1,682 research outputs found
Origin of the Scaling Law in Human Mobility: Hierarchical Organization of Traffic Systems
Uncovering the mechanism leading to the scaling law in human trajectories is
of fundamental importance in understanding many spatiotemporal phenomena. We
propose a hierarchical geographical model to mimic the real traffic system,
upon which a random walker will generate a power-law travel displacement
distribution with exponent -2. When considering the inhomogeneities of cities'
locations and attractions, this model reproduces a power-law displacement
distribution with an exponential cutoff, as well as a scaling behavior in the
probability density of having traveled a certain distance at a certain time.
Our results agree very well with the empirical observations reported in [D.
Brockmann et al., Nature 439, 462 (2006)].Comment: 6 figures, 4 page
An XMM-Newton study of the mixed-morphology supernova remnant W28 (G6.4-0.1)
We have performed an XMM-Newton imaging and spectroscopic study of supernova
remnant (SNR) W28, a prototype mixed-morphology or thermal composite SNR,
believed to be interacting with a molecular cloud. The observed hot X-ray
emitting plasma is characterized by low metal abundances, showing no evidence
of ejecta. The X-rays arising from the deformed northeast shell consist of a
thermal component with a temperature of keV plus a hard component of
either thermal (temperature keV) or non-thermal (photon index
-2.4) origin. The X-ray emission in the SNR interior is blobby and the
corresponding spectra are best described as the emission from a cold
( keV) plasma in non-equilibrium ionization with an ionization
timescale of cm s plus a hot ( keV)
gas in collisional ionization equilibrium. Applying the two-temperature model
to the smaller central regions, we find non-uniform interstellar absorption,
temperature and density distribution, which indicates that the remnant is
evolving in a non-uniform environment with denser material in the east and
north. The cloudlet evaporation mechanism can essentially explain the
properties of the X-ray emission in the center and thermal conduction may also
play a role for length scales comparable to the remnant radius. A recombining
plasma model with an electron temperature of keV is also feasible
for describing the hot central gas with the recombination age of the gas
estimated at yr.Comment: 16 pages, 7 figures, 5 tables, ApJ in pres
A Study of Fermi-LAT GeV gamma-ray Emission towards the Magnetar-harboring Supernova Remnant Kesteven 73 and Its Molecular Environment
We report our independent GeV gamma-ray study of the young shell-type
supernova remnant (SNR) Kes 73 which harbors a central magnetar, and CO-line
millimeter observations toward the SNR. Using 7.6 years of Fermi-LAT
observation data, we detected an extended gamma-ray source ("source A") with
the centroid on the west of the SNR, with a significance of 21.6 sigma in
0.1-300 GeV and an error circle of 5.4 arcminute in angular radius. The
gamma-ray spectrum cannot be reproduced by a pure leptonic emission or a pure
emission from the magnetar, and thus a hadronic emission component is needed.
The CO-line observations reveal a molecular cloud (MC) at V_LSR~90 km/s, which
demonstrates morphological correspondence with the western boundary of the SNR
brightened in multiwavelength. The 12CO (J=2-1)/12CO (J=1-0) ratio in the left
(blue) wing 85-88 km/s is prominently elevated to ~1.1 along the northwestern
boundary, providing kinematic evidence of the SNR-MC interaction. This SNR-MC
association yields a kinematic distance 9 kpc to Kes 73. The MC is shown to be
capable of accounting for the hadronic gamma-ray emission component. The
gamma-ray spectrum can be interpreted with a pure hadronic emission or a
magnetar+hadronic hybrid emission. In the case of pure hadronic emission, the
spectral index of the protons is 2.4, very similar to that of the
radio-emitting electrons, essentially consistent with the diffusive shock
acceleration theory. In the case of magnetar+hadronic hybrid emission, a
magnetic field decay rate >= 10^36 erg/s is needed to power the magnetar's
curvature radiation.Comment: 7 figures, published in Ap
Modeling Human Dynamics with Adaptive Interest
Recently, increasing empirical evidence indicates the extensive existence of
heavy tails in the interevent time distributions of various human behaviors.
Based on the queuing theory, the Barab\'asi model and its variations suggest
the highest-priority-first protocol a potential origin of those heavy tails.
However, some human activity patterns, also displaying the heavy-tailed
temporal statistics, could not be explained by a task-based mechanism. In this
paper, different from the mainstream, we propose an interest-based model. Both
the simulation and analysis indicate a power-law interevent time distribution
with exponent -1, which is in accordance with some empirical observations in
human-initiated systems.Comment: 3 figures and 4 page
A Model of Two-Way Selection System for Human Behavior
We propose a model of two-way selection system. It appears in the processes
like choosing a mate between men and women, making contracts between job
hunters and recruiters, and trading between buyers and sellers. In this paper,
we propose a model of two-way selection system, and present its analytic
solution for the expectation of successful matching total and the regular
pattern that the matching rate trends toward an inverse proportion to either
the ratio between the two sides or the ratio of the state total to the smaller
people number. The proposed model is verified by empirical data of the
matchmaking fairs. Results indicate that the model well predicts this typical
real-world two- way selection behavior to the bounded error extent, thus it is
helpful for understanding the dynamics mechanism of the real-world two-way
selection system.Comment: 8 pages, 4 figure
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