S. Thoudam
A5009878905- Astrophysics and Cosmic Phenomena
- Radio Astronomy Observations and Technology
- Dark Matter and Cosmic Phenomena
- Gamma-ray bursts and supernovae
- Radio Wave Propagation Studies
- Particle Detector Development and Performance
- Neutrino Physics Research
- Lightning and Electromagnetic Phenomena
- Electromagnetic Compatibility and Measurements
- Solar and Space Plasma Dynamics
- Galaxies: Formation, Evolution, Phenomena
- Radiation Detection and Scintillator Technologies
- Precipitation Measurement and Analysis
- Ionosphere and magnetosphere dynamics
- Particle physics theoretical and experimental studies
- Pulsars and Gravitational Waves Research
- Astronomy and Astrophysical Research
- Astronomical Observations and Instrumentation
- Particle Accelerators and Free-Electron Lasers
- Scientific Research and Discoveries
- Computational Physics and Python Applications
- GNSS positioning and interference
- Geophysics and Gravity Measurements
- Power Line Communications and Noise
- Fire effects on ecosystems
Khalifa University of Science and Technology
2020-2025
Friedrich-Alexander-Universität Erlangen-Nürnberg
2023
National Institute for Subatomic Physics
2023
Radboud University Nijmegen
2011-2023
Can Tho University
2023
Karlsruhe Institute of Technology
2023
Deutsches Elektronen-Synchrotron DESY
2023
Vrije Universiteit Brussel
2023
University of Groningen
2023
Netherlands Institute for Radio Astronomy
2016-2023
The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve long-standing issues such as the transition from Galactic extragalactic origin, and nature cutoff observed at highest energies. Composition can be studied by measuring atmospheric depth shower maximum Xmax air showers generated high-energy hitting Earth's atmosphere. We present a new method reconstruct based on radio measurements. emission mechanism is complex process...
We have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up ~10^18 eV. Our shows that single Galactic component subsequent cut-offs in individual spectra different elements, optimised explain below ~10^14 eV knee all-particle spectrum, cannot above ~2x10^16 discuss two approaches for second -- re-acceleration at wind termination shock, supernova explosions Wolf-Rayet stars, show latter scenario can almost all features eV,...
We present LOFAR measurements of radio emission from extensive air showers. find that this is strongly polarized, with a median degree polarization nearly $99\%$, and the angle between direction electric field Lorentz force acting on particles, depends observer location in shower plane. This can be understood as superposition radially polarized charge-excess mechanism, first proposed by Askaryan geomagnetic mechanism Kahn Lerche. calculate relative strengths both contributions, quantified...
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components the sky brightness. Recovering original brightness distribution aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods Aims. Recent papers have established clear link between nature interferometry measurement "compressed sensing" (CS)...
The low frequency array (LOFAR), is the first radio telescope designed with capability to measure emission from cosmic-ray induced air showers in parallel interferometric observations. In $\sim 2\,\mathrm{years}$ of observing, 405 events energy range $10^{16} - 10^{18}\,\mathrm{eV}$ have been detected band $30 80\,\mathrm{MHz}$. Each these registered up $\sim1000$ independent antennas resulting measurements unprecedented detail. This article describes dataset, as well analysis pipeline, and...
We have conducted two pilot surveys for radio pulsars and fast transients with the Low-Frequency Array (LOFAR) around 140 MHz here report on first low-frequency fast-radio burst limit discovery of new pulsars. The survey, LOFAR Pilot Pulsar Survey (LPPS), observed a large fraction northern sky, ~1.4 x 10^4 sq. deg, 1-hr dwell times. Each observation covered ~75 deg using 7 independent fields formed by incoherently summing high-band antenna fields. second Tied-Array (LOTAS), spanned ~600...
Recent cosmic-ray measurements have found an anomaly in the energy spectrum at GeV-TeV energies. Although origin of is not clearly understood, suggested explanations include effect source spectrum, propagation effects, and nearby sources. In this paper, we propose that spectral might be re-acceleration cosmic rays by weak shocks Galaxy. After acceleration strong supernova remnant shock waves, undergo diffusive through During propagation, may again encounter expanding get re-accelerated. As...
(abridged). We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with International Telescope, which include baselines >1000 km. Suitable must contain significant correlated flux density (50-100mJy) at frequencies around 110--190~MHz on scales of a few hundred mas. At least 200--300-km international baselines, we find 1 calibrator source per square degree over large part northern...
We present an updated cosmic-ray mass composition analysis in the energy range ${10}^{16.8}$ to ${10}^{18.3}\text{ }\text{ }\mathrm{eV}$ from 334 air showers measured with LOFAR radio telescope and selected for minimal bias. In this range, origin of cosmic rays is expected shift galactic extragalactic sources. The based on improved method infer depth maximum ${X}_{\mathrm{max}}$ extensive measurements shower simulations. show results average standard deviation versus primary analyze dataset...
We present the results of a four-month campaign searching for low-frequency radio transients near North Celestial Pole with Low-Frequency Array (LOFAR), as part Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April comprised 2149 11-minute snapshots, each covering 175 deg^2. have found one convincing candidate astrophysical transient, duration few minutes flux density at 60 MHz 15-25 Jy. transient does not repeat has no obvious optical or...
The low-frequency radio spectra of the hotspots within powerful galaxies can provide valuable information about physical processes operating at site jet termination. These are responsible for dissipation kinetic energy, particle acceleration, and magnetic-field generation. Here, we report new observations galaxy Cygnus A using Low Frequency Array (LOFAR) between 109 183 MHz, an angular resolution ∼3.5 arcsec. emission lobes is found to have a complex spectral index distribution, with...
Context. Observing Jupiter's synchrotron emission from the Earth remains today sole method to scrutinize distribution and dynamical behavior of ultra energetic electrons magnetically trapped around planet (because in-situ particle data are limited in inner magnetosphere). Aims. We perform first resolved low-frequency imaging with LOFAR at 127 MHz. The radiation comes low energy (~1-30 MeV) which map a broad region magnetosphere. Methods (see article for complete abstract) Results. images...
Recent measurements of cosmic ray spectra several individual nuclear species by the CREAM, TRACER and ATIC experiments indicate a change in spectral index power laws at TeV energies. Possible explanations among others include non-linear diffusive shock acceleration rays, different propagation properties higher lower energies Galaxy presence nearby sources. In this paper, we show that if supernova remnants are main sources rays our Galaxy, effect can be responsible for observed changes. Using...
The Sun is an active source of radio emission which often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low frequencies (<100 MHz), the has not been imaged extensively because instrumental limitations previous telescopes. Here, combined high spatial, spectral temporal resolution Low Frequency Array (LOFAR) was used to study Type III bursts at 30-90 MHz their association CMEs. 126 simultaneous tied-array beams within 5 radii centre. This method...
We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns these are radically different those measured fair-weather conditions. With the use a simple two-layer model for atmospheric electric field, can be well reproduced by state-of-the-art simulation codes. This in turn provides novel way to study fields.
Air showers induced by cosmic rays create nanosecond pulses detectable at radio frequencies. These have been measured successfully in the past few years LOw Frequency ARray (LOFAR) and are used to study properties of rays. For a complete understanding this phenomenon underlying physical processes, an absolute calibration detecting antenna system is needed. We present three approaches that were check improve model LOFAR provide whole for air shower measurements. Two methods based on...
Cassiopeia A is one of the best-studied supernova remnants. Its shocked ejecta emits brightly in radio and X-rays. unshocked can be studied through infrared emission, radio-active decay $^{44}$Ti, low frequency free-free absorption due to cold gas internal shell. Free-free affected by mass, geometry, temperature, ionisation conditions absorbing gas. Observations at lowest frequencies constrain a combination these properties. We use LOFAR LBA observations 30-77 MHz L-band VLA compare...
With LOFAR we have been able to image the development of lightning flashes with meter-scale accuracy and unprecedented detail. We discuss primary steps behind our most recent mapping method. To demonstrate capabilities technique show interpret images first few milliseconds two intra-cloud flashes. In all negative leaders propagate in charge layer below main charge. Among several interesting features that about 2~ms after initiation Primary Initial Leader triggers formation a multitude (more...
We present high resolution observations of negative leaders at altitude using the LOFAR radio telescope. show that structure (altitudes larger than 7 km) differs in several respects from lower altitudes. In particular, High Altitude Negative Leaders (HANLs) very distinct steps a few hundred meters, stepping times order milliseconds and filamentary extends outward over hundreds meters; as opposed to (≲ 5 leaders, which have distances around 0.01 ms 10 m. Similar emit copious VHF radiation...
Measurements of cosmic rays by experiments such as ATIC, CREAM and PAMELA indicate a hardening the ray energy spectrum at TeV energies. In our recent work, we showed that can be due to effect nearby supernova remnants. We it for case protons helium nuclei. this paper, present an improved more detailed version previous extend study heavier species boron, carbon, oxygen iron Unlike study, work involves calculation background follows consistent treatment source parameters between components....
The pattern of the radio emission air showers is finely sampled with Low-Frequency ARray (LOFAR). A set 382 measured used to test a fast, analytic parameterization distribution pulse powers. Using this we are able reconstruct shower axis and give estimators for energy as well distance maximum.