13,804 research outputs found
Chandra View of Pulsar Wind Nebula Tori
The results from a systematic study of eleven pulsar wind nebulae with a
torus structure observed with the Chandra X-ray observatory are presented. A
significant observational correlation is found between the radius of the tori,
r, and the spin-down luminosity of the pulsars, Edot. A logarithmic linear fit
between the two parameters yields log r = (0.57 +- 0.22) log Edot -22.3 +- 8.0
with a correlation coefficient of 0.82, where the units of r and Edot are pc
and ergs s^-1, respectively. The value obtained for the Edot dependency of r is
consistent with a square root law, which is theoretically expected. This is the
first observational evidence of this dependency, and provides a useful tool to
estimate the spin-down energies of pulsars without direct detections of
pulsation. Applications of this dependency to some other samples are also
shown.Comment: 8 pages, 2 figures, ApJ in pres
X-ray Evolution of SNR 1987A: The Radial Expansion
We present the evolution of the radial expansion of SNR 1987A as measured
using Chandra X-ray observations taken over the last 10 years. To characterize
the complex structure of the remnant and isolate the expansion measurement, we
fit the images to several empirical models including: a simple circular torus,
a torus with bilateral lobes, and a torus with four tangentially extended
lobes. We discuss the results of this measure in the context of the overall
evolution of the supernova remnant, for which we believe we have measured the
end of the free expansion phase and its transition to the adiabatic phase (at
least along the equatorial ring). The timing of this event is in agreement with
early predictions of the remnant evolution.Comment: Accepted for publication in ApJ, 21 pages, 4 figures, 3 table
E/B Separation in CMB Interferometry
We study the problem of separating E and B modes in interferometric
observations of the polarization of the cosmic microwave background. The E and
B band powers and their mixings are measured from both single-dish and
interferometric mock observations using the quadratic estimator of the maximum
likelihood analysis. We find that the interferometer can separate E and B modes
in a single-pointing measurement and is thus well suited for detecting the
faint lensing induced and gravity-wave induced B modes. In mosaicking
observation, compared to the single dish, the interferometer is in general more
efficient in separating E and B modes, and for high signal-to-noise per pixel
it needs about three times fewer pixels to measure extremely blue polarization
power spectra.Comment: 21 pages, 11 figures, ApJ in pres
Kondo effects in a C_60 single-molecule transistor
We have used the electromigration technique to fabricate a
single-molecule transistor (SMT). We present a full
experimental study as a function of temperature, down to 35 mK, and as a
function of magnetic field up to 8 T in a SMT with odd number of electrons,
where the usual spin-1/2 Kondo effect occurs, with good agreement with theory.
In the case of even number of electrons, a low temperature magneto-transport
study is provided, which demonstrates a Zeeman splitting of the zero-bias
anomaly at energies well below the Kondo scale.Comment: 4 pages, 3 figure
Secure agent data integrity shield
In the rapidly expanding field of E-Commerce, mobile agent is the emerging technology that addresses the requirement of intelligent filtering/processing of information. This paper will address the area of mobile agent data integrity protection. We propose the use of Secure Agent Data Integrity Shield (SADIS) as a scheme that protects the integrity of data collected during agent roaming. With the use of a key seed negotiation protocol and integrity protection protocol, SADIS protects the secrecy as well as the integrity of agent data. Any illegal data modification, deletion, or insertion can be detected either by the subsequent host or the agent butler. Most important of all, the identity of each malicious host can be established. To evaluate the feasibility of our design, a prototype has been developed using Java. The result of benchmarking shows improvement both in terms of data and time efficiency
Entropy of black holes in the deformed Ho\v{r}ava-Lifshitz gravity
We find the entropy of Kehagias-Sfetsos black hole in the deformed
Ho\v{r}ava-Lifshitz gravity by using the first law of thermodynamics. When
applying generalized uncertainty principle (GUP) to Schwarzschild black hole,
the entropy may be interpreted as the GUP-inspired
black hole entropy. Hence, it implies that the duality in the entropy between
the Kehagias-Sfetsos black hole and GUP-inspired Schwarzschild black hole is
present.Comment: 11 pages, version to appear in PL
Evolution of the Chandra CCD Spectra of SNR 1987A: Probing the Reflected-Shock Picture
We continue to explore the validity of the reflected shock structure (RSS)
picture in SNR 1987A that was proposed in our previous analyses of the X-ray
emission from this object. We used an improved version of our RSS model in a
global analysis of 14 CCD spectra from the monitoring program with Chandra. In
the framework of the RSS picture, we are able to match both the expansion
velocity curve deduced from the analysis of the X-ray images and light curve.
Using a simplified analysis, we also show that the X-rays and the non-thermal
radio emission may originate from the same shock structure (the blast wave). We
believe that using the RSS model in the analysis of grating data from the
Chandra monitoring program of SNR 1987A that cover a long enough time interval,
will allow us to build a more realistic physical picture and model of SNR
1987A.Comment: 14 pages, 1 Table, 8 figures, accepted for publication in MNRA
Faster k-Medoids Clustering: Improving the PAM, CLARA, and CLARANS Algorithms
Clustering non-Euclidean data is difficult, and one of the most used
algorithms besides hierarchical clustering is the popular algorithm
Partitioning Around Medoids (PAM), also simply referred to as k-medoids. In
Euclidean geometry the mean-as used in k-means-is a good estimator for the
cluster center, but this does not hold for arbitrary dissimilarities. PAM uses
the medoid instead, the object with the smallest dissimilarity to all others in
the cluster. This notion of centrality can be used with any (dis-)similarity,
and thus is of high relevance to many domains such as biology that require the
use of Jaccard, Gower, or more complex distances.
A key issue with PAM is its high run time cost. We propose modifications to
the PAM algorithm to achieve an O(k)-fold speedup in the second SWAP phase of
the algorithm, but will still find the same results as the original PAM
algorithm. If we slightly relax the choice of swaps performed (at comparable
quality), we can further accelerate the algorithm by performing up to k swaps
in each iteration. With the substantially faster SWAP, we can now also explore
alternative strategies for choosing the initial medoids. We also show how the
CLARA and CLARANS algorithms benefit from these modifications. It can easily be
combined with earlier approaches to use PAM and CLARA on big data (some of
which use PAM as a subroutine, hence can immediately benefit from these
improvements), where the performance with high k becomes increasingly
important.
In experiments on real data with k=100, we observed a 200-fold speedup
compared to the original PAM SWAP algorithm, making PAM applicable to larger
data sets as long as we can afford to compute a distance matrix, and in
particular to higher k (at k=2, the new SWAP was only 1.5 times faster, as the
speedup is expected to increase with k)
High Resolution X-Ray Spectroscopy of SN 1987A: Monitoring with XMM-Newton
We report the results of our XMM-Newton monitoring of SN 1987A. The ongoing
propagation of the supernova blast wave through the inner circumstellar ring
caused a drastic increase in X-ray luminosity during the last years, enabling
detailed high resolution X-ray spectroscopy with the Reflection Grating
Spectrometer. The observations can be used to follow the detailed evolution of
the arising supernova remnant. The fluxes and broadening of the numerous
emission lines seen in the dispersed spectra provide information on the
evolution of the X-ray emitting plasma and its dynamics. These were analyzed in
combination with the EPIC-pn spectra, which allow a precise determination of
the higher temperature plasma. We modeled individual emission lines and fitted
plasma emission models. Especially from the observations between 2003 and 2007
we can see a significant evolution of the plasma parameters and a deceleration
of the radial velocity of the lower temperature plasma regions. We found an
indication (3-sigma-level) of an iron K feature in the co-added EPIC-pn
spectra. The comparison with Chandra grating observations in 2004 yields a
clear temporal coherence of the spectral evolution and the sudden deceleration
of the expansion velocity seen in X-ray images ~6100 days after the explosion.Comment: 10 pages, 8 Figures; accepted by A&
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