5,327 research outputs found
Low-Complexity Iterative Detection for Orthogonal Time Frequency Space Modulation
We elaborate on the recently proposed orthogonal time frequency space (OTFS)
modulation technique, which provides significant advantages over orthogonal
frequency division multiplexing (OFDM) in Doppler channels. We first derive the
input--output relation describing OTFS modulation and demodulation (mod/demod)
for delay--Doppler channels with arbitrary number of paths, with given delay
and Doppler values. We then propose a low-complexity message passing (MP)
detection algorithm, which is suitable for large-scale OTFS taking advantage of
the inherent channel sparsity. Since the fractional Doppler paths (i.e., not
exactly aligned with the Doppler taps) produce the inter Doppler interference
(IDI), we adapt the MP detection algorithm to compensate for the effect of IDI
in order to further improve performance. Simulations results illustrate the
superior performance gains of OTFS over OFDM under various channel conditions.Comment: 6 pages, 7 figure
Importance Sketching of Influence Dynamics in Billion-scale Networks
The blooming availability of traces for social, biological, and communication
networks opens up unprecedented opportunities in analyzing diffusion processes
in networks. However, the sheer sizes of the nowadays networks raise serious
challenges in computational efficiency and scalability.
In this paper, we propose a new hyper-graph sketching framework for inflence
dynamics in networks. The central of our sketching framework, called SKIS, is
an efficient importance sampling algorithm that returns only non-singular
reverse cascades in the network. Comparing to previously developed sketches
like RIS and SKIM, our sketch significantly enhances estimation quality while
substantially reducing processing time and memory-footprint. Further, we
present general strategies of using SKIS to enhance existing algorithms for
influence estimation and influence maximization which are motivated by
practical applications like viral marketing. Using SKIS, we design high-quality
influence oracle for seed sets with average estimation error up to 10x times
smaller than those using RIS and 6x times smaller than SKIM. In addition, our
influence maximization using SKIS substantially improves the quality of
solutions for greedy algorithms. It achieves up to 10x times speed-up and 4x
memory reduction for the fastest RIS-based DSSA algorithm, while maintaining
the same theoretical guarantees.Comment: 12 pages, to appear in ICDM 2017 as a regular pape
On the 3-D structure and dissipation of reconnection-driven flow-bursts
The structure of magnetic reconnection-driven outflows and their dissipation
are explored with large-scale, 3-D particle-in-cell (PIC) simulations. Outflow
jets resulting from 3-D reconnection with a finite length x-line form fronts as
they propagate into the downstream medium. A large pressure increase ahead of
this ``reconnection jet front'' (RJF), due to reflected and transmitted ions,
slows the front so that its velocity is well below the velocity of the ambient
ions in the core of the jet. As a result, the RJF slows and diverts the
high-speed flow into the direction perpendicular to the reconnection plane. The
consequence is that the RJF acts as a thermalization site for the ion bulk flow
and contributes significantly to the dissipation of magnetic energy during
reconnection even though the outflow jet is subsonic. This behavior has no
counterpart in 2-D reconnection. A simple analytic model predicts the front
velocity and the fraction of the ion bulk flow energy that is dissipated
Discovery of Radio Emission from the Tight M8 Binary: LP 349-25
We present radio observations of 8 ultracool dwarfs with a narrow spectral
type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8
binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system
detected in radio and its trigonometric parallax pi = 67.6 mas, recently
measured by Gatewood et al., makes it the furthest ultracool system detected by
the Very Large Array to date, and the most radio-luminous outside of obvious
flaring activity or variability. With a separation of only 1.8 AU, masses of
the components of LP 349-25 can be measured precisely without any theoretical
assumptions (Forveille et al.), allowing us to clarify their fully-convective
status and hence the kind of magnetic dynamo in these components which may play
an important role to explain our detection of radio emission from these
objects. This also makes LP 349-25 an excellent target for further studies with
better constraints on the correlations between X-ray, radio emission and
stellar parameters such as mass, age, temperature, and luminosity in ultracool
dwarfs.Comment: accepted by ApJ, referee's comments included, typo in equation 1
correcte
SMA observations of the proto brown dwarf candidate SSTB213 J041757
Context. The previously identified source SSTB213 J041757 is a proto brown
dwarf candidate in Taurus, which has two possible components A and B. It was
found that component B is probably a class 0/I proto brown dwarf associated
with an extended envelope.
Aims. Studying molecular outflows from young brown dwarfs provides important
insight into brown dwarf formation mechanisms, particularly brown dwarfs at the
earliest stages such as class 0, I. We therefore conducted a search for
molecular outflows from SSTB213 J041757.
Methods. We observed SSTB213 J041757 with the Submillimeter Array to search
for CO molecular outflow emission from the source.
Results. Our CO maps do not show any outflow emission from the proto brown
dwarf candidate.
Conclusions. The non-detection implies that the molecular outflows from the
source are weak; deeper observations are therefore needed to probe the outflows
from the source.Comment: 7 pages, 4 figures, accepted for publication in A&
On the Cause of Supra-Arcade Downflows in Solar Flares
A model of supra-arcade downflows (SADs), dark low density regions also known
as tadpoles that propagate sunward during solar flares, is presented. It is
argued that the regions of low density are flow channels carved by
sunward-directed outflow jets from reconnection. The solar corona is
stratified, so the flare site is populated by a lower density plasma than that
in the underlying arcade. As the jets penetrate the arcade, they carve out
regions of depleted plasma density which appear as SADs. The present
interpretation differs from previous models in that reconnection is localized
in space but not in time. Reconnection is continuous in time to explain why
SADs are not filled in from behind as they would if they were caused by
isolated descending flux tubes or the wakes behind them due to temporally
bursty reconnection. Reconnection is localized in space because outflow jets in
standard two-dimensional reconnection models expand in the normal (inflow)
direction with distance from the reconnection site, which would not produce
thin SADs as seen in observations. On the contrary, outflow jets in spatially
localized three-dimensional reconnection with an out-of-plane (guide) magnetic
field expand primarily in the out-of-plane direction and remain collimated in
the normal direction, which is consistent with observed SADs being thin.
Two-dimensional proof-of-principle simulations of reconnection with an
out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion
regions and the necessity of density stratification. Three-dimensional
simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in
August, 2013. This version is the accepted versio
Super-Alfv\'enic propagation of reconnection signatures and Poynting flux during substorms
The propagation of reconnection signatures and their associated energy are
examined using kinetic particle-in-cell simulations and Cluster satellite
observations. It is found that the quadrupolar out-of-plane magnetic field near
the separatrices is associated with a kinetic Alfv\'en wave. For magnetotail
parameters, the parallel propagation of this wave is super-Alfv\'enic
(V_parallel ~ 1500 - 5500 km/s) and generates substantial Poynting flux (S ~
10^-5 - 10^-4 W/m^2) consistent with Cluster observations of magnetic
reconnection. This Poynting flux substantially exceeds that due to frozen-in
ion bulk outflows and is sufficient to generate white light aurora in the
Earth's ionosphere.Comment: Submitted to PRL on 11/1/2010. Resubmitted on 4/5/201
Detection of lithium in nearby young late-M dwarfs
Late M-type dwarfs in the solar neighborhood include a mixture of very
low-mass stars and brown dwarfs which is difficult to disentangle due to the
lack of constraints on their age such as trigonometric parallax, lithium
detection and space velocity.
We search for young brown dwarf candidates among a sample of 28 nearby late-M
dwarfs with spectral types between M5.0 and M9.0, and we also search for debris
disks around three of them.
Based on theoretical models, we used the color , the -band absolute
magnitude and the detection of the Li I 6708 doublet line as a strong
constraint to estimate masses and ages of our targets. For the search of debris
disks, we observed three targets at submillimeter wavelength of 850 m.
We report here the first clear detections of lithium absorption in four
targets and a marginal detection in one target. Our mass estimates indicate
that two of them are young brown dwarfs, two are young brown dwarf candidates
and one is a young very low-mass star. The closest young field brown dwarf in
our sample at only 15 pc is an excellent benchmark for further studying
physical properties of brown dwarfs in the range 100150 Myr. We did not
detect any debris disks around three late-M dwarfs, and we estimated upper
limits to the dust mass of debris disks around them.Comment: 10 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
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