43,596 research outputs found
Inferring probabilistic stellar rotation periods using Gaussian processes
Variability in the light curves of spotted, rotating stars is often
non-sinusoidal and quasi-periodic --- spots move on the stellar surface and
have finite lifetimes, causing stellar flux variations to slowly shift in
phase. A strictly periodic sinusoid therefore cannot accurately model a
rotationally modulated stellar light curve. Physical models of stellar surfaces
have many drawbacks preventing effective inference, such as highly degenerate
or high-dimensional parameter spaces. In this work, we test an appropriate
effective model: a Gaussian Process with a quasi-periodic covariance kernel
function. This highly flexible model allows sampling of the posterior
probability density function of the periodic parameter, marginalising over the
other kernel hyperparameters using a Markov Chain Monte Carlo approach. To test
the effectiveness of this method, we infer rotation periods from 333 simulated
stellar light curves, demonstrating that the Gaussian process method produces
periods that are more accurate than both a sine-fitting periodogram and an
autocorrelation function method. We also demonstrate that it works well on real
data, by inferring rotation periods for 275 Kepler stars with previously
measured periods. We provide a table of rotation periods for these 1132 Kepler
objects of interest and their posterior probability density function samples.
Because this method delivers posterior probability density functions, it will
enable hierarchical studies involving stellar rotation, particularly those
involving population modelling, such as inferring stellar ages, obliquities in
exoplanet systems, or characterising star-planet interactions. The code used to
implement this method is available online.Comment: Submitted to MNRAS. Replaced 27/06/2017: corrections made to
koi_periods.cs
Plausible Mobility: Inferring Movement from Contacts
We address the difficult question of inferring plausible node mobility based
only on information from wireless contact traces. Working with mobility
information allows richer protocol simulations, particularly in dense networks,
but requires complex set-ups to measure, whereas contact information is easier
to measure but only allows for simplistic simulation models. In a contact trace
a lot of node movement information is irretrievably lost so the original
positions and velocities are in general out of reach. We propose a fast
heuristic algorithm, inspired by dynamic force-based graph drawing, capable of
inferring a plausible movement from any contact trace, and evaluate it on both
synthetic and real-life contact traces. Our results reveal that (i) the quality
of the inferred mobility is directly linked to the precision of the measured
contact trace, and (ii) the simple addition of appropriate anticipation forces
between nodes leads to an accurate inferred mobility.Comment: 8 pages, 8 figures, 1 tabl
A Complete Spectroscopic Survey of the Milky Way satellite Segue 1: Dark matter content, stellar membership and binary properties from a Bayesian analysis
We introduce a comprehensive analysis of multi-epoch stellar line-of-sight
velocities to determine the intrinsic velocity dispersion of the ultrafaint
satellites of the Milky Way. Our method includes a simultaneous Bayesian
analysis of both membership probabilities and the contribution of binary
orbital motion to the observed velocity dispersion within a 14-parameter
likelihood. We apply our method to the Segue 1 dwarf galaxy and conclude that
Segue 1 is a dark-matter-dominated galaxy at high probability with an intrinsic
velocity dispersion of 3.7^{+1.4}_{-1.1} km/sec. The dark matter halo required
to produce this dispersion must have an average density of 2.5^{+4.1}_{-1.9}
solar mass/pc^3 within a sphere that encloses half the galaxy's stellar
luminosity. This is the highest measured density of dark matter in the Local
Group. Our results show that a significant fraction of the stars in Segue 1 may
be binaries with the most probable mean period close to 10 years, but also
consistent with the 180 year mean period seen in the solar vicinity at about 1
sigma. Despite this binary population, the possibility that Segue 1 is a bound
star cluster with the observed velocity dispersion arising from the orbital
motion of binary stars is disfavored by the multi-epoch stellar velocity data
at greater than 99% C.L. Finally, our treatment yields a projected
(two-dimensional) half-light radius for the stellar profile of Segue 1 of
28^{+5}_{-4} pc, in excellent agreement with photometric measurements.Comment: 15 pages, 19 figure
Experimental Bayesian Quantum Phase Estimation on a Silicon Photonic Chip
Quantum phase estimation is a fundamental subroutine in many quantum
algorithms, including Shor's factorization algorithm and quantum simulation.
However, so far results have cast doubt on its practicability for near-term,
non-fault tolerant, quantum devices. Here we report experimental results
demonstrating that this intuition need not be true. We implement a recently
proposed adaptive Bayesian approach to quantum phase estimation and use it to
simulate molecular energies on a Silicon quantum photonic device. The approach
is verified to be well suited for pre-threshold quantum processors by
investigating its superior robustness to noise and decoherence compared to the
iterative phase estimation algorithm. This shows a promising route to unlock
the power of quantum phase estimation much sooner than previously believed
The Dawes Review 8: Measuring the Stellar Initial Mass Function
The birth of stars and the formation of galaxies are cornerstones of modern
astrophysics. While much is known about how galaxies globally and their stars
individually form and evolve, one fundamental property that affects both
remains elusive. This is problematic because this key property, the birth mass
distribution of stars, referred to as the stellar initial mass function (IMF),
is a key tracer of the physics of star formation that underpins almost all of
the unknowns in galaxy and stellar evolution. It is perhaps the greatest source
of systematic uncertainty in star and galaxy evolution. The past decade has
seen a growing number and variety of methods for measuring or inferring the
shape of the IMF, along with progressively more detailed simulations,
paralleled by refinements in the way the concept of the IMF is applied or
conceptualised on different physical scales. This range of approaches and
evolving definitions of the quantity being measured has in turn led to
conflicting conclusions regarding whether or not the IMF is universal. Here I
review and compare the growing wealth of approaches to our understanding of
this fundamental property that defines so much of astrophysics. I summarise the
observational measurements from stellar analyses, extragalactic studies and
cosmic constraints, and highlight the importance of considering potential IMF
variations, reinforcing the need for measurements to quantify their scope and
uncertainties carefully, in order for this field to progress. I present a new
framework to aid the discussion of the IMF and promote clarity in the further
development of this fundamental field.Comment: Accepted for publication in PASA. 52 pages, 10 figures. A bug in
pasa-mnras.bst causes references beginning with lower-case letters (e.g.,
"de", "van") to be placed at the end of the reference list, rather than
alphabetically. Kindly and skilled people are encouraged to correct this and
share with the PASA editor
PinMe: Tracking a Smartphone User around the World
With the pervasive use of smartphones that sense, collect, and process
valuable information about the environment, ensuring location privacy has
become one of the most important concerns in the modern age. A few recent
research studies discuss the feasibility of processing data gathered by a
smartphone to locate the phone's owner, even when the user does not intend to
share his location information, e.g., when the Global Positioning System (GPS)
is off. Previous research efforts rely on at least one of the two following
fundamental requirements, which significantly limit the ability of the
adversary: (i) the attacker must accurately know either the user's initial
location or the set of routes through which the user travels and/or (ii) the
attacker must measure a set of features, e.g., the device's acceleration, for
potential routes in advance and construct a training dataset. In this paper, we
demonstrate that neither of the above-mentioned requirements is essential for
compromising the user's location privacy. We describe PinMe, a novel
user-location mechanism that exploits non-sensory/sensory data stored on the
smartphone, e.g., the environment's air pressure, along with publicly-available
auxiliary information, e.g., elevation maps, to estimate the user's location
when all location services, e.g., GPS, are turned off.Comment: This is the preprint version: the paper has been published in IEEE
Trans. Multi-Scale Computing Systems, DOI: 0.1109/TMSCS.2017.275146
Understanding Internet topology: principles, models, and validation
Building on a recent effort that combines a first-principles approach to modeling router-level connectivity with a more pragmatic use of statistics and graph theory, we show in this paper that for the Internet, an improved understanding of its physical infrastructure is possible by viewing the physical connectivity as an annotated graph that delivers raw connectivity and bandwidth to the upper layers in the TCP/IP protocol stack, subject to practical constraints (e.g., router technology) and economic considerations (e.g., link costs). More importantly, by relying on data from Abilene, a Tier-1 ISP, and the Rocketfuel project, we provide empirical evidence in support of the proposed approach and its consistency with networking reality. To illustrate its utility, we: 1) show that our approach provides insight into the origin of high variability in measured or inferred router-level maps; 2) demonstrate that it easily accommodates the incorporation of additional objectives of network design (e.g., robustness to router failure); and 3) discuss how it complements ongoing community efforts to reverse-engineer the Internet
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