4,111 research outputs found
Calibration of the LOFAR low-band antennas using the Galaxy and a model of the signal chain
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.Comment: 23 pages, 10 figure
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
The LOFAR Transients Pipeline
Current and future astronomical survey facilities provide a remarkably rich
opportunity for transient astronomy, combining unprecedented fields of view
with high sensitivity and the ability to access previously unexplored
wavelength regimes. This is particularly true of LOFAR, a
recently-commissioned, low-frequency radio interferometer, based in the
Netherlands and with stations across Europe. The identification of and response
to transients is one of LOFAR's key science goals. However, the large data
volumes which LOFAR produces, combined with the scientific requirement for
rapid response, make automation essential. To support this, we have developed
the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image
data from LOFAR or other instruments and searches it for transients and
variables, providing automatic alerts of significant detections and populating
a lightcurve database for further analysis by astronomers. Here, we discuss the
scientific goals of the TraP and how it has been designed to meet them. We
describe its implementation, including both the algorithms adopted to maximize
performance as well as the development methodology used to ensure it is robust
and reliable, particularly in the presence of artefacts typical of radio
astronomy imaging. Finally, we report on a series of tests of the pipeline
carried out using simulated LOFAR observations with a known population of
transients.Comment: 30 pages, 11 figures; Accepted for publication in Astronomy &
Computing; Code at https://github.com/transientskp/tk
Progress with the LOFAR Imaging Pipeline
One of the science drivers of the new Low Frequency Array (LOFAR) is
large-area surveys of the low-frequency radio sky. Realizing this goal requires
automated processing of the interferometric data, such that fully calibrated
images are produced by the system during survey operations. The LOFAR Imaging
Pipeline is the tool intended for this purpose, and is now undergoing
significant commissioning work. The pipeline is now functional as an automated
processing chain. Here we present several recent LOFAR images that have been
produced during the still ongoing commissioning period. These early LOFAR
images are representative of some of the science goals of the commissioning
team members.Comment: 11 pages, 6 figures. Accepted for publication in proceedings of
"ISKAF2010 Science Meeting", PoS(ISKAF2010)05
A deep campaign to characterize the synchronous radio/X-ray mode switching of PSR B0943+10
We report on simultaneous X-ray and radio observations of the mode-switching
pulsar PSR B0943+10 obtained with the XMM-Newton satellite and the LOFAR, LWA
and Arecibo radio telescopes in November 2014. We confirm the synchronous
X-ray/radio switching between a radio-bright (B) and a radio-quiet (Q) mode, in
which the X-ray flux is a factor ~2.4 higher than in the B-mode. We discovered
X-ray pulsations, with pulsed fraction of 38+/-5% (0.5-2 keV), during the
B-mode, and confirm their presence in Q-mode, where the pulsed fraction
increases with energy from ~20% up to ~65% at 2 keV. We found marginal evidence
for an increase in the X-ray pulsed fraction during B-mode on a timescale of
hours. The Q-mode X-ray spectrum requires a fit with a two-component model
(either a power-law plus blackbody or the sum of two blackbodies), while the
B-mode spectrum is well fit by a single blackbody (a single power-law is
rejected). With a maximum likelihood analysis, we found that in Q-mode the
pulsed emission has a thermal blackbody spectrum with temperature ~3.4x10^6 K
and the unpulsed emission is a power-law with photon index ~2.5, while during
B-mode both the pulsed and unpulsed emission can be fit by either a blackbody
or a power law with similar values of temperature and photon index. A Chandra
image shows no evidence for diffuse X-ray emission. These results support a
scenario in which both unpulsed non-thermal emission, likely of magnetospheric
origin, and pulsed thermal emission from a small polar cap (~1500 m^2) with a
strong non-dipolar magnetic field (~10^{14} G), are present during both radio
modes and vary in intensity in a correlated way. This is broadly consistent
with the predictions of the partially screened gap model and does not
necessarily imply global magnetospheric rearrangements to explain the mode
switching.Comment: To be published on The Astrophysical Journa
Imaging Jupiter's radiation belts down to 127 MHz with LOFAR
Context. Observing Jupiter's synchrotron emission from the Earth remains
today the sole method to scrutinize the distribution and dynamical behavior of
the ultra energetic electrons magnetically trapped around the planet (because
in-situ particle data are limited in the inner magnetosphere). Aims. We perform
the first resolved and low-frequency imaging of the synchrotron emission with
LOFAR at 127 MHz. The radiation comes from low energy electrons (~1-30 MeV)
which map a broad region of Jupiter's inner magnetosphere. Methods (see article
for complete abstract) Results. The first resolved images of Jupiter's
radiation belts at 127-172 MHz are obtained along with total integrated flux
densities. They are compared with previous observations at higher frequencies
and show a larger extent of the synchrotron emission source (>=4 ). The
asymmetry and the dynamic of east-west emission peaks are measured and the
presence of a hot spot at lambda_III=230 {\deg} 25 {\deg}. Spectral flux
density measurements are on the low side of previous (unresolved) ones,
suggesting a low-frequency turnover and/or time variations of the emission
spectrum. Conclusions. LOFAR is a powerful and flexible planetary imager. The
observations at 127 MHz depict an extended emission up to ~4-5 planetary radii.
The similarities with high frequency results reinforce the conclusion that: i)
the magnetic field morphology primarily shapes the brightness distribution of
the emission and ii) the radiating electrons are likely radially and
latitudinally distributed inside about 2 . Nonetheless, the larger extent
of the brightness combined with the overall lower flux density, yields new
information on Jupiter's electron distribution, that may shed light on the
origin and mode of transport of these particles.Comment: 10 pages, 12 figures, accepted for publication in A&A (27/11/2015) -
abstract edited because of limited character
Observing pulsars and fast transients with LOFAR
Low frequency radio waves, while challenging to observe,are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric âradio windowâ: 10â240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth.LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals.We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR
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