1,658 research outputs found
The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
We present an analysis of the distribution of H-alpha emission measures for
the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin
H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic
latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude
classical H II regions, leaving only sightlines that sample the diffuse WIM.
The distribution of EM sin |b| for the full sample is poorly characterized by a
single normal distribution, but is extraordinarily well fit by a lognormal
distribution, with = 0.146 +/- 0.001 and standard deviation
0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic
latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The
distribution may widen slightly at low Galactic latitude. We compare the
observed EM distribution function to the predictions of three-dimensional
magnetohydrodynamic simulations of isothermal turbulence within a
non-stratified interstellar medium. We find that the distribution of EM sin |b|
is well described by models of mildy supersonic turbulence with a sonic Mach
number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field
strength. The model also successfully predicts the distribution of dispersion
measures of pulsars and H-alpha line profiles. In the best fitting model, the
turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8
kpc scale height of the layer. The WIM gas has a lognormal distribution of
densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also
discuss the implications of these results for interpreting the filling factor,
the power requirement, and the magnetic field of the WIM.Comment: 16 pages, 13 figures, ApJ in press. Replacement reflects version
accepted for publicatio
High-Lundquist Number Scaling in Three-Dimensional Simulations of Parker's Model of Coronal Heating
Parker's model is one of the most discussed mechanisms for coronal heating
and has generated much debate. We have recently obtained new scaling results in
a two-dimensional (2D) version of this problem suggesting that the heating rate
becomes independent of resistivity in a statistical steady state [Ng and
Bhattacharjee, Astrophys. J., 675, 899 (2008)]. Our numerical work has now been
extended to 3D by means of large-scale numerical simulations. Random
photospheric footpoint motion is applied for a time much longer than the
correlation time of the motion to obtain converged average coronal heating
rates. Simulations are done for different values of the Lundquist number to
determine scaling. In the high-Lundquist number limit, the coronal heating rate
obtained so far is consistent with a trend that is independent of the Lundquist
number, as predicted by previous analysis as well as 2D simulations. In the
same limit the average magnetic energy built up by the random footpoint motion
tends to have a much weaker dependence on the Lundquist number than that in the
2D simulations, due to the formation of strong current layers and subsequent
disruption when the equilibrium becomes unstable. We will present scaling
analysis showing that when the dissipation time is comparable or larger than
the correlation time of the random footpoint motion, the heating rate tends to
become independent of Lundquist number, and that the magnetic energy production
is also reduced significantly.Comment: Accepted for publication in Astrophysical Journa
Percolation with excluded small clusters and Coulomb blockade in a granular system
We consider dc-conductivity of a mixture of small conducting and
insulating grains slightly below the percolation threshold, where finite
clusters of conducting grains are characterized by a wide spectrum of sizes.
The charge transport is controlled by tunneling of carriers between neighboring
conducting clusters via short ``links'' consisting of one insulating grain.
Upon lowering temperature small clusters (up to some -dependent size) become
Coulomb blockaded, and are avoided, if possible, by relevant hopping paths. We
introduce a relevant percolational problem of next-nearest-neighbors (NNN)
conductivity with excluded small clusters and demonstrate (both numerically and
analytically) that decreases as power law of the size of excluded
clusters. As a physical consequence, the conductivity is a power-law function
of temperature in a wide intermediate temperature range. We express the
corresponding index through known critical indices of the percolation theory
and confirm this relation numerically.Comment: 7 pages, 6 figure
Magnetic Field Effects near the launching region of Astrophysical Jets
One of the fundamental properties of astrophysical magnetic fields is their
ability to change topology through reconnection and in doing so, to release
magnetic energy, sometimes violently. In this work, we review recent results on
the role of magnetic reconnection and associated heating and particle
acceleration in jet/accretion disk systems, namely young stellar objects
(YSOs), microquasars, and active galactic nuclei (AGNs).Comment: 9 pages, 3 figures, invited paper for the Procs. of the Conference on
High Energy Phenomena in Relativistic Outflows II, Buenos Aires, October
2009. Submitted to International Journal of Modern Physics
Compressive Sensing of Signals Generated in Plastic Scintillators in a Novel J-PET Instrument
The J-PET scanner, which allows for single bed imaging of the whole human
body, is currently under development at the Jagiellonian University. The dis-
cussed detector offers improvement of the Time of Flight (TOF) resolution due
to the use of fast plastic scintillators and dedicated electronics allowing for
sam- pling in the voltage domain of signals with durations of few nanoseconds.
In this paper we show that recovery of the whole signal, based on only a few
samples, is possible. In order to do that, we incorporate the training signals
into the Tikhonov regularization framework and we perform the Principal
Component Analysis decomposition, which is well known for its compaction
properties. The method yields a simple closed form analytical solution that
does not require iter- ative processing. Moreover, from the Bayes theory the
properties of regularized solution, especially its covariance matrix, may be
easily derived. This is the key to introduce and prove the formula for
calculations of the signal recovery error. In this paper we show that an
average recovery error is approximately inversely proportional to the number of
acquired samples
Application of the Compress Sensing Theory for Improvement of the TOF Resolution in a Novel J-PET Instrument
Nowadays, in Positron Emission Tomography (PET) systems, a Time of Flight
information is used to improve the image reconstruction process. In Time of
Flight PET (TOF-PET), fast detectors are able to measure the difference in the
arrival time of the two gamma rays, with the precision enabling to shorten
significantly a range along the line-of-response (LOR) where the annihilation
occurred. In the new concept, called J-PET scanner, gamma rays are detected in
plastic scintillators. In a single strip of J-PET system, time values are
obtained by probing signals in the amplitude domain. Owing to Compress Sensing
theory, information about the shape and amplitude of the signals is recovered.
In this paper we demonstrate that based on the acquired signals parameters, a
better signal normalization may be provided in order to improve the TOF
resolution. The procedure was tested using large sample of data registered by a
dedicated detection setup enabling sampling of signals with 50 ps intervals.
Experimental setup provided irradiation of a chosen position in the plastic
scintillator strip with annihilation gamma quanta
Can jets make the radioactively powered emission from neutron star mergers bluer?
Neutron star mergers eject neutron-rich matter in which heavy elements are synthesized. The decay of these freshly synthesized elements powers electromagnetic transients ('macronovae' or 'kilonovae') whose luminosity and colour strongly depend on their nuclear composition. If the ejecta are very neutron-rich (electron fraction Ye < 0.25), they contain fair amounts of lanthanides and actinides that have large opacities and therefore efficiently trap the radiation inside the ejecta so that the emission peaks in the red part of the spectrum. Even small amounts of this high-opacity material can obscure emission from lower lying material and therefore act as a 'lanthanide curtain'. Here, we investigate how a relativistic jet that punches through the ejecta can potentially push away a significant fraction of the high opacity material before the macronova begins to shine. We use the results of detailed neutrino-driven wind studies as initial conditions and explore with 3D special relativistic hydrodynamic simulations how jets are propagating through these winds. Subsequently, we perform Monte Carlo radiative transfer calculations to explore the resulting macronova emission. We find that the hole punched by the jet makes the macronova brighter and bluer for on-axis observers during the first few days of emission, and that more powerful jets have larger impacts on the macronova
Analysis framework for the J-PET scanner
J-PET analysis framework is a flexible, lightweight, ROOT-based software
package which provides the tools to develop reconstruction and calibration
procedures for PET tomography. In this article we present the implementation of
the full data-processing chain in the J-PET framework which is used for the
data analysis of the J-PET tomography scanner. The Framework incorporates
automated handling of PET setup parameters' database as well as high level
tools for building data reconstruction procedures. Each of these components is
briefly discussed.Comment: 6 pages, 1 figur
Hit time and hit position reconstruction in the J-PET detector based on a library of averaged model signals
In this article we present a novel method of hit time and hit position
reconstruction in long scintillator detectors. We take advantage of the fact
that for this kind of detectors amplitude and shape of registered signals
depends strongly on the position where particle hit the detector. The
reconstruction is based on determination of the degree of similarity between
measured and averaged signals stored in a library for a set of well-defined
positions along the scintillator. Preliminary results of validation of the
introduced method with experimental data obtained by means of the double strip
prototype of the J-PET detector are presented
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