1,504 research outputs found
Ca(r)veat Emptor: Crowdsourcing Data to Challenge the Testimony of In-Car Technology
This Article addresses the situation in which a car acts as a witness against its human driver in a court of law. This possibility has become a reality due to technology embedded in modern-day vehicles that captures data prior to a crash event. The authors contend that it is becoming increasingly difficult for drivers to defend themselves in a meaningful way against the testimony of cars and suggest that crowdsourcing data could be a viable option for assessing the trustworthiness of such evidence. The Article further explores whether crowdsourced data could be used as an additional source of information in the fact-finding process and if such data could provide a counterbalance to the prevailing tendency to fault human drivers rather than their vehicles or the manufactures of their vehicles. The practical importance of this issue in the age of driving automation is highlighted, and lawyers, judges, and lawmakers are urged to remain open-minded regarding the use of this new strategy
Local polynomial regression for circular predictors
We consider local smoothing of datasets where the design space is the d-dimensional (d >= 1) torus and the response variable is real-valued. Our purpose is to extend least squares local polynomial fitting to this situation. We give both theoretical and empirical results
Faint-end Quasar Luminosity Functions from Cosmological Hydrodynamic Simulations
We investigate the predictions for the faint-end quasar luminosity function
(QLF) and its evolution using fully cosmological hydrodynamic simulations which
self-consistently follow star formation, black hole growth and associated
feedback processes. We find remarkably good agreement between predicted and
observed faint end of the optical and X-ray QLFs (the bright end is not
accessible in our simulated volumes) at z < 2. At higher redshifts our
simulations tend to overestimate the QLF at the faintest luminosities. We show
that although the low (high) luminosity ranges of the faint-end QLF are
dominated by low (high) mass black holes, a wide range of black hole masses
still contributes to any given luminosity range. This is consistent with the
complex lightcurves of black holes resulting from the detailed hydrodynamics
followed in the simulations. Consistent with the results on the QLFs, we find
good agreement for the evolution of the comoving number density (in optical,
soft and hard X-ray bands) of AGN for luminosities above 10^43 erg/s. However,
the luminosity density evolution from the simulation appears to imply a peak at
higher redshift than constrained from hard X-ray data (but not in optical). Our
predicted excess at the faintest fluxes at z >= 2 does not lead to an
overestimate to the total X-ray background and its contribution is at most a
factor of two larger than the unresolved fraction of the 2-8 keV background.
Even though this could be explained by some yet undetected, perhaps heavily
obscured faint quasar population, we show that our predictions for the faint
sources at high redshifts (which are dominated by the low mass black holes) in
the simulations are likely affected by resolution effects.Comment: 12 pages, 9 figures; submitted and reviewed by MNRA
Quasars Are Not Light-Bulbs: Testing Models of Quasar Lifetimes with the Observed Eddington Ratio Distribution
We use the observed distribution of Eddington ratios as a function of
supermassive black hole (BH) mass to constrain models of AGN lifetimes and
lightcurves. Given the observed AGN luminosity function, a model for AGN
lifetimes (time above a given luminosity) translates directly to a predicted
Eddington ratio distribution. Models for self-regulated BH growth, in which
feedback produces a 'blowout' decay phase after some peak luminosity (shutting
down accretion) make specific predictions for the lifetimes distinct from those
expected if AGN are simply gas starved (without feedback) and very different
from simple phenomenological 'light bulb' models. Present observations of the
Eddington ratio distribution, spanning 5 decades in Eddington ratio, 3 in BH
mass, and redshifts z=0-1, agree with the predictions of self-regulated models,
and rule out 'light-bulb', pure exponential, and gas starvation models at high
significance. We compare the Eddington ratio distributions at fixed BH mass and
fixed luminosity (both are consistent, but the latter are much less
constraining). We present empirical fits to the lifetime distribution and show
how the Eddington ratio distributions place tight limits on AGN lifetimes at
various luminosities. We use this to constrain the shape of the typical AGN
lightcurve, and provide simple analytic fits. Given independent constraints on
episodic lifetimes, most local BHs must have gained their mass in no more than
a couple of bright episodes, in agreement with merger-driven fueling models.Comment: 21 pages, 13 figures, accepted to ApJ (revised to match accepted
version; modeling and tests of redshift evolution added
Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton
We have identified four new black hole candidates in M31 globular clusters
using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The
X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the
highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts
from the neutron star transients Aql X-1 or 4U 1608-452; the outburst
apparently started ~45 days earlier than the observed peak, hence the
luminosity could have been considerably higher. We identified XB082, XB153 and
XB185 as BHCs by observing "low state" emission spectra at luminosities that
exceed the threshold for neutron star binaries. The probability that these are
neutron star systems with anisotropic emission beamed toward us is <4E-4, and
their variability suggests emission from a single source. We therefore conclude
that these systems likely contain black holes rather than neutron stars. We
have now identified 4 persistently bright BHCs in the region; the probability
that these are all background AGN is <1E-20. According to theory, the donors
could be tidally captured main sequence stars, or white dwarves in
ultra-compact binaries. We find that GCs that are particularly massive (XB082)
or metal rich (XB144) can host bright X-ray sources in addition to those that
are both (XB163). Our method may reveal BHCs in other bright X-ray sources.Comment: Accepted for publication in ApJ. 17 pages, 5 figure
Kernel density classification and boosting: an L2 sub analysis
Kernel density estimation is a commonly used approach to classification. However, most of the theoretical results for kernel methods apply to estimation per se and not necessarily to classification. In this paper we show that when estimating the difference between two densities, the optimal smoothing parameters are increasing functions of the sample size of the complementary group, and we provide a small simluation study which examines the relative performance of kernel density methods when the final goal is classification. A relative newcomer to the classification portfolio is “boosting”, and this paper proposes an algorithm for boosting kernel density classifiers. We note that boosting is closely linked to a previously proposed method of bias reduction in kernel density estimation and indicate how it will enjoy similar properties for classification. We show that boosting kernel classifiers reduces the bias whilst only slightly increasing the variance, with an overall reduction in error. Numerical examples and simulations are used to illustrate the findings, and we also suggest further areas of research
Single or Double Degenerate Progenitors? Searching for Shock Emission in the SDSS-II Type Ia Supernovae
From the set of nearly 500 spectroscopically confirmed type~Ia supernovae and
around 10,000 unconfirmed candidates from SDSS-II, we select a subset of 108
confirmed SNe Ia with well-observed early-time light curves to search for
signatures from shock interaction of the supernova with a companion star. No
evidence for shock emission is seen; however, the cadence and photometric noise
could hide a weak shock signal. We simulate shocked light curves using SN Ia
templates and a simple, Gaussian shock model to emulate the noise properties of
the SDSS-II sample and estimate the detectability of the shock interaction
signal as a function of shock amplitude, shock width, and shock fraction. We
find no direct evidence for shock interaction in the rest-frame -band, but
place an upper limit on the shock amplitude at 9% of supernova peak flux ( mag). If the single degenerate channel dominates type~Ia progenitors,
this result constrains the companion stars to be less than about 6
on the main sequence, and strongly disfavors red giant companions.Comment: 28 pages, 3 figure
Kernel density estimation on the torus
Kernel density estimation for multivariate, circular data has been formulated only when the sample space is the sphere, but theory for the torus would also be useful. For data lying on a d-dimensional torus (d >= 1), we discuss kernel estimation of a density, its mixed partial derivatives, and their squared functionals. We introduce a specific class of product kernels whose order is suitably defined in such a way to obtain L-2-risk formulas whose structure can be compared to their Euclidean counterparts. Our kernels are based on circular densities; however, we also discuss smaller bias estimation involving negative kernels which are functions of circular densities. Practical rules for selecting the smoothing degree, based on cross-validation, bootstrap and plug-in ideas are derived. Moreover, we provide specific results on the use of kernels based on the von Mises density. Finally, real-data examples and simulation studies illustrate the findings
SN 2011hw: Helium-Rich Circumstellar Gas and the Luminous Blue Variable to Wolf-Rayet Transition in Supernova Progenitors
We present optical photometry and spectroscopy of the peculiar Type IIn/Ibn
supernova SN2011hw. Its light curve exhibits a slower decline rate than normal
SNeIbc, with a peak absolute magnitude of -19.5 (unfiltered) and a secondary
peak of -18.3 mag (R). Spectra of SN2011hw are unusual compared to normal SN
types, most closely resembling the spectra of SNeIbn. We center our analysis on
comparing SN 2011hw to the well-studied TypeIbn SN2006jc. While the two SNe
have many important similarities, the differences are quite telling: compared
to SN2006jc, SN2011hw has weaker HeI and CaII lines and relatively stronger H
lines, its light curve has a higher luminosity and slower decline rate, and
emission lines associated with the progenitor's CSM are narrower. One can
reproduce the unusual continuum shape of SN2011hw with equal contributions of a
6000K blackbody and a spectrum of SN2006jc. We attribute this emission
component and many other differences between the two SNe to extra opacity from
a small amount of additional H in SN2011hw, analogous to the small H mass that
makes SNeIIb differ from SNeIb. Slower speeds in the CSM and elevated H content
suggest a connection between the progenitor of SN2011hw and the class of
Ofpe/WN9 stars, which have been associated with LBVs in their hot quiescent
phases between outbursts, and are H-poor - but not H-free like classical
Wolf-Rayet (WR) stars. We conclude that the similarities and differences
between SN2011hw and SN2006jc can be largely understood if their progenitors
exploded at different points in the transitional evolution from an LBV to a WR
star.Comment: 11 pages, 7 figures, submitted to MNRA
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