390 research outputs found
Hard X-ray footpoint sizes and positions as diagnostics of flare accelerated energetic electrons in the low solar atmosphere
The hard X-ray (HXR) emission in solar flares comes almost exclusively from a
very small part of the flaring region, the footpoints of magnetic loops. Using
RHESSI observations of solar flare footpoints, we determine the radial
positions and sizes of footpoints as a function of energy in six near-limb
events to investigate the transport of flare accelerated electrons and the
properties of the chromosphere. HXR visibility forward fitting allows to find
the positions/heights and the sizes of HXR footpoints along and perpendicular
to the magnetic field of the flaring loop at different energies in the HXR
range. We show that in half of the analyzed events, a clear trend of decreasing
height of the sources with energy is found. Assuming collisional thick-target
transport, HXR sources are located between 600 and 1200 km above the
photosphere for photon energies between 120 and 25 keV respectively. In the
other events, the position as a function of energy is constant within the
uncertainties. The vertical sizes (along the path of electron propagation)
range from 1.3 to 8 arcseconds which is up to a factor 4 larger than predicted
by the thick-target model even in events where the positions/heights of HXR
sources are consistent with the collisional thick-target model. Magnetic
mirroring, collisional pitch angle scattering and X-ray albedo are discussed as
potential explanations of the findings.Comment: 10 pages, 8 figures, accepted for publication in Ap
Tomographic approach to resolving the distribution of LISA Galactic binaries
The space based gravitational wave detector LISA is expected to observe a
large population of Galactic white dwarf binaries whose collective signal is
likely to dominate instrumental noise at observational frequencies in the range
10^{-4} to 10^{-3} Hz. The motion of LISA modulates the signal of each binary
in both frequency and amplitude, the exact modulation depending on the source
direction and frequency. Starting with the observed response of one LISA
interferometer and assuming only doppler modulation due to the orbital motion
of LISA, we show how the distribution of the entire binary population in
frequency and sky position can be reconstructed using a tomographic approach.
The method is linear and the reconstruction of a delta function distribution,
corresponding to an isolated binary, yields a point spread function (psf). An
arbitrary distribution and its reconstruction are related via smoothing with
this psf. Exploratory results are reported demonstrating the recovery of binary
sources, in the presence of white Gaussian noise.Comment: 13 Pages and 9 figures high resolution figures can be obtains from
http://www.phys.utb.edu/~rajesh/lisa_tomography.pd
Wind mapping in Venus' upper mesosphere with the IRAM-Plateau de Bure interferometer
The dynamics of the upper mesosphere of Venus (~85-115 km) have been
characterized as a combination of a retrograde superrotating zonal wind (RSZ)
with a subsolar-to-antisolar flow (SSAS). Numerous mm-wave single-dish
observations have been obtained and could directly measure mesospheric
line-of-sight winds by mapping Doppler-shifts on CO rotational lines, but their
limited spatial resolution makes their interpretation difficult. By using
interferometric facilities, one can obtain better resolution on Doppler-shifts
maps, allowing in particular to put firmer constraints on the respective
contributions of the SSAS and RSZ circulations to the global mesospheric wind
field. We report on interferometric observations of the CO(1-0) line obtained
with the IRAM-Plateau de Bure interferometer in November 2007 and June 2009,
that could map the upper mesosphere dynamics on the morning hemisphere with a
very good spatial resolution (3.5-5.5"). All the obtained measurements show,
with a remarkably good temporal stability, that the wind globally flows in the
(sky) East-West direction, corresponding in the observed geometry either to an
unexpected prograde zonal wind or a SSAS flow. A very localized inversion of
the wind direction, that could correspond to a RSZ wind, is also repeatedly
detected in the night hemisphere. The presence of significant meridional winds
is not evidenced. Using models with different combinations of zonal and SSAS
winds, we find that the data is best reproduced by a dominant SSAS flow with a
maximal velocity at the terminator of ~200 m/s, displaying large diurnal and
latitudinal asymmetries, combined with an equatorial RSZ wind of 70-100 m/s,
overall indicating a wind-field structure consistent with but much more complex
than the usual representation of the mesospheric dynamics.Comment: Accepted for publication in A&
Electric field representation of pulsar intensity spectra
Pulsar dynamic spectra exhibit high visibility fringes arising from
interference between scattered radio waves. These fringes may be random or
highly ordered patterns, depending on the nature of the scattering or
refraction. Here we consider the possibility of decomposing pulsar dynamic
spectra -- which are intensity measurements -- into their constituent scattered
waves, i.e. electric field components. We describe an iterative method of
achieving this decomposition and show how the algorithm performs on data from
the pulsar B0834+06. The match between model and observations is good, although
not formally acceptable as a representation of the data. Scattered wave
components derived in this way are immediately useful for qualitative insights
into the scattering geometry. With some further development this approach can
be put to a variety of uses, including: imaging the scattering and refracting
structures in the interstellar medium; interstellar interferometric imaging of
pulsars at very high angular resolution; and mitigating pulse arrival time
fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA
On the reliability of polarization estimation using Rotation Measure Synthesis
We benchmark the reliability of the Rotation Measure (RM) synthesis algorithm
using the 1005 Centaurus A field sources of Feain et al. (2009). The RM
synthesis solutions are compared with estimates of the polarization parameters
using traditional methods. This analysis provides verification of the
reliability of RM synthesis estimates. We show that estimates of the
polarization parameters can be made at lower S/N if the range of RMs is
bounded, but reliable estimates of individual sources with unusual RMs require
unconstrainted solutions and higher S/N.
We derive from first principles the statistical properties of the
polarization amplitude associated with RM synthesis in the presence of noise.
The amplitude distribution depends explicitly on the amplitude of the
underlying (intrinsic) polarization signal. Hence it is necessary to model the
underlying polarization signal distribution in order to estimate the
reliability and errors in polarization parameter estimates. We introduce a
Bayesian method to derive the distribution of intrinsic amplitudes based on the
distribution of measured amplitudes.
The theoretically-derived distribution is compared with the empirical data to
provide quantitative estimates of the probability that an RM synthesis solution
is correct as a function of S/N. We provide quantitative estimates of the
probability that any given RM synthesis solution is correct as a function of
measured polarized amplitude and the intrinsic polarization amplitude compared
to the noise.Comment: accepted for publication in the Astrophysical Journa
LISA Data Analysis using MCMC methods
The Laser Interferometer Space Antenna (LISA) is expected to simultaneously
detect many thousands of low frequency gravitational wave signals. This
presents a data analysis challenge that is very different to the one
encountered in ground based gravitational wave astronomy. LISA data analysis
requires the identification of individual signals from a data stream containing
an unknown number of overlapping signals. Because of the signal overlaps, a
global fit to all the signals has to be performed in order to avoid biasing the
solution. However, performing such a global fit requires the exploration of an
enormous parameter space with a dimension upwards of 50,000. Markov Chain Monte
Carlo (MCMC) methods offer a very promising solution to the LISA data analysis
problem. MCMC algorithms are able to efficiently explore large parameter
spaces, simultaneously providing parameter estimates, error analyses and even
model selection. Here we present the first application of MCMC methods to
simulated LISA data and demonstrate the great potential of the MCMC approach.
Our implementation uses a generalized F-statistic to evaluate the likelihoods,
and simulated annealing to speed convergence of the Markov chains. As a final
step we super-cool the chains to extract maximum likelihood estimates, and
estimates of the Bayes factors for competing models. We find that the MCMC
approach is able to correctly identify the number of signals present, extract
the source parameters, and return error estimates consistent with Fisher
information matrix predictions.Comment: 14 pages, 7 figure
The remnant of SN1987A revealed at (sub-)mm wavelengths
Context: Supernova 1987A (SN1987A) exploded in the Large Magellanic Cloud
(LMC). Its proximity and rapid evolution makes it a unique case study of the
early phases in the development of a supernova remnant. One particular aspect
of interest is the possible formation of dust in SN1987A, as SNe could
contribute significantly to the dust seen at high redshifts. Aims: We explore
the properties of SN1987A and its circumburst medium as seen at mm and sub-mm
wavelengths, bridging the gap between extant radio and infrared (IR)
observations of respectively the synchrotron and dust emission. Methods:
SN1987A was observed with the Australia Telescope Compact Array (ATCA) at 3.2
mm in July 2005, and with the Atacama Pathfinder EXperiment (APEX) at 0.87 mm
in May 2007. We present the images and brightness measurements of SN1987A at
these wavelengths for the first time. Results: SN1987A is detected as an
unresolved point source of 11.2 +/- 2.0 mJy at 3.2 mm (5" beam) and 21 +/- 4
mJy at 0.87 mm (18" beam). These flux densities are in perfect agreement with
extrapolations of the powerlaw radio spectrum and modified-blackbody dust
emission, respectively. This places limits on the presence of free-free
emission, which is similar to the expected free-free emission from the ionized
ejecta from SN1987A. Adjacent, fainter emission is observed at 0.87 mm
extending ~0.5' towards the south-west. This could be the impact of the
supernova progenitor's wind when it was still a red supergiant upon a dense
medium. Conclusions: We have established a continuous spectral energy
distribution for the emission from SN1987A and its immediate surroundings,
linking the IR and radio data. This places limits on the contribution from
ionized plasma. Our sub-mm image reveals complexity in the distribution of cold
dust surrounding SN1987A, but leaves room for freshly synthesized dust in the
SN ejecta.Comment: Accepted for publication in Astronomy and Astrophysics Letters on 28
April 2011. A better quality figure 1 can be had from
http://www.astro.keele.ac.uk/~jacco/research/SN1987A087mm.ep
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
Reconstructing Images from Projections Using the Maximum-Entropy Method. Numerical Simulations of Low-Aspect Astrotomography
The reconstruction of images from a small number of projections using the
maximum-entropy method (MEM) with the Shannon entropy is considered. MEM
provides higher-quality image reconstruction for sources with extended
components than the Hogbom CLEAN method, which is also used in low-aspect
astrotomography. The quality of image reconstruction for sources with mixed
structure containing bright, compact features embedded in a comparatively weak,
extended base can be further improved using a difference-mapping method, which
requires a generalization of MEM for the reconstruction of sign-variable
functions.We draw conclusions based on the results of numerical simulations for
a number of model radio sources with various morphologies.Comment: 11 pages, 9 figure
HI distribution and kinematics of NGC 1569
We present WSRT observations of high sensitivity and resolution of the
neutral hydrogen in the starburst dwarf galaxy NGC 1569. Assuming a distance of
2.2 Mpc, we find a total HI mass of 1.3 * 10^8 M_sun to be distributed in the
form of a dense, clumpy ridge surrounded by more extended diffuse HI containing
a few additional discrete features, such as a Western HI Arm and an HI bridge
reaching out to a small counterrotating companion cloud. About 10% by mass of
all HI in NGC 1569 is at unusually high velocities. Some of this HI may be
associated with the mass outflow evident from H-alpha measurements, but some
may also be associated with NGC 1569's HI companion and intervening HI bridge,
in which case, infall rather than outflow might be the cause of the discrepant
velocities. No indication of a large bubble structure was found in
position-velocity maps of the high-velocity HI. The galaxy as a whole is in
modest overall rotation, but the HI gas lacks any sign of rotation within 60''
(0.6 kpc) from the center, i.e. over most of the optical galaxy. Here,
turbulent motions resulting from the starburst appear to dominate over
rotation. In the outer disk, the rotational velocities reach a maximum of 35
\pm 6 km/s, but turbulent motion remains significant. Thus, starburst effects
are still noticeable in the outer HI disk, although they are no longer dominant
beyond 0.6 kpc. Even excluding the most extreme high-velocity HI clouds, NGC
1569 still has an unusually high mean HI velocity dispersion of sigma_v=21.3
km/s, more than double that of other dwarf galaxies.Comment: Figure 11a,b and Figure 14 separately in jpg forma
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