| 1. |
- 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.
|
|
| 2. |
- Buitink, Stijn, et al.
(författare)
-
Searching for neutrino radio flashes from the Moon with LOFAR
- 2013
-
Ingår i: 5th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities. - : American Institute of Physics (AIP). ; , s. 27-31
-
Konferensbidrag (refereegranskat)abstract
- Ultra-high-energy neutrinos and cosmic rays produce short radio flashes through the Askaryan effect when they impact on the Moon. Earthbound radio telescopes can search the Lunar surface for these signals. A new generation of lowfrequency, digital radio arrays, spearheaded by LOFAR, will allow for searches with unprecedented sensitivity. In the first stage of the NuMoon project, low-frequency observations were carried out with the Westerbork Synthesis Radio Telescope, leading to the most stringent limit on the cosmic neutrino flux above 1023 eV. With LOFAR we will be able to reach a sensitivity of over an order of magnitude better and to decrease the threshold energy.
|
|
| 3. |
- Nelles, Anna, et al.
(författare)
-
Detecting radio emission from air showers with LOFAR
- 2013
-
Ingår i: 5th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities. - : American Institute of Physics (AIP). ; , s. 105-110
-
Konferensbidrag (refereegranskat)abstract
- LOFAR (the Low Frequency Array) is the largest radio telescope in the world for observing low frequency radio emission from 10 to 240 MHz. In addition to its use as an interferometric array, LOFAR is now routinely used to detect cosmic ray induced air showers by their radio emission. The LOFAR core in the Netherlands has a higher density of antennas than any dedicated cosmic ray experiment in radio. On an area of 12 km2 more than 2300 antennas are installed. They measure the radio emission from air showers with unprecedented precision and, therefore, give the perfect opportunity to disentangle the physical processes which cause the radio emission in air showers. In parallel to ongoing astronomical observations LOFAR is triggered by an array of particle detectors to record time-series containing cosmic-ray pulses. Cosmic rays have been measured with LOFAR since June 2011. We present the results of the first year of data.
|
|
| 4. |
- Thoudam, Satyendra, et al.
(författare)
-
A possible correlation between the high-energy electron spectrum and the cosmic ray secondary-to-primary ratios
- 2011
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 414, s. 1432-1438
-
Tidskriftsartikel (refereegranskat)abstract
- Recent observations of high-energy cosmic ray electrons by the Fermi-Large Area Telescope (LAT) and the High Energy Stereoscopic System (HESS) experiments between 20 GeV and 5 TeV have found that the energy spectrum closely follows a broken power law with a break at around 1 TeV. On the other hand, measurements of cosmic ray secondary-to-primary ratios like the boron-to-carbon ratio seem to indicate a possible change in the slope at energies around 100 GeV n−1. In this paper, we discuss one possible explanation for the observed break in the electron spectrum and its possible correlation with the flattening in the secondary-to-primary ratios at higher energies. In our model, we assume that cosmic rays after acceleration by supernova remnant shock waves, escape downstream of the shock and remain confined within the remnant until the shock slows down. During this time, the high-energy electrons suffer from radiative energy losses and the cosmic ray nuclei undergo nuclear fragmentations due to their interactions with the matter. Once the cosmic rays are released from the supernova remnants, they follow diffusive propagation in the Galaxy where they further suffer from radiative or fragmentation losses.
|
|
| 5. |
- 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.
|
|
| 6. |
- Thoudam, Satyendra, et al.
(författare)
-
Cosmic-ray spectral anomaly at GeV-TeV energies as due to re-acceleration by weak shocks in the Galaxy
- 2013
-
Ingår i: Proceedings of the 33rd International Cosmic Rays Conference. ; , s. 1-4
-
Konferensbidrag (refereegranskat)abstract
- Recent cosmic-ray measurements by the ATIC, CREAM and PAMELA experiments have found an apparent hardening of the energy spectrum at TeV energies. Although the origin of the hardening is not clearly understood, possible explanations include hardening in the cosmic-ray source spectrum, changes in the cosmic-ray propagation properties in the Galaxy and the effect of nearby sources. In this contribution, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the young ones due to their bigger size, re-acceleration is expected to be produced mainly by weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.
|
|
| 7. |
- Thoudam, Satyendra, et al.
(författare)
-
GeV-TeV cosmic-ray spectral anomaly as due to reacceleration by weak shocks in the Galaxy
- 2014
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567, s. 1-10
-
Tidskriftsartikel (refereegranskat)abstract
- Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include the effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of reacceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the younger remnants because of their bigger sizes, reacceleration is expected to be produced mainly by weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that the re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, thereby explaining the observed GeV-TeV spectral anomaly.
|
|
| 8. |
- Thoudam, Satyendra, et al.
(författare)
-
Nearby supernova remnants and the cosmic ray spectral hardening at high energies
- 2012
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 421:2, s. 1209-1214
-
Tidskriftsartikel (refereegranskat)abstract
- Recent measurements of cosmic ray spectra of several individual nuclear species by the CREAM, TRACER and ATIC experiments indicate a change in the spectral index of the power laws at TeV energies. Possible explanations among others include non-linear diffusive shock acceleration of cosmic rays, different cosmic ray propagation properties at higher and lower energies in the Galaxy and the presence of nearby sources. In this paper, we show that if supernova remnants are the main sources of cosmic rays in our Galaxy, the effect of the nearby remnants can be responsible for the observed spectral changes. Using a rigidity-dependent escape of cosmic rays from the supernova remnants, we explain the apparent observed property that the hardening of the helium spectrum occurs at relatively lower energies as compared to the protons and also that the spectral hardening does not persist beyond ∼(20–30) TeV energies.
|
|
| 9. |
- Thoudam, Satyendra, et al.
(författare)
-
On the point-source approximation of nearby cosmic ray sources
- 2012
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 419:1, s. 624-637
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we check in detail the validity of the widely adopted point-source approximation for nearby cosmic ray (CR) sources. Under an energy-independent escape model for CRs from the sources, we show that for young nearby sources, the point-source approximation breaks down at lower energies and the CR spectrum depends on the size and the morphology of the source. When applied to the nearby supernova remnants (SNRs), we find that the approximation breaks down for some of the individual remnants like the Vela, but interestingly it still holds good for their combined total spectrum at the Earth. Moreover, we also find that the results obtained under this simple approximation are quite different from those calculated under an energy-dependent escape model which is favoured by diffusive shock acceleration models inside SNRs. Our study suggests that if SNRs are the main sources of CRs in our Galaxy, then the commonly adopted point-source model (with an energy-independent escape scenario) appears flawed for CR studies from the nearby SNRs.
|
|
| 10. |
- Thoudam, Satyendra, et al.
(författare)
-
Revisiting the hardening of the cosmic ray energy spectrum at TeV energies
- 2013
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 435:3, s. 2532-2542
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of cosmic rays by experiments such as ATIC, CREAM and PAMELA indicate a hardening of the cosmic ray energy spectrum at TeV energies. In our recent work, we showed that the hardening can be due to the effect of nearby supernova remnants. We showed it for the case of protons and helium nuclei. In this paper, we present an improved and more detailed version of our previous work, and extend our study to heavier cosmic ray species such as boron, carbon, oxygen and iron nuclei. Unlike our previous study, the present work involves a detailed calculation of the background cosmic rays and follows a consistent treatment of cosmic ray source parameters between the background and the nearby components. Moreover, we also present a detailed comparison of our results on the secondary-to-primary ratios, secondary spectra and the diffuse gamma-ray spectrum with the results expected from other existing models, which can be checked by future measurements at high energies.
|
|
| 11. |
- 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).
|
|
