144 research outputs found
Fluctuation Properties and Polar Emission Mapping of Pulsar B0834+06 at Decameter Wavelengths
Recent results regarding subpulse-drift in pulsar B0943+10 have led to the
identification of a stable system of sub-beams circulating around the magnetic
axis of the star. Here, we present single-pulse analysis of pulsar B0834+06 at
35 MHz, using observations from the Gauribidanur Radio Telescope. Certain
signatures in the fluctuation spectra and correlations allow estimation of the
circulation time and drift direction of the underlying emission pattern
responsible for the observed modulation. We use the `cartographic transform'
mapping technique to study the properties of the polar emission pattern. These
properties are compared with those for the other known case of B0943+10, and
the implications are discussed.Comment: (v3: minor changes); 9 pages, 6 figures (11 ps files); To appear in
MNRA
Topology and Polarisation of Subbeams Associated With Pulsar B0943+10's `Drifting'-Subpulse Emission: II. Analysis of Gauribidanur 35-MHz Observations
In the previous paper of this series Deshpande & Rankin (2001) reported
results regarding subpulse-drift phenomenon in pulsar B0943+10 at 430 MHz and
111 MHz. This study has led to the identification of a stable system of
subbeams circulating around the magnetic axis of this star. Here, we present a
single-pulse analysis of our observations of this pulsar at 35 MHz. The
fluctuation properties seen at this low frequency, as well as our independent
estimates of the number of subbeams required and their circulation time, agree
remarkably well with the reported behavior at higher frequencies. We use the
`cartographic'-transform mapping technique developed in Paper-I to study the
emission pattern in the polar region of this pulsar. The significance of our
results in the context of radio emission mechanisms is also discussed.Comment: 6 pages, 7 figures (2 color figs), MNRAS, 326 (4), 1249-1254 (2001
ISM Simulations: An Overview of Models
Until recently the dynamical evolution of the interstellar medium (ISM) was
simulated using collisional ionization equilibrium (CIE) conditions. However,
the ISM is a dynamical system, in which the plasma is naturally driven out of
equilibrium due to atomic and dynamic processes operating on different
timescales. A step forward in the field comprises a multi-fluid approach taking
into account the joint thermal and dynamical evolutions of the ISM gas.Comment: Overview paper (3 pages) presented by M. Avillez at the Special
Session "Modern views of the interstellar medium", XXVIIIth IAU General
Assembly, August 27-30, 2012, Beijing. Chin
Electron Distribution in the Galactic Disk - Results From a Non-Equilibrium Ionization Model of the ISM
Using three-dimensional non-equilibrium ionization (NEI) hydrodynamical
simulation of the interstellar medium (ISM), we study the electron density,
, in the Galactic disk and compare it with the values derived from
dispersion measures towards pulsars with known distances located up to 200 pc
on either side of the Galactic midplane.
The simulation results, consistent with observations, can be summarized as
follows: (i) the DMs in the simulated disk lie between the maximum and minimum
observed values, (ii) the log derived from lines of sight crossing the
simulated disk follows a Gaussian distribution centered at \mu=-1.4 with a
dispersion \sigma=0.21, thus, the Galactic midplane =0.04\pm 0.01^{-3}$, (iii) the highest electron concentration by mass (up to 80%) is in
the thermally unstable regime (200<T<10^{3.9} K), (iv) the volume occupation
fraction of the warm ionized medium is 4.9-6%, and (v) the electrons have a
clumpy distribution along the lines of sight.Comment: Letter accepted for publication in Monthly Notices of the Royal
Astronomical Societ
Vacuum Gap Model for PSR B0943+10
PSR B0943+10 is known to show remarkably stable drifting subpulses, which can
be interpreted in terms of a circumferential motion of 20 sparks, each
completing one circulation around the periphery of the polar cap in 37 pulsar
periods. We use this observational constraint and argue that the vacuum gap
model can adequately describe the observed drift patterns. Further we
demonstrate that {\em only} the presence of strong non-dipolar surface magnetic
field can favor such vacuum gap formation. Subsequently, for the first time we
are able to constrain the parameters of the surface magnetic field, and model
the expected magnetic structure on the polar cap of PSR B0943+10 considering
the inverse Compton scattering photon dominated vacuum gap.Comment: 6 pages, 3 figures, acccepted for publication in Astronomy and
Astrophysic
Volt-per-Ă ngstrom terahertz fields from X-ray free-electron lasers
The electron linear accelerators driving modern X-ray free-electron lasers can emit intense, tunable, quasi-monochromatic terahertz (THz) transients with peak electric fields of V Ă
â»Âč and peak magnetic fields in excess of 10 T when a purpose-built, compact, superconducting THz undulator is implemented. New research avenues such as X-ray movies of THz-driven mode-selective chemistry come into reach by making dual use of the ultra-short GeV electron bunches, possible by a rather minor extension of the infrastructure
LOFAR tied-array imaging and spectroscopy of solar S bursts
Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes.
Aims. Here, LOw Frequency ARray (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms.
Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (~50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second.
Results. On 9 July 2013, over 3000 S bursts were observed over a time period of ~8 h. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (~2.5 MHz) features, the majority drifting at ~3.5 MHzâs-1 and a wide range of circular polarisation degrees (2â8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere.
Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however, they possess some of the characteristics of electron-cyclotron maser emission
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
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,
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