SwePub
Sök i SwePub databas

  Utökad sökning

Träfflista för sökning "WFRF:(Thoudam Satyendra) srt2:(2015-2019)"

Sökning: WFRF:(Thoudam Satyendra) > (2015-2019)

  • Resultat 1-50 av 51
Sortera/gruppera träfflistan
   
NumreringReferensOmslagsbildHitta
1.
  • Acero, F., et al. (författare)
  • Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7-3946
  • 2017
  • Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 840:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7-3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti) correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H I emission. We present a series of simulated images of RX J1713.7-3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton, whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H I observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.
  •  
2.
  • Arias, M., et al. (författare)
  • Low-frequency radio absorption in Cassiopeia A
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 612
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Cassiopeia A is one of the best-studied supernova remnants. Its bright radio and X-ray emission is due to shocked ejecta. Cas A is rather unique in that the unshocked ejecta can also be studied: through emission in the infrared, the radio-active decay of Ti-44, and the low-frequency free-free absorption caused by cold ionised gas, which is the topic of this paper. Aims. Free-free absorption processes are affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies can constrain a combination of these properties. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30-77 MHz and Very Large Array (VLA) L-band observations at 1-2 GHz to fit for internal absorption as parametrised by the emission measure. We simultaneously fit multiple UV-matched images with a common resolution of 17 '' (this corresponds to 0.25 pc for a source at the distance of Cas A). The ample frequency coverage allows us separate the relative contributions from the absorbing gas, the unabsorbed front of the shell, and the absorbed back of the shell to the emission spectrum. We explored the effects that a temperature lower than the similar to 100-500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material, such as its mass and density. We also compiled integrated radio flux density measurements, fit for the absorption processes that occur in the radio band, and considered their effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 +/- 0.48 M-circle dot for an assumed gas temperature of T = 100 K. This estimate is reduced for colder gas temperatures and, most significantly, if the ejecta are clumped. We measure the reverse shock to have a radius of 114 '' +/- 6 '' and be centred at 23:23:26, +58:48:54 (J2000). We also find that a decrease in the amount of mass in the unshocked ejecta (as more and more material meets the reverse shock and heats up) cannot account for the observed low-frequency behaviour of the secular decline rate. Conclusions. To reconcile our low-frequency absorption measurements with models that reproduce much of the observed behaviour in Cas A and predict little mass in the unshocked ejecta, the ejecta need to be very clumped or the temperature in the cold gas needs to be low (similar to 10 K). Both of these options are plausible and can together contribute to the high absorption value that we find.
  •  
3.
  • Becherini, Yvonne, et al. (författare)
  • Very-High-Energy gamma-ray astronomy with the ALTO observatory
  • 2018
  • Ingår i: 35th International Cosmic Ray Conference;ICRC2017, Busan, Korea, July 10-20, 2017. - Trieste : Sissa Medialab.
  • Konferensbidrag (refereegranskat)abstract
    • ALTO is a concept/project in the exploratory phase since 2013 aiming to build a wide-field Very-High-Energy gamma-ray observatory at very high altitude in the Southern hemisphere. The operation of such an observatory will complement the Northern hemisphere observations performed by HAWC and will make possible the exploration of the central region of our Galaxy and the hunt for PeVatrons, and to search for extended Galactic objects such as the Vela Supernova Remnant and the Fermi bubbles. The ALTO project is aiming for a substantial improvement of the Water Cherenkov Detection Technique by increasing the altitude of the observatory in order to lower the energy threshold, by using a layer of scintillator below the water tank to optimize the signal over background discrimination, by minimizing the size of the tanks and having a more compact array to sample the air-shower footprints with better precision, and by using precise electronics which will provide time-stamped waveforms to improve the angular and energy resolution. ALTO is designed to have as low an energy threshold as possible so as to act as a fast trigger alert to other observatories -- primarily to the Southern part of CTA -- for transient Galactic and extra-galactic phenomena. The wide field-of-view resulting from the detection technique allows the survey of a large portion of the sky continuously, thus giving the possibility to access emission from Gamma-Ray Bursts, Active Galactic Nuclei and X-ray binary flares, and extended emissions of both Galactic (Vela SNR, Fermi bubbles) and extra-galactic (AGN radio lobes) origin. The ALTO observatory will be composed of about a thousand detection units, each of which consists of a Water Cherenkov Detector positioned above a liquid Scintillation Detector, distributed within an area of about 200 m in diameter. The project is in the design study phase which is soon to be followed by a prototyping phase. The ALTO concept, design study and expected sensitivity together with the prototype status and plans for final deployment in the Southern hemisphere will be the subjects of this presentation.
  •  
4.
  • Bonardi, A., et al. (författare)
  • Study of the LOFAR radio self-trigger and single-station acquisition mode
  • 2018
  • Ingår i: 35th International Cosmic Ray Conference, ICRC 2017, 10-20 July 2017. - : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • The LOw Frequency ARay (LOFAR) observatory is a multipurpose radio antenna array aimed to detect radio signals in the frequency range 10-240 MHz. Radio antennas are clustered into over 50 stations, and are spread along Central and Northern Europe. The LOFAR core, where the density of stations is highest, is instrumented with the LOfar Radboud air shower Array (LORA), covering an area of about 300 m diameter centered at the LOFAR core position. Since 2011 the LOFAR core has been used for detecting radio-signals associated to cosmic-ray air showers in the energy range 1016 - 1018 eV. Data acquisition is triggered by the LORA scintillator array, which provides energy, arrival direction, and core position estimates of the detected air shower too. Thus only the core of the LOFAR array is currently used for cosmic-ray detection. In order to extend the energy range of the detected cosmic rays, it is necessary to expand the effective collecting area to the whole LOFAR array. On this purpose, a detailed study about the LOFAR potentialities of working in self-trigger mode, i.e. with the cosmic-ray data acquisition trigger provided by the radio-antenna only, is presented here. A new method based on the intensity and the frequency spectrum for determining the air shower position to be implemented on LOFAR remote stations is presented too. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
5.
  • Bonardi, Antonio, et al. (författare)
  • Towards real-time cosmic-ray identification with the LOw Frequency ARay
  • 2019
  • Ingår i: 8th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2018). - : EDP Sciences. - 9782759890804 ; , s. 1-3
  • Konferensbidrag (refereegranskat)abstract
    • The radio signals emitted by Extensive Air Showers have been successfully used for the last decade by LOFAR to reconstruct the properties of the primary cosmic rays. Since an effective real-time recognition system for the very short radio pulses is lacking, cosmic-ray acquisition is currently triggered by an external array of particle detector, called LORA, limiting the LOFAR collecting area to the area covered by LORA. A new algorithm for the real-time cosmic-ray detection has been developed for the LOFAR Low Band Antenna, which are sensitive between 10 and 90 MHz, and is here presented together with the latest results.
  •  
6.
  • Borwankar, Chinmay, et al. (författare)
  • Simulation studies of MACE-I : Trigger rates and energy thresholds
  • 2016
  • Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 84, s. 97-106
  • Tidskriftsartikel (refereegranskat)abstract
    • The MACE (Major Atmospheric Cherenkov Experiment) is an upcoming Very High Energy (VHE) gamma-ray telescope, based on imaging atmospheric Cherenkov technique, being installed at Hanle, a high altitude astronomical site in Ladakh, India. Here we present Monte Carlo simulation studies of trigger rates and threshold energies of MACE in the zenith angle range of 0 degrees-60 degrees for on-axis gamma-ray coming from point source and various cosmic ray species. We have simulated the telescope's response to gamma-rays, proton, electron and alpha initiated atmospheric Extensive Air Showers (EAS) in the broad energy range of 5 GeV to 20 TeV. For gamma-rays we consider power law and log parabolic spectra while other particles are simulated with their respective cosmic ray spectrum. Trigger rates and threshold energies are estimated for the trigger configuration of 4 Close Cluster Nearest Neighbour(CCNN) pixels as implemented in MACE hardware, in combination with single channel discriminator threshold ranging from 6-10 photo electrons (pe). We find that MACE can achieve the gamma-ray trigger energy threshold of similar to 17 GeV (4 CCNN, 9 pe) at 0 degrees zenith angle for power law spectrum. The total trigger rate at 0 degrees zenith is expected to be similar to 650 Hz, with protons contributing similar to 80% to it. For the zenith range of 0 degrees-40 degrees we find that the telescope can achieve gamma-gray trigger threshold energies of similar to 22 GeV at 20 degrees zenith angle and similar to 40 GeV at 40 degrees zenith angle. Integral rates are also almost constant for this zenith angle range. At zenith angle of 60 degrees, trigger energy threshold increases to similar to 173 GeV and total integral rate falls down to similar to 305 Hz. (C) 2016 Elsevier B.V. All rights reserved.
  •  
7.
  • Buitink, S., et al. (författare)
  • A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations
  • 2016
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 531:7592, s. 70-73
  • Tidskriftsartikel (refereegranskat)abstract
    • Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 1017–1018 electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal1 comes from accelerators capable of producing cosmic rays of these energies2. Cosmic rays initiate air showers—cascades of secondary particles in the atmosphere—and their masses can be inferred from measurements of the atmospheric depth of the shower maximum3 (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground4. Current measurements5 have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays6, 7, 8 is a rapidly developing technique9 for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front6, 12. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 1017–1017.5 electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 1017.5 electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 1017–1017.5 electronvolt range.
  •  
8.
  • Buitink, S., et al. (författare)
  • Cosmic ray mass composition with LOFAR
  • 2018
  • Ingår i: 35th International Cosmic Ray Conference — ICRC2017. 10–20 July, 2017. Bexco, Busan, Korea. - Trieste, Italy : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • The LOFAR radio telescope measures the radio emission from extensive air showers with unprecedented precision. In the dense core individual air showers are detected by hundreds of dipole antennas. By fitting the complex radiation pattern to Monte Carlo radio simulation codes we obtain measurements of the atmospheric depth of the shower maximum X max with a precision of < 20 g/cm 2 . This quantity is sensitive to the mass composition of cosmic rays. We discuss the first mass composition results of LOFAR and the improvements that are currently being made to enhance the accuracy of future analysis. Firstly, a more realistic treatment of the atmosphere will decrease the systematic uncertainties due to the atmosphere. Secondly, a series of upgrades to the LOFAR system will lead to increased effective area, duty cycle, and the possibility to extend the composition analysis down to the energy of 10 16.5 eV. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
9.
  • Corstanje, A., et al. (författare)
  • The effect of the atmospheric refractive index on the radio signal of extensive air showers
  • 2017
  • Ingår i: Astroparticle physics. - : Elsevier. - 0927-6505 .- 1873-2852. ; 89, s. 23-29
  • Tidskriftsartikel (refereegranskat)abstract
    • For the interpretation of measurements of radio emission from extensive air showers, an important systematic uncertainty arises from natural variations of the atmospheric refractive index n. At a given altitude, the refractivity N = 10(6) (n - 1) can have relative variations on the order of 10% depending on temperature, humidity, and air pressure. Typical corrections to be applied to N are about 4%. Using CoREAS simulations of radio emission from air showers, we have evaluated the effect of varying N on measurements of the depth of shower maximum X-max. For an observation band of 30-80 MHz, a difference of 4% in refractivity gives rise to a systematic error in the inferred X-max between 3.5 and 11 g/cm(2), for proton showers with zenith angles ranging from 15 to 50 degrees. At higher frequencies, from 120 to 250 MHz, the offset ranges from 10 to 22 g/cm(2). These offsets were found to be proportional to the geometric distance to X-max. We have compared the results to a simple model based on the Cherenkov angle. For the 120-250 MHz band, the model is in qualitative agreement with the simulations. In typical circumstances, we find a slight decrease in X-max compared to the default refractivity treatment in CoREAS. While this is within commonly treated systematic uncertainties, accounting for it explicitly improves the accuracy of X-max measurements. (C) 2017 Elsevier B.V. All rights reserved.
  •  
10.
  • Corstanje, A., et al. (författare)
  • The effect of the atmospheric refractive index on the radio signal of extensive air showers using Global Data Assimilation System (GDAS)
  • 2018
  • Ingår i: 35th International Cosmic Ray Conference, ICRC 2017, 10-20 July 2017, Bexco, Busan, Korea. - : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • One of the major systematic uncertainties in the measurement of Xmax from radio emission of EAS arises from variations of the refractive index in the atmosphere. The refractive index n varies with temperature, humidity and pressure, and the variations can be on the order of 10% for (n-1) at a given altitude. The effect of a varying refractive index on Xmax measurements is evaluated using CoREAS: a microscopic simulation of the radio emission from the individual particles in the cascade simulated with CORSIKA. We discuss the resulting offsets in Xmax for different frequency regimes, and compare them to a simple physical model. Under typical circumstances, the offsets in Xmax range from 4-11 g/cm2 for the 30-80 MHz frequency band. Therefore, for precise measurements it is required to include atmospheric data at the time and place of observation of the air shower into the simulations. The aim is to implement this in the next version of CoREAS/CORSIKA using the Global Data Assimilation System (GDAS), a global atmospheric model based on meteorological measurements and numerical weather predictions. This can then be used to re-evaluate the air shower measurements of the LOFAR radio telescope. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
11.
  • Corstanje, A., et al. (författare)
  • The shape of the radio wavefront of extensive air showers as measured with LOFAR
  • 2015
  • Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 61, s. 22-31
  • Tidskriftsartikel (refereegranskat)abstract
    • Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.
  •  
12.
  • Corstanje, A., et al. (författare)
  • Timing calibration and spectral cleaning of LOFAR time series data
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 590
  • Tidskriftsartikel (refereegranskat)abstract
    • We describe a method for spectral cleaning and timing calibration of short voltage time series data from individual radio interferometer receivers. It makes use of the phase differences in Fast Fourier Transform (FFT) spectra across antenna pairs. For strong, localized terrestrial sources these are stable over time, while being approximately uniform-random for a sum over many sources or for noise. Using only milliseconds-long datasets, the method finds the strongest interfering transmitters, a first-order solution for relative timing calibrations, and faulty data channels. No knowledge of gain response or quiescent noise levels of the receivers is required. With relatively small data volumes, this approach is suitable for use in an online system monitoring setup for interferometric arrays. We have applied the method to our cosmic-ray data collection, a collection of measurements of short pulses from extensive air showers, recorded by the LOFAR radio telescope. Per air shower, we have collected 2 ms of raw time series data for each receiver. The spectral cleaning has a calculated optimal sensitivity corresponding to a power signal-to-noise ratio of 0.08 (or -11 dB) in a spectral window of 25 kHz, for 2 ms of data in 48 antennas. This is well sufficient for our application. Timing calibration across individual antenna pairs has been performed at 0.4 ns precision; for calibration of signal clocks across stations of 48 antennas the precision is 0.1 ns. Monitoring differences in timing calibration per antenna pair over the course of the period 2011 to 2015 shows a precision of 0.08 ns, which is useful for monitoring and correcting drifts in signal path synchronizations. A cross-check method for timing calibration is presented, using a pulse transmitter carried by a drone flying over the array. Timing precision is similar, 0.3 ns.
  •  
13.
  • Garsden, H., et al. (författare)
  • LOFAR sparse image reconstruction
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 575:A90
  • Tidskriftsartikel (refereegranskat)abstract
    • 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 of the sky brightness. Recovering the original brightness distribution with aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods.Aims. Recent papers have established a clear link between the discrete nature of radio interferometry measurement and the “compressed sensing” (CS) theory, which supports sparse reconstruction methods to form an image from the measured visibilities. Empowered by proximal theory, CS offers a sound framework for efficient global minimization and sparse data representation using fast algorithms. Combined with instrumental direction-dependent effects (DDE) in the scope of a real instrument, we developed and validated a new method based on this framework.Methods. We implemented a sparse reconstruction method in the standard LOFAR imaging tool and compared the photometric and resolution performance of this new imager with that of CLEAN-based methods (CLEAN and MS-CLEAN) with simulated and real LOFAR data.Results. We show that i) sparse reconstruction performs as well as CLEAN in recovering the flux of point sources; ii) performs much better on extended objects (the root mean square error is reduced by a factor of up to 10); and iii) provides a solution with an effective angular resolution 2−3 times better than the CLEAN images.Conclusions. Sparse recovery gives a correct photometry on high dynamic and wide-field images and improved realistic structures of extended sources (of simulated and real LOFAR datasets). This sparse reconstruction method is compatible with modern interferometric imagers that handle DDE corrections (A- and W-projections) required for current and future instruments such as LOFAR and SKA.
  •  
14.
  • Girard, J. N., et al. (författare)
  • Imaging Jupiter’s radiation belts down to 127 MHz with LOFAR
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. With the limited amount of in situ particle data available for the innermost region of Jupiter’s magnetosphere, Earth-based observations of the giant planets synchrotron emission remain the sole method today of scrutinizing the distribution and dynamical behavior of the ultra energetic electrons magnetically trapped around the planet. Radio observations ultimately provide key information about the origin and control parameters of the harsh radiation environment.Aims. We perform the first resolved and low-frequency imaging of the synchrotron emission with LOFAR. At a frequency as low as 127 MHz, the radiation from electrons with energies of ~1–30 MeV are expected, for the first time, to be measured and mapped over a broad region of Jupiter’s inner magnetosphere.Methods. Measurements consist of interferometric visibilities taken during a single 10-hour rotation of the Jovian system. These visibilities were processed in a custom pipeline developed for planetary observations, combining flagging, calibration, wide-field imaging, direction-dependent calibration, and specific visibility correction for planetary targets. We produced spectral image cubes of Jupiter’s radiation belts at the various angular, temporal, and spectral resolutions from which flux densities were measured.Results. The first resolved images of Jupiter’s radiation belts at 127–172 MHz are obtained with a noise level ~20–25 mJy/beam, along with total integrated flux densities. They are compared with previous observations at higher frequencies. A greater extent of the synchrotron emission source (≥4 RJ) is measured in the LOFAR range, which is the signature – as at higher frequencies – of the superposition of a “pancake” and an isotropic electron distribution. Asymmetry of east-west emission peaks is measured, as well as the longitudinal dependence of the radial distance of the belts, and the presence of a hot spot at λIII = 230° ± 25°. Spectral flux density measurements are on the low side of previous (unresolved) ones, suggesting a low-frequency turnover and/or time variations of the Jovian synchrotron spectrum.Conclusions. LOFAR proves to be a powerful and flexible planetary imager. In the case of Jupiter, observations at 127 MHz depict the distribution of ~1–30 MeV energy electrons up to ~4–5 planetary radii. The similarities of the observations at 127 MHz with those at higher frequencies reinforce the conclusion that the magnetic field morphology primarily shapes the brightness distribution features of Jupiter’s synchrotron emission, as well as how the radiating electrons are likely radially and latitudinally distributed inside about 2 planetary radii. Nonetheless, the detection of an emission region that extends to larger distances than at higher frequencies, combined with the overall lower flux density, yields new information on Jupiter’s electron distribution, and this information may ultimately shed light on the origin and mode of transport of these particles.
  •  
15.
  • Hare, B. M., et al. (författare)
  • LOFAR Lightning Imaging : Mapping Lightning With Nanosecond Precision
  • 2018
  • Ingår i: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 123:5, s. 2861-2876
  • Tidskriftsartikel (refereegranskat)abstract
    • Lightning mapping technology has proven instrumental in understanding lightning. In this work we present a pipeline that can use lightning observed by the LOw-Frequency ARray (LOFAR) radio telescope to construct a 3-D map of the flash. We show that LOFAR has unparalleled precision, on the order of meters, even for lightning flashes that are over 20km outside the area enclosed by LOFAR antennas (approximate to 3,200km(2)), and can potentially locate over 10,000 sources per lightning flash. We also show that LOFAR is the first lightning mapping system that is sensitive to the spatial structure of the electrical current during individual lightning leader steps.
  •  
16.
  • Heald, G. H., et al. (författare)
  • The LOFAR Multifrequency Snapshot Sky Survey (MSSS) : I. Survey description and first results
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 582, s. 1-22
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the Multifrequency Snapshot Sky Survey (MSSS), the first northern-sky Low Frequency Array (LOFAR) imaging survey. In this introductory paper, we first describe in detail the motivation and design of the survey. Compared to previous radio surveys, MSSS is exceptional due to its intrinsic multifrequency nature providing information about the spectral properties of the detected sources over more than two octaves (from 30 to 160 MHz). The broadband frequency coverage, together with the fast survey speed generated by LOFAR’s multibeaming capabilities, make MSSS the first survey of the sort anticipated to be carried out with the forthcoming Square Kilometre Array (SKA). Two of the sixteen frequency bands included in the survey were chosen to exactly overlap the frequency coverage of large-area Very Large Array (VLA) and Giant Metrewave Radio Telescope (GMRT) surveys at 74 MHz and 151 MHz respectively. The survey performance is illustrated within the MSSS Verification Field (MVF), a region of 100 square degrees centered at (α,δ)J2000 = (15h,69°). The MSSS results from the MVF are compared with previous radio survey catalogs. We assess the flux and astrometric uncertainties in the catalog, as well as the completeness and reliability considering our source finding strategy. We determine the 90% completeness levels within the MVF to be 100 mJy at 135 MHz with 108″ resolution, and 550 mJy at 50 MHz with 166″ resolution. Images and catalogs for the full survey, expected to contain 150 000–200 000 sources, will be released to a public web server. We outline the plans for the ongoing production of the final survey products, and the ultimate public release of images and source catalogs.
  •  
17.
  • Horandel, Jorg R., et al. (författare)
  • The mass composition of cosmic rays measured with LOFAR
  • 2017
  • Ingår i: RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS. - : EDP Sciences.
  • Konferensbidrag (refereegranskat)abstract
    • High-energy cosmic rays, impinging on the atmosphere of the Earth initiate cascades of secondary particles, the extensive air showers. The electrons and positrons in the air shower emit electromagnetic radiation. This emission is detected with the LOFAR radio telescope in the frequency range from 30 to 240 MHz. The data are used to determine the properties of the incoming cosmic rays. The radio technique is now routinely used to measure the arrival direction, the energy, and the particle type (atomic mass) of cosmic rays in the energy range from 10(17) to 10(18) eV. This energy region is of particular astrophysical interest, since in this regime a transition from a Galactic to an extra-galactic origin of cosmic rays is expected. For illustration, the LOFAR results are used to set constraints on models to describe the origin of high-energy cosmic rays.
  •  
18.
  • Moldón, J., et al. (författare)
  • The LOFAR long baseline snapshot calibrator survey
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz.Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.Results. More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree.Conclusions. The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator.
  •  
19.
  • Mulrey, K., et al. (författare)
  • Calibration of the LOFAR low-band antennas using the Galaxy and a model of the signal chain
  • 2019
  • Ingår i: Astroparticle physics. - : Elsevier. - 0927-6505 .- 1873-2852. ; 111, s. 1-11
  • Tidskriftsartikel (refereegranskat)abstract
    • The LOw-Frequency ARray (LOFAR) is used to make precise measurements of radio emission from extensive air showers, yielding information about the primary cosmic ray. Interpreting the measured data requires an absolute and frequency-dependent calibration of the LOFAR system response. This is particularly important for spectral analyses, because the shape of the detected signal holds information about the shower development. We revisit the calibration of the LOFAR antennas in the range of 30-80 MHz. Using the Galactic emission and a detailed model of the LOFAR signal chain, we find an improved calibration that provides an absolute energy scale and allows for the study of frequency dependent features in measured signals. With the new calibration, systematic uncertainties of 13% are reached, and comparisons of the spectral shape of calibrated data with simulations show promising agreement. (C) 2019 Elsevier B.V. All rights reserved.
  •  
20.
  • Mulrey, K., et al. (författare)
  • Expansion of the LOFAR radboud air shower array
  • 2018
  • Ingår i: Proceedings of Science. - : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • The LOFAR Radboud Air Shower Array (LORA) consists of 20 plastic scintillators and is situated at the core of the LOFAR radio telescope. LORA detects particles from extensive air showers and triggers the read-out of the LOFAR antennas. The dense LOFAR antenna spacing allows for detailed sampling of the radio emission generated in extensive air showers, which yields high precision reconstruction of cosmic ray properties and information about the shower development. We discuss the proposed expansion of LORA, including the addition of scintillator units and the implementation of triggering algorithms that will probe more details of the radio emission and detect lower energy showers without introducing a composition bias, which is important for studying the origin of cosmic rays.
  •  
21.
  • Mulrey, K., et al. (författare)
  • Updated Calibration of the LOFAR Low-Band Antennas
  • 2019
  • Ingår i: 8th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2018). - : EDP Sciences. - 9782759890804 ; , s. 1-3
  • Konferensbidrag (refereegranskat)abstract
    • The LOw-Frequency ARray (LOFAR) telescope measures radio emission from air showers. In order to interpret the data, an absolute, frequency dependent calibration is required. Due to a growing need for a better understanding of the measured frequency spectrum, we revisit the calibration of the LOFAR antennas in the range of 30-80 MHz. Using the galactic radio emission and a detailed model of the LOFAR signal chain, we find a calibration that provides an absolute energy scale and allows us to study frequency dependent features in measured air shower signals.
  •  
22.
  • Nelles, A., et al. (författare)
  • A new way of air shower detection : measuring the properties of cosmic rays with LOFAR
  • 2015
  • Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 632:1, s. 1-11
  • Tidskriftsartikel (refereegranskat)abstract
    • High-energy cosmic rays impinging onto the atmosphere of the Earth initiate cascades of secondary particles: extensive air showers. Many of the particles in a shower are electrons and positrons. During the development of the air shower and by interacting with the geomagnetic field, the electromagnetic cascade creates radiation, which we detect at frequencies of tens of MHz with the LOFAR radio telescope in the Netherlands. After many years of struggling to understand the emission mechanisms, the radio community has achieved the breakthrough. We are now able to determine direction, energy, and type of the shower- inducing primary particle from the radio measurements. The large number of antennas at LOFAR allows us to have a high precision and very detailed measurements. We will elaborate on the shower reconstruction, a precise description of the intensity of the radio signal at ground level (at frequencies from 10 to 240 MHz), a precise measurement of the shape of the radio wavefront, and on the reconstruction of the shower energy.
  •  
23.
  • Nelles, A., et al. (författare)
  • Calibrating the absolute amplitude scale for air showers measured at LOFAR
  • 2015
  • Ingår i: Journal of Instrumentation. - 1748-0221. ; 10
  • Tidskriftsartikel (refereegranskat)abstract
    • Air showers induced by cosmic rays create nanosecond pulses detectable at radio frequencies. These pulses have been measured successfully in the past few years at the LOw-Frequency ARray (LOFAR) and are used to study the properties of cosmic rays. For a complete understanding of this phenomenon and the underlying physical processes, an absolute calibration of the detecting antenna system is needed. We present three approaches that were used to check and improve the antenna model of LOFAR and to provide an absolute calibration of the whole system for air shower measurements. Two methods are based on calibrated reference sources and one on a calibration approach using the diffuse radio emission of the Galaxy, optimized for short data-sets. An accuracy of 19% in amplitude is reached. The absolute calibration is also compared to predictions from air shower simulations. These results are used to set an absolute energy scale for air shower measurements and can be used as a basis for an absolute scale for the measurement of astronomical transients with LOFAR.
  •  
24.
  • Nelles, A., et al. (författare)
  • Measuring a Cherenkov ring in the radio emission from air showers at 110-190 MHz with LOFAR
  • 2015
  • Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 65, s. 11-21
  • Tidskriftsartikel (refereegranskat)abstract
    • Measuring radio emission from air showers offers a novel way to determine properties of the primary cosmic rays such as their mass and energy. Theory predicts that relativistic time compression effects lead to a ring of amplified emission which starts to dominate the emission pattern for frequencies above ∼100∼100 MHz. In this article we present the first detailed measurements of this structure. Ring structures in the radio emission of air showers are measured with the LOFAR radio telescope in the frequency range of 110–190 MHz. These data are well described by CoREAS simulations. They clearly confirm the importance of including the index of refraction of air as a function of height. Furthermore, the presence of the Cherenkov ring offers the possibility for a geometrical measurement of the depth of shower maximum, which in turn depends on the mass of the primary particle.
  •  
25.
  •  
26.
  • Orrù, E., et al. (författare)
  • Wide-field LOFAR imaging of the field around the double-double radio galaxy B1834+620 : A fresh view on a restarted AGN and doubeltjes
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 584, s. 1-12
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The existence of double-double radio galaxies (DDRGs) is evidence for recurrent jet activity in active galactic nuclei (AGN), as expected from standard accretion models. A detailed study of these rare sources provides new perspectives for investigating the AGN duty cycle, AGN-galaxy feedback, and accretion mechanisms. Large catalogues of radio sources, on the other hand, provide statistical information about the evolution of the radio-loud AGN population out to high redshifts.Aims. Using wide-field imaging with the LOFAR telescope, we study both a well-known DDRG as well as a large number of radio sources in the field of view.Methods. We present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using LOFAR commissioning data. Our image covers about 100 square degrees and contains over 1000 sources.Results. The four components of the DDRG B1834+620 have been resolved for the first time at 144 MHz. Inner lobes were found to point towards the direction of the outer lobes, unlike standard FR II sources. Polarized emission was detected at +60 rad m-2 in the northern outer lobe. The high spatial resolution allows the identification of a large number of small double-lobed radio sources; roughly 10% of all sources in the field are doubles with a separation smaller than 1′.Conclusions. The spectral fit of the four components is consistent with a scenario in which the outer lobes are still active or the jets recently switched off, while emission of the inner lobes is the result of a mix-up of new and old jet activity. From the presence of the newly extended features in the inner lobes of the DDRG, we can infer that the mechanism responsible for their formation is the bow shock that is driven by the newly launched jet. We find that the density of the small doubles exceeds the density of FR II sources with similar properties at 1.4 GHz, but this difference becomes smaller for low flux densities. Finally, we show that the significant challenges of wide-field imaging (e.g., time and frequency variation of the beam, directional dependent calibration errors) can be solved using LOFAR commissioning data, thus demonstrating the potential of the full LOFAR telescope to discover millions of powerful AGN at redshift z ~ 1.
  •  
27.
  • Pilia, M., et al. (författare)
  • Wide-band, low-frequency pulse profiles of 100 radio pulsars with LOFAR
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 586
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. LOFAR offers the unique capability of observing pulsars across the 10−240  MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively.Aims. The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium.Methods. We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: high band (120–167 MHz, 100 profiles) and low band (15–62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and 1400 MHz) to study the profile evolution. The profiles were aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR.Results. We find that the profile evolution with decreasing radio frequency does not follow a specific trend; depending on the geometry of the pulsar, new components can enter into or be hidden from view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories.
  •  
28.
  • Rossetto, L., et al. (författare)
  • Characterisation of the radio frequency spectrum emitted by high energy air showers with LOFAR
  • 2018
  • Ingår i: 35th International Cosmic Ray Conference, ICRC2017. - Trieste : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • The high number density of radio antennas at the LOFAR core in Northern Netherlands allows to detect radio signals emitted by extensive air showers in the energy range 1016 - 1018 e V, and to characterise the geometry of the observed cascade in a detailed way. The radio signal emitted by extensive air showers along their propagation in the atmosphere has been studied in the 30 - 70 MHz frequency range. The study has been conducted on real data and simulated showers. Regarding real data, cosmic ray radio signals detected by LOFAR since 2011 have been analysed. For simulated showers, the CoREAS code, a plug-in of the CORSIKA particle simulation code, has been used. The results show a clear dependence of the frequency spectrum on the distance to the shower axis for both real data and simulations. In particular, the spectrum flatten at a distance around 100 m from the shower axis, where the coherence of the radio signal is maximum. This behaviour could also be used to reconstruct the position of the shower axis at ground. A correlation between the frequency spectrum and the geometrical distance to the depth of the shower maximum Xmax has also been investigated. The final aim of this study is to find a method to improve the inferred information of primary cosmic rays with radio antennas, in view of affirming the radio detection technique as reliable method for the study of extensive air showers. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
29.
  • Rossetto, Laura, et al. (författare)
  • Latest results on the analysis of the radio frequency spectrum emitted by high energy air showers with LOFAR
  • 2019
  • Ingår i: 8th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2018). - : EDP Sciences. - 9782759890804 ; , s. 1-3
  • Konferensbidrag (refereegranskat)abstract
    • The LOw Frequency ARay (LOFAR) is a multi-purpose radio antenna array aimed to detect radio signals in the frequency range 10 - 240 MHz, covering a large surface in Northern Europe with a higher density in the Netherlands. Analytical calculations and simulation studies performed in the 2000s indicate a dependence of the radio frequency spectrum on cosmic-ray characteristics. The high number density of radio antennas at the LOFAR core allows to characterise the observed cascade in a detailed way. The radio signal emitted by air showers in the atmosphere has been studied accurately in the 30 - 80 MHz frequency range. The analysis has been conducted on simulated events and on real data detected by LOFAR since 2011. The final aim of this study is to find an independent method to infer information of primary cosmic rays for improving the reconstruction of primary particle parameters. Results show a strong dependence of the frequency spectrum on the distance to the shower axis for both real data and simulations. Furthermore, results show that this method is very sensitive to the precision in reconstructing the position of the shower axis at ground, and to different antenna calibration procedures. A correlation between the frequency spectrum and geometrical distance to the shower maximum development X-max has also been investigated.
  •  
30.
  • Rossetto, L., et al. (författare)
  • Measurement of cosmic rays with LOFAR
  • 2016
  • Ingår i: XIV INTERNATIONAL CONFERENCE ON TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS (TAUP 2015), PTS 1-7. - : Institute of Physics Publishing (IOPP).
  • Konferensbidrag (refereegranskat)abstract
    • The LOw Frequency ARay (LOFAR) is a multipurpose radio -antenna array aimed to detect radio signals in the 10 - 240 MHz frequency range, covering a large surface in Northern Europe with a higher density in the Northern Netherlands. Radio emission in the atmosphere is produced by cosmic -ray induced air showers through the interaction of charged particles with the Earth magnetic field. The detection of radio signals allows to reconstruct several properties of the observed cascade. We review here all important results achieved in the last years. We proved that the radio -signal distribution at ground level is described by a two-dimensional pattern, which is well fitted by a double Gaussian function. The radio -signal arrival time and polarization have been measured, thus providing additional information on the extensive air shower geometry, and on the radio emission processes. We also showed that the radio signal reaches ground in a thin, curved wavefront which is best parametrized by a hyperboloid shape centred around the shower axis. Radio emission has also been studied under thunderstorm conditions and compared to fair weather conditions. Moreover, by using a hybrid reconstruction technique, we performed mass composition measurements in the energy range 10(17) - 10(18) eV.
  •  
31.
  • Schellart, P., et al. (författare)
  • Probing Atmospheric Electric Fields in Thunderstorms through Radio Emission from Cosmic-Ray-Induced Air Showers
  • 2015
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 114:16, s. 1-5
  • Tidskriftsartikel (refereegranskat)abstract
    • We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.
  •  
32.
  • Scholten, O., et al. (författare)
  • Circular polarization in radio emission from extensive air showers
  • 2017
  • Ingår i: 35th International Cosmic Ray Conference, ICRC 2017; Bexco, Busan; South Korea; 10 July 2017 through 20 July 2017. - Trieste, Italy : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • At LOFAR we measure the radio emission from extensive air showers (EAS) in the frequency band of 30 - 80 MHz in dual-polarized antennas. Through an accurate antenna calibration we can determine the complete set of four Stokes parameters that uniquely determine the linear and circular polarization of the radio signal for an EAS. The observed dependency of the circular polarization on azimuth angle and distance to the shower axis is explained as due to the interfering contributions from the two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the Askaryan effect. The measured data show a quantitative agreement with microscopic CORSIKA/CoREAS calculations. Having a very detailed understanding of radio emission from EAS, opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.
  •  
33.
  • Scholten, Olaf, et al. (författare)
  • Lightning Imaging with LOFAR
  • 2017
  • Ingår i: 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2016). - : EDP Sciences.
  • Konferensbidrag (refereegranskat)abstract
    • We show that LOFAR can be used as a lightning mapping array with a resolution that is orders of magnitude better than existing arrays. In addition the polarization of the radiation can be used to track the direction of the stepping discharges. © 2017 The Authors, published by EDP Sciences.
  •  
34.
  • Scholten, O., et al. (författare)
  • Measurement of the circular polarization in radio emission from extensive air showers confirms emission mechanisms
  • 2016
  • Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 94:10
  • Tidskriftsartikel (refereegranskat)abstract
    • We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The observed dependency of the circular polarization on azimuth angle and distance to the shower axis is a clear signature of the interfering contributions from two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the build-up of excess charge at the shower front. The data, as measured at LOFAR, agree very well with a calculation from first principles. This opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.
  •  
35.
  • Scholten, Olaf, et al. (författare)
  • Precision study of radio emission from air showers at LOFAR
  • 2017
  • Ingår i: RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS. - : EDP Sciences.
  • Konferensbidrag (refereegranskat)abstract
    • Radio detection as well as modeling of cosmic rays has made enormous progress in the past years. We show this by using the subtle circular polarization of the radio pulse from air showers measured in fair weather conditions and the intensity of radio emission from an air shower under thunderstorm conditions.
  •  
36.
  • Shulevski, A., et al. (författare)
  • The peculiar radio galaxy 4C 35.06 : a case for recurrent AGN activity?
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 579, s. 1-10
  • Tidskriftsartikel (refereegranskat)abstract
    • Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~ 4″), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~ 30″), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α< − 1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated fluxdensity spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 kms-1), similar to what is found in other clusters. The derived column density is NHI ~ 4 × 1020 cm-2 for a Tspin = 100 K. This detection supports the connection – already suggested for other restarted radio sources – between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.
  •  
37.
  • Sobey, C., et al. (författare)
  • LOFAR discovery of a quiet emission mode in PSR B0823+26
  • 2015
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451, s. 2493-2506
  • Tidskriftsartikel (refereegranskat)abstract
    • PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over time-scales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR (Low-Frequency Array) discovery that PSR B0823+26 has a weak and sporadically emitting ‘quiet’ (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting ‘bright’ (B) mode. Previously, the pulsar has been undetected in the Q mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q mode and B mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.
  •  
38.
  • Stewart, A. J., et al. (författare)
  • LOFAR MSSS : detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole
  • 2016
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:3, s. 2321-2342
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-min snapshots, each covering 175 deg2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15–25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9−3.7+14.7×10−4" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">3.9+14.7−3.7×10−4 d−1 deg−2, and a transient surface density of 1.5 × 10−5 deg−2, at a 7.9-Jy limiting flux density and ∼10-min time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration.
  •  
39.
  • Thoudam, Satyendra, et al. (författare)
  • Anomaly in the cosmic-ray energy spectrum at GeV-TeV energies
  • 2015
  • Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 632:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent measurements of cosmic rays by various experiments have found that the energy spectrum of cosmic rays is harder in the TeV region than at GeV energies. The origin of the spectral hardening is not clearly understood. In this paper, we discuss the possibility that the spectral hardening might be due to the effect of re-acceleration of cosmic rays by weak shocks associated with old supernova remnants in the Galaxy.
  •  
40.
  • Thoudam, Satyendra, et al. (författare)
  • Cosmic-ray energy spectrum and composition up to the ankle : the case for a second Galactic component
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to about 10(18) eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below similar to 10(14) eV and the "knee" in the all-particle spectrum, cannot explain the observed all-particle spectrum above similar to 2 x 10(16) eV. We discuss two approaches for a second component of Galactic cosmic rays re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to similar to 10(18) eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at similar to 3 x 10(15) eV and the "second knee" at similar to 10(17) eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the "ankle" (at similar to 4 x 10(18) eV), and find that extragalactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between similar to 10(17) and 10(18) eV.
  •  
41.
  • Thoudam, Satyendra, et al. (författare)
  • Measurement of the cosmic-ray energy spectrum above 1016 eV with the LOFAR Radboud Air Shower Array
  • 2016
  • Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 73, s. 34-43
  • Tidskriftsartikel (refereegranskat)abstract
    • The energy reconstruction of extensive air showers measured with the LOFAR Radboud Air Shower Array (LORA) is presented in detail. LORA is a particle detector array located in the center of the LOFAR radio telescope in the Netherlands. The aim of this work is to provide an accurate and independent energy measurement for the air showers measured through their radio signal with the LOFAR antennas. The energy reconstruction is performed using a parameterized relation between the measured shower size and the cosmic-ray energy obtained from air shower simulations. In order to illustrate the capabilities of LORA, the all-particle cosmic-ray energy spectrum has been reconstructed, assuming that cosmic rays are composed only of protons or iron nuclei in the energy range between ∼2 × 1016 and 2 × 1018 eV. The results are compatible with literature values and a changing mass composition in the transition region from a Galactic to an extragalactic origin of cosmic rays.
  •  
42.
  • Thoudam, Satyendra, et al. (författare)
  • Simulation study for the proposed wide field-of-view gamma-ray detector array ALTO
  • 2018
  • Ingår i: Proceedings of Science. - Trieste, Italy : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • ALTO is a wide field-of-view air shower detector array for very-high-energy (VHE) gamma-ray astronomy, proposed to be installed in the Southern Hemisphere at an altitude of ∼5.1 km above sea level. The array will use water Cherenkov detectors, as in the HAWC observatory, but combined with scintillator detectors, to detect air showers induced by VHE gamma rays in the atmosphere. It is being designed to attain a lower energy threshold, better energy and angular resolution, and improved sensitivity. The array will consist of ∼1250 small-sized (3.6 m diameter) detector units distributed over a circular area of ∼160 m in diameter. Each detector unit will consist of a water Cherenkov detector and a liquid scintillation detector underneath which will preferentially identify muons, facilitating the background (cosmic ray) rejection, thereby improving the sensitivity. The background rejection will be further enhanced by the close-packed arrangement and the small size of the detectors which will allow a fine sampling of air-shower footprints at the ground. In this contribution, we present the Monte-Carlo simulation of the experiment performed using CORSIKA and GEANT4 simulation packages. The expected performance of the array in terms of reconstruction accuracies of the shower core and arrival direction, as well as preliminary estimate of the trigger energy threshold after preliminary selection cuts for a point-like gamma-ray source are presented. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
43.
  • Trinh, T. N. G., et al. (författare)
  • Circular polarization of radio emission from air showers in thunderstorm conditions
  • 2017
  • Ingår i: 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2016). - : E D P SCIENCES. - 9782759890156
  • Konferensbidrag (refereegranskat)abstract
    • We present measured radio emission from cosmic-ray-induced air showers under thunderstorm conditions. We observe for these events large differences in intensity, linear polarization and circular polarization from the events measured under fair-weather conditions. This can be explained by the effects of atmospheric electric fields in thunderclouds. Therefore, measuring the intensity and polarization of radio emission from cosmic ray extensive air showers during thunderstorm conditions provides a new tool to probe the atmospheric electric fields present in thunderclouds.
  •  
44.
  • Trinh, T. N. G., et al. (författare)
  • Influence of atmospheric electric fields on the radio emission from extensive air showers
  • 2016
  • Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 93:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The atmospheric electric fields in thunderclouds have been shown to significantly modify the intensity and polarization patterns of the radio footprint of cosmic-ray-induced extensive air showers. Simulations indicated a very nonlinear dependence of the signal strength in the frequency window of 30–80 MHz on the magnitude of the atmospheric electric field. In this work we present an explanation of this dependence based on Monte Carlo simulations, supported by arguments based on electron dynamics in air showers and expressed in terms of a simplified model. We show that by extending the frequency window to lower frequencies, additional sensitivity to the atmospheric electric field is obtained.
  •  
45.
  • Trinh, T. N. G., et al. (författare)
  • Thunderstorm electric fields probed by extensive air showers through their polarized radio emission
  • 2017
  • Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 95:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We observe a large fraction of circular polarization in radio emission from extensive air showers recorded during thunderstorms, much higher than in the emission from air showers measured during fair-weather circumstances. We show that the circular polarization of the air showers measured during thunderstorms can be explained by the change in the direction of the transverse current as a function of altitude induced by atmospheric electric fields. Thus by using the full set of Stokes parameters for these events, we obtain a good characterization of the electric fields in thunderclouds. We also measure a large horizontal component of the electric fields in the two events that we have analyzed.
  •  
46.
  • Vedantham, H. K., et al. (författare)
  • Lunar occultation of the diffuse radio sky : LOFAR measurements between 35 and 80 MHz
  • 2015
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 450, s. 2291-2305
  • Tidskriftsartikel (refereegranskat)abstract
    • We present radio observations of the Moon between 35 and 80 MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies 35 < ν < 80 MHz since it is ‘colder’ than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the centre of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than 80 dB to achieve an RFI temperature <1 mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than 20 mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds 20 mK if the aggregate scattering cross-section of visible satellites exceeds 175 m2 at 800 km height, or 15 m2 at 400 km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10–15 per cent level). Further refinement of our technique may yield constraints on the redshifted global 21 cm signal from Cosmic Dawn (40 > z > 12) and the Epoch of Reionization (12 > z > 5).
  •  
47.
  • Winchen, T., et al. (författare)
  • Cosmic ray physics with the LOFAR radio telescope
  • 2019
  • Ingår i: 26th Extended European Cosmic Ray Symposium, 6–10 July 2018, Altai State University, Barnaul-Belokurikha, Russian Federation. - : Institute of Physics (IOP). ; , s. 1-6
  • Konferensbidrag (refereegranskat)abstract
    • The LOFAR radio telescope is able to measure the radio emission from cosmic ray induced air showers with hundreds of individual antennas. This allows for precision testing of the emission mechanisms for the radio signal as well as determination of the depth of shower maximum X max , the shower observable most sensitive to the mass of the primary cosmic ray, to better than 20 g cm -2 . With a densely instrumented circular area of roughly 320 m 2 , LOFAR is targeting for cosmic ray astrophysics in the energy range 10 16 -10 18 eV. In this contribution we give an overview of the status, recent results, and future plans of cosmic ray detection with the LOFAR radio telescope. © Published under licence by IOP Publishing Ltd.
  •  
48.
  • Winchen, Tobias, et al. (författare)
  • Overview and status of the lunar detection of cosmic particles with LOFAR
  • 2018
  • Ingår i: Proceedings of Science. - Trieste, Italy : Sissa Medialab Srl.
  • Konferensbidrag (refereegranskat)abstract
    • When a cosmic particle hits matter it produces radio emission via the Askaryan effect. This allows to use Earth's moon as detector for cosmic particles by searching for these ns-pulses with radio telescopes. This technique potentially increases the available collective area by several orders of magnitude compared to current experiments. The LOw Frequency ARray (LOFAR) is the largest radio telescope operating in the optimum frequency regime for corresponding searches. In this contribution, we report on the design and status of the implementation of the lunar detection mode at LOFAR. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).
  •  
49.
  • Winchen, T., et al. (författare)
  • Properties of the Lunar Detection Mode for ZeV-Scale Particles with LOFAR
  • 2019
  • Ingår i: 8th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2018). - : EDP Sciences. - 9782759890804 ; , s. 1-3
  • Konferensbidrag (refereegranskat)abstract
    • The steep decrease of the flux of ultra-high energy cosmic rays (UHECR) provides a challenge to answer the long standing question about their origin and nature. A significant increase in detector volume may be achieved by employing Earth's moon as a detector that is read out using existing Earth-bound radio telescopes by searching for the radio pulses emitted by the particle shower in the lunar rock. In this contribution we will report on the properties of a corresponding detection mode currently under development for the LOFAR Radio telescope.
  •  
50.
  • Winchen, T., et al. (författare)
  • Status of the lunar detection mode for cosmic particles of LOFAR
  • 2019
  • Ingår i: Journal of Physics. - : Institute of Physics (IOP). ; , s. 1-7
  • Konferensbidrag (refereegranskat)abstract
    • Cosmic particles hitting Earth's moon produce radio emission via the Askaryan effect. If the resulting radio ns-pulse can be detected by radio telescopes, this technique potentially increases the available collective area for ZeV scale particles by several orders of magnitude compared to current experiments. The LOw Frequency ARray (LOFAR) is the largest radio telescope operating in the optimum frequency regime for this technique. In this contribution, we report on the status of the implementation of the lunar detection mode at LOFAR. © Published under licence by IOP Publishing Ltd.
  •  
Skapa referenser, mejla, bekava och länka
  • Resultat 1-50 av 51

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.

 
pil uppåt Stäng

Kopiera och spara länken för att återkomma till aktuell vy