 5.0 per hour. The cumulative distribution of pulse strength $S$ is a steep power law, $N(>S)\propto S^{-4.7}$, but much less steep than would be expected if we were observing the tail of a Gaussian distribution of normal pulses. We detected no other transient pulses in a dispersion measure range from 1 to 90 pc cm$^{-3}$, in the beam tracking PSR B0950+08. The giant pulses have a narrower temporal width than the mean pulse (17.8 ms, on average, vs.\ 30.5 ms). The pulse widths are consistent with a previously observed weak dependence on observing frequency, which may be indicative of a deviation from a Kolmogorov spectrum of electron density irregularities along the line of sight. The rate and strength of these giant pulses is less than has been observed at $\sim$100 MHz. Additionally, the mean (normal) pulse flux density we observed is less than at $\sim$100 MHz. These results suggest this pulsar is weaker and produces less frequent giant pulses at 39 MHz than at 100 MHz.
    The Astronomical Journal 02/2015; 149(2). DOI:10.1088/0004-6256/149/2/65 · 4.02 Impact Factor
  • N. E. Kassim · S. D. Hyman · H. Intema · T. J. W. Lazio ·
    [Show abstract] [Hide abstract]
    ABSTRACT: An upgrade of the low frequency observing system of the VLA developed by NRL and NRAO, called low band (LB), will open a new era of Galactic center (GC) transient monitoring. Our previous searches using the VLA and GMRT have revealed a modest number of radio-selected transients, but have been severely sensitivity and observing time limited. The new LB system, currently accessing the 236--492 MHz frequency range, promises ≥5 × improved sensitivity over the legacy VLA system. The new system is emerging from commissioning in time to catch any enhanced sub-GHz emission from the G2 cloud event, and we review existing limits based on recent observations. We also describe a proposed 24/7 commensal system, called the LOw Band Observatory (LOBO). LOBO offers over 100 VLA GC monitoring hours per year, possibly revealing new transients and helping validate ASTRO2010's anticipation of a new era of transient radio astronomy. A funded LOBO pathfinder called the VLA Low Frequency Ionosphere and Transient Experiment (VLITE) is under development. Finally, we consider the impact of LB and LOBO on our GC monitoring program.
    Proceedings of the International Astronomical Union 10/2014; 9(S303):458-460. DOI:10.1017/S1743921314001148
  • Source
    [Show abstract] [Hide abstract]
    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.
    Monthly Notices of the Royal Astronomical Society 04/2014; 440(1). DOI:10.1093/mnras/stu256 · 5.11 Impact Factor
  • Source
    Tracey DeLaney · Namir E. Kassim · Lawrence Rudnick · R. A. Perley ·
    [Show abstract] [Hide abstract]
    ABSTRACT: Characterizing the ejecta in young supernova remnants is a requisite step towards a better understanding of stellar evolution. In Cassiopeia A the density and total mass remaining in the unshocked ejecta are important parameters for modeling its explosion and subsequent evolution. Low frequency (<100 MHz) radio observations of sufficient angular resolution offer a unique probe of unshocked ejecta revealed via free-free absorption against the synchrotron emitting shell. We have used the Very Large Array plus Pie Town Link extension to probe this cool, ionized absorber at 9 arcseconds and 18.5 arcseconds resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 electrons per cubic centimeters and a total mass of 0.39 Solar masses with uncertainties of a factor of about 2. This is a significant improvement over the 100 electrons per cubic centimeter upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our estimates are sensitive to a number of factors including temperature and geometry. However using reasonable values for each, our unshocked mass estimate agrees with predictions from dynamical models. We also consider the presence, or absence, of cold iron- and carbon-rich ejecta and how these affect our calculations. Finally we reconcile the intrinsic absorption from unshocked ejecta with the turnover in Cas A's integrated spectrum documented decades ago at much lower frequencies. These and other recent observations below 100 MHz confirm that spatially resolved thermal absorption, when extended to lower frequencies and higher resolution, will offer a powerful new tool for low frequency astrophysics.
    The Astrophysical Journal 02/2014; 785(1). DOI:10.1088/0004-637X/785/1/7 · 5.99 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
    Publications of the Astronomical Society of the Pacific 01/2014; 126(936). DOI:10.1086/675262 · 3.50 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
  • Scott D. Hyman · N. E. Kassim · S. E. Cutchin · J. Lazio · H. Intema ·
    [Show abstract] [Hide abstract]
    ABSTRACT: We present initial results from a September 2013 - May 2014 VLA program to monitor for low frequency Galactic center radio transients (GCRTs). In our earlier search for radio-selected transients toward the Galactic center with the legacy VLA and the GMRT, we found three transients. They had disparate properties: minutes-to-months timescales, single to multiple bursts, and peak flux densities from 0.05 to 1.5 Jy, but none of these had X-ray counterparts. The new VLA low-band system's wide bandpass, covering frequencies from ~250 to 450 MHz, is expected to be ~5 times more sensitive than the narrow-band 330 MHz legacy VLA system. Projections of transient rates are presented based on preliminary results of the current monitoring program, and are compared with rates extrapolated from our program on the legacy VLA. Our current program, with one-tenth the observing time as our previous ones combined, could yield ~1-3x as many new transient detections. The improved statistics from continued monitoring, along with spectral information available from the wide bandpass, will help elucidate the nature of the underlying GCRT population(s). In addition, we report on any activity observed from the 2013-14 G2 gas cloud interaction with Sgr A*, which some models predict will produce enhancements in emission of several Jy's at < 1 GHz frequencies. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
  • Sean E. Cutchin · S. D. Hyman · N. E. Kassim · J. Lazio ·
    [Show abstract] [Hide abstract]
    ABSTRACT: We present preliminary results from a pilot study to monitor the Galactic Center (GC) for radio transients at low-frequencies with the First Station of the Long Wavelength Array (LWA1). Observations were conducted between May 23 - June 8, 2013 utilizing the full LWA1 station beam correlated with an outlier dipole to form a two-element interferometer in order to resolve out extended Galactic emission. This study focuses on radio-selected transients with timescales from minutes to days, longer than those probed by complementary single-dispersed-pulse type searches. A monitoring campaign with LWA1 builds upon successful GC monitoring programs at 330 MHz and 235 MHz with both the VLA and the GMRT, respectively. While there exists a long-established phenomenology of variability in the radio sky, wide-field radio transient surveys of the sky have been rare, and most variable radio sources have been found as a result of following-up X-ray transients and gamma-ray bursts. Long wavelength (> 1 m) observations provide a naturally wide field-of-view, and are well suited for detecting the generally non-thermal emission from radio transients. Our preliminary results indicate we can constrain enhanced emission below 100 MHz from within a few degrees towards the GC that are significant with respect to reasonable extrapolations of previously detected VLA and GMRT non-thermal transients at higher frequencies.
  • Wendy M. Peters · W. D. Cotton · N. E. Kassim ·
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the Very Large Array (VLA) Low-frequency Sky Survey Redux (VLSSr), which covers the sky above declinations δ > -30 degrees at a frequency of 74 MHz with 75" resolution and an average RMS noise of 0.1 Jy/beam. The theoretical largest angular size imaged is 36', and there are approximately 95,000 cataloged sources. We have completely re-imaged all data from the original VLSS survey leading to improvements in a number of areas. These include the application of a more accurate primary beam correction which removes substantial radially dependent flux errors present in the VLSS, and smart-windowing to reduce the clean bias by half. We look ahead to the possibility of an expanded, "VLSS generation 2", made by piggybacking observations of the planned VLA Sky Survey (VLASS) using a proposed 24/7 commensal system, called the LOw Band Observatory (LOBO). Catalogs and images for the VLSSr are available at <http://www.cv.nrao.edu/vlss/VLSSlist.shtml>.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The merger of a binary neutron star pair is expected to generate a strong transient radio signal. This emission will be strongest at low-frequencies and will disperse as it transverses the interstellar medium, arriving at Earth after coincidentally emitted gravitational or (higher frequency) electromagnetic signals. The rate of compact object merger events is poorly constrained by observations. The first station of Long Wavelength Array (LWA-1) telescope is a low-frequency radio telescope located near Socorro, NM, which is sensitive to a frequency range of 10-88 MHz. I will discuss the sensitivity of LWA-1 to transient radio emission from binary neutron star mergers and a limit set by LWA-1 observations to constrain the rate of such merger events.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the detection of giant pulse emission from PSR B0950+08 at 40 MHz with the First Station of the Long Wavelength Array. 1029 pulses were detected in 24 hours of observations conducted between March and April 2012. The range of flux density is from ~50 Jy to ~300 Jy over a 16 MHz bandwidth for a ~10 millisecond pulse duration.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Low Frequency All Sky Monitor (LoFASM) is a distributed array of dipole antennas that are sensitive to radio frequencies from 10 to 88 MHz. LoFASM consists of antennas and front end electronics that were originally developed for the Long Wavelength Array (LWA) by the U.S. Naval Research Lab, the University of New Mexico, Virginia Tech, and the Jet Propulsion Laboratory. LoFASM, funded by the U.S. Department of Defense, will initially consist of 4 stations, each consisting of 12 dual-polarization dipole antenna stands. The primary science goals of LoFASM will be the detection and study of low-frequency radio transients, a high priority science goal as deemed by the National Research Council's decadal survey. The data acquisition system for the LoFASM antenna array will be using Field Programmable Gate Array (FPGA) technology to implement a real time full Stokes spectrometer and data recorder. This poster presents an overview of the current design and digital architecture of a single station of the LoFASM array as well as the status of the entire project.
  • [Show abstract] [Hide abstract]
    ABSTRACT: A low-frequency single dispersed pulse search was recently conducted with the first station of the Long Wavelength Array (LWA-1), a low-frequency radio telescope located near Socorro, NM, which is sensitive to a frequency range of 10-88 MHz. Possible candidate progenitors for such signals include: compact object mergers, supernovae, cosmic string cusp events, and exploding primordial black holes. I will summarize the observations conducted, the method used to reduce the data collected, and some relevant limits which can be set as a result of this work.
  • [Show abstract] [Hide abstract]
    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.
    09/2013; 48(5). DOI:10.1002/rds.20056
  • N. E. Kassim · S. D. Hyman · H. Intema · T. E. Clarke · R. Subrahmanyan ·
    [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Low Frequency All Sky Monitor (LoFASM) is a distributed array of dipole antennas that are sensitive to radio frequencies from 5 to 88 MHz. The primary science goals will be the detection and study of low-frequency radio transients. LoFASM consists of antennas and front end electronics that were originally developed for the Long Wavelength Array (LWA) by the U.S. Naval Research Lab, the University of New Mexico, Virginia Tech, and the Jet Propulsion Laboratory. LoFASM, funded by the U.S. Department of Defense, will initially consist of 4 stations, each consisting of 12 dual-polarization dipole antenna stands. The signals received by LoFASM are digitized and processed using Reconfigurable Open Architecture Computing Hardware (ROACH) boards. This poster will describe the LoFASM project with an emphasis on the ROACH data processing pipe-line.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present an update on the Jansky Very Large Array Low Band (VLA-LB) project, currently undergoing scientific commissioning and expected to be fully available in 2013. VLA-LB is a joint NRL and NRAO initiative to equip the VLA with broadband low frequency receivers that cover the spectrum between 66 and 470 MHz. The current system can already access the 66 to 86 MHz and 230 to 436 MHz sub-bands by working with existing 74 and 330 MHz feeds, respectively. The bandwidth at 74 MHz will increase by more than an order of magnitude while the 330 MHz bandwidth increases by approximately a factor of 6. The improved bandwidth and system temperature, coupled with the power of the WIDAR correlator, promise significantly enhanced performance compared to past VLA capabilities. Early commissioning results at “P band” (330 MHz) with a handful of antennas accessing the larger bandwidth indicate sensitivity rivaling that of the legacy 27-antenna, narrow-band old VLA capability. New feeds that can exploit a larger fraction of the available receiver bandwidth are being explored. While VLA-LB is useful as a conventional system, we are looking to enhance its power by leveraging the VLA’s capability to detect radiation at its prime and Cassegrain foci simultaneously. The ability to observe with more than one band in parallel is a powerful multiplier of a telescope’s function, and many instruments (e.g. the GMRT, WSRT and VLA) offer this. A variant is being explored for VLA-LB: observing from the prime focus during all normal Cassegrain observations. This proposed VLA-LB commensal system would piggyback normal VLA observing time to survey at low frequencies with relatively large field of views. Shared fields with other multi-beaming, dipole-based arrays that view the same sky with the VLA, e.g. the first station of the Long Wavelength Array (LWA1), would be possible. The collected data will be assembled into a database of spectra and wide-field images, suitable for studies of individual objects as well as searches for transients and high redshift spectral features (eg. HI absorption or OH mega-masers). We describe how the VLA-LB commensal system might be implemented, and explore early ideas for its scientific promise.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Low Frequency All Sky Monitor is a system of geographically separated radio arrays dedicated to the study of radio transients. LoFASM consists of four stations, each comprised of 12 cross-dipole antennas designed to operate between 5-88MHz. The antennas and front end electronics for LoFASM were designed by the Naval Research Laboratory for the Long Wavelength Array project. Over the last year, undergraduate students from the University of Texas at Brownsville’s Center for Advanced Radio Astronomy have been establishing these stations around the continental US, consisting of sites located in Port Mansfield, Texas, the LWA North Arm site of the LWA1 Radio Observatory in New Mexico, adjacent to the North Arm of the Very Large Array, the Green Bank Radio Observatory, West Virginia, and NASA’s Goldstone tracking complex in California. In combination with the establishment of these sites was the development of the analog hardware, which consists of commercial off-the-shelf RF splitter/combiners and a custom amplifier and filter chain designed by colleagues at the University of New Mexico. This poster will expound on progress in site installation and development of the analog signal chain.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The LWA1 Northern Celestial Cap (LNCC) pulsar survey, which is targeting the frequency range from 30 to 62 MHz, will be the first part of an all sky pulsar/radio transient survey using the first station (LWA1) of the Long Wavelength Array (LWA) telescope. This poster will focus on the search parameters for the pipelines that will process the LNCC survey data. The current status of the survey will also be discussed as well as preliminary results. Based on simulations it is estimated that about 75 pulsars will be detected with perhaps as many as 10 of them being new pulsars.

Publication Stats

3k Citations
610.31 Total Impact Points


  • 2012
    • West Virginia University
      • Department of Physics
      Morgantown, West Virginia, United States
    • Sweet Briar College
      Worcester, Massachusetts, United States
  • 2005-2012
    • SpecTIR™ Remote Sensing Division
      Reno, Nevada, United States
    • University of Tasmania
      Hobart Town, Tasmania, Australia
  • 1997-2010
    • United States Naval Research Laboratory
      Washington, Washington, D.C., United States
  • 2005-2006
    • University of Texas at Austin
      Austin, Texas, United States
  • 1999-2004
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
    • Cornell University
      • Department of Astronomy
      Ithaca, New York, United States
    • Raman Research Institute
      Bengalūru, Karnātaka, India
  • 2003
    • University of Wisconsin, Madison
      • Department of Astronomy
      Mississippi, United States
    • Leiden University
      • Leiden Observartory
      Leyden, South Holland, Netherlands
  • 1998-2002
    • Kennesaw State University
      Кеннесо, Georgia, United States
  • 2000
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
  • 1985-1989
    • University of Maryland, College Park
      CGS, Maryland, United States