21 research outputs found
Broadband Meter-Wavelength Observations of Ionospheric Scintillation
Intensity scintillations of cosmic radio sources are used to study
astrophysical plasmas like the ionosphere, the solar wind, and the interstellar
medium. Normally these observations are relatively narrow band. With Low
Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging
Receiver Array (KAIRA) station in northern Finland we have observed
scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were
discovered in interstellar scintillations of pulsars, can provide precise
estimates of the distance and velocity of the scattering plasma. Here we report
the first observations of such arcs in the ionosphere and the first broad-band
observations of arcs anywhere, raising hopes that study of the phenomenon may
similarly improve the analysis of ionospheric scintillations. These
observations were made of the strong natural radio source Cygnus-A and covered
the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen
early in the observations, before transit, and disappeared after transit
although scintillations continued to be obvious during the entire observation.
We show that this can be attributed to the structure of Cygnus-A. Initial
results from modeling these scintillation arcs are consistent with simultaneous
ionospheric soundings taken with other instruments, and indicate that
scattering is most likely to be associated more with the topside ionosphere
than the F-region peak altitude. Further modeling and possible extension to
interferometric observations, using international LOFAR stations, are
discussed.Comment: 11 pages, 17 figure
First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256
Abell 2256 is one of the best known examples of a galaxy cluster hosting
large-scale diffuse radio emission that is unrelated to individual galaxies. It
contains both a giant radio halo and a relic, as well as a number of head-tail
sources and smaller diffuse steep-spectrum radio sources. The origin of radio
halos and relics is still being debated, but over the last years it has become
clear that the presence of these radio sources is closely related to galaxy
cluster merger events. Here we present the results from the first LOFAR Low
band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our
knowledge, the image presented in this paper at 63 MHz is the deepest ever
obtained at frequencies below 100 MHz in general. Both the radio halo and the
giant relic are detected in the image at 63 MHz, and the diffuse radio emission
remains visible at frequencies as low as 20 MHz. The observations confirm the
presence of a previously claimed ultra-steep spectrum source to the west of the
cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz.
The steep spectrum suggests that this source is an old part of a head-tail
radio source in the cluster. For the radio relic we find an integrated spectral
index of -0.81 \pm 0.03, after removing the flux contribution from the other
sources. This is relatively flat which could indicate that the efficiency of
particle acceleration at the shock substantially changed in the last \sim 0.1
Gyr due to an increase of the shock Mach number. In an alternative scenario,
particles are re-accelerated by some mechanism in the downstream region of the
shock, resulting in the relatively flat integrated radio spectrum. In the radio
halo region we find indications of low-frequency spectral steepening which may
suggest that relativistic particles are accelerated in a rather inhomogeneous
turbulent region.Comment: 13 pages, 13 figures, accepted for publication in A\&A on April 12,
201
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
KAIRA: The Kilpisjärvi Atmospheric Imaging Receiver Array-System Overview and First Results
KAIRA: The Kilpisjarvi Atmospheric Imaging Receiver Array-System Overview and First Results
Wide-band Simultaneous Observations of Pulsars: Disentangling Dispersion Measure and Profile Variations
20 Pages, 14 Figures, Accepted for publication in Astronomy & AstrophysicsInternational audienceDispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 100000 across our observing band. We use this fact to constrain some of the properties of the ISM along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays