63 research outputs found
Maximum Fidelity
The most fundamental problem in statistics is the inference of an unknown
probability distribution from a finite number of samples. For a specific
observed data set, answers to the following questions would be desirable: (1)
Estimation: Which candidate distribution provides the best fit to the observed
data?, (2) Goodness-of-fit: How concordant is this distribution with the
observed data?, and (3) Uncertainty: How concordant are other candidate
distributions with the observed data? A simple unified approach for univariate
data that addresses these traditionally distinct statistical notions is
presented called "maximum fidelity". Maximum fidelity is a strict frequentist
approach that is fundamentally based on model concordance with the observed
data. The fidelity statistic is a general information measure based on the
coordinate-independent cumulative distribution and critical yet previously
neglected symmetry considerations. An approximation for the null distribution
of the fidelity allows its direct conversion to absolute model concordance (p
value). Fidelity maximization allows identification of the most concordant
model distribution, generating a method for parameter estimation, with
neighboring, less concordant distributions providing the "uncertainty" in this
estimate. Maximum fidelity provides an optimal approach for parameter
estimation (superior to maximum likelihood) and a generally optimal approach
for goodness-of-fit assessment of arbitrary models applied to univariate data.
Extensions to binary data, binned data, multidimensional data, and classical
parametric and nonparametric statistical tests are described. Maximum fidelity
provides a philosophically consistent, robust, and seemingly optimal foundation
for statistical inference. All findings are presented in an elementary way to
be immediately accessible to all researchers utilizing statistical analysis.Comment: 66 pages, 32 figures, 7 tables, submitte
Analytical model for macromolecular partitioning during yeast cell division
Background: Asymmetric cell division, whereby a parent cell generates two sibling cells with unequal content and thereby distinct fates, is central to cell differentiation, organism development and ageing. Unequal partitioning of the macromolecular content of the parent cell â which includes proteins, DNA, RNA, large proteinaceous assemblies and organelles â can be achieved by both passive (e.g. diffusion, localized retention sites) and active (e.g. motor-driven transport) processes operating in the presence of external polarity cues, internal asymmetries, spontaneous symmetry breaking, or stochastic effects. However, the quantitative contribution of different processes to the partitioning of macromolecular content is difficult to evaluate. Results: Here we developed an analytical model that allows rapid quantitative assessment of partitioning as a function of various parameters in the budding yeast Saccharomyces cerevisiae. This model exposes quantitative degeneracies among the physical parameters that govern macromolecular partitioning, and reveals regions of the solution space where diffusion is sufficient to drive asymmetric partitioning and regions where asymmetric partitioning can only be achieved through additional processes such as motor-driven transport. Application of the model to different macromolecular assemblies suggests that partitioning of protein aggregates and episomes, but not prions, is diffusion-limited in yeast, consistent with previous reports. Conclusions: In contrast to computationally intensive stochastic simulations of particular scenarios, our analytical model provides an efficient and comprehensive overview of partitioning as a function of global and macromolecule-specific parameters. Identification of quantitative degeneracies among these parameters highlights the importance of their careful measurement for a given macromolecular species in order to understand the dominant processes responsible for its observed partitioning
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X-Ray Spectroscopy of Astrophysical Plasmas
We provide a qualitative review of key Xâray spectral diagnostics of astrophysical plasmas. We begin with a brief discussion of the two major types of equilibria, collisional ionization and photoionization, and then consider the behaviour of hydrogenâlike, heliumâlike, iron Lâshell and iron Kâshell transitions for these separate cases. Where possible, we discuss explicit examples using highâresolution spectra acquired by the grating instruments on the Chandra and XMMâNewton observatories
New Constraint on Open Cold-Dark-Matter Models
We calculate the large-angle cross-correlation between the
cosmic-microwave-background (CMB) temperature and the x-ray-background (XRB)
intensity expected in an open Universe with cold dark matter (CDM) and a nearly
scale-invariant spectrum of adiabatic density perturbations. Results are
presented as a function of the nonrelativistic-matter density (in
units of the critical density) and the x-ray bias (evaluated at a
redshift in evolving-bias models) for both an open Universe and a
flat cosmological-constant Universe. Recent experimental upper limits to the
amplitude of this cross-correlation provide a new constraint to the
- parameter space that open-CDM models (and the open-inflation
models that produce them) must satisfy.Comment: 4 pages, LaTeX. Revised version contains additional figure that
clarifies new constraint. (To appear in PRL.
Optical and Radio Properties of Extragalactic Sources Observed by the FIRST and SDSS Surveys
We discuss the optical and radio properties of 30,000 FIRST sources
positionally associated with an SDSS source in 1230 deg of sky. The
majority (83%) of the FIRST sources identified with an SDSS source brighter
than r=21 are optically resolved. We estimate an upper limit of 5% for the
fraction of quasars with broad-band optical colors indistinguishable from those
of stars. The distribution of quasars in the radio flux -- optical flux plane
supports the existence of the "quasar radio-dichotomy"; 8% of all quasars with
i<18.5 are radio-loud and this fraction seems independent of redshift and
optical luminosity. The radio-loud quasars have a redder median color by 0.08
mag, and a 3 times larger fraction of objects with red colors. FIRST galaxies
represent 5% of all SDSS galaxies with r<17.5, and 1% for r<20, and are
dominated by red galaxies. Magnitude and redshift limited samples show that
radio galaxies have a different optical luminosity distribution than non-radio
galaxies selected by the same criteria; when galaxies are further separated by
their colors, this result remains valid for both blue and red galaxies. The
distributions of radio-to-optical flux ratio are similar for blue and red
galaxies in redshift-limited samples; this similarity implies that the
difference in their luminosity functions, and resulting selection effects, are
the dominant cause for the preponderance of red radio galaxies in flux-limited
samples. We confirm that the AGN-to-starburst galaxy number ratio increases
with radio flux, and find that radio emission from AGNs is more concentrated
than radio emission from starburst galaxies (abridged).Comment: submitted to AJ, color gif figures, PS figures available from
[email protected]
XMM-Newton Reflection Grating Spectrometer Observations of Discrete Soft-X-ray Emission Features from NGC 1068
We present the first high-resolution, soft-X-ray spectrum of the prototypical
Seyfert 2 galaxy, NGC 1068. This spectrum was obtained with the XMM-Newton
Reflection Grating Spectrometer. Emission lines from H-like and He-like low-Z
ions (from C to Si) and Fe-L-shell ions dominate the spectrum. Strong, narrow
radiative recombination continua (RRC) for several ions are also present,
implying that most of the observed soft-X-ray emission arises in
low-temperature (few eV) plasma. This plasma is photoionized by the inferred
nuclear continuum (obscured along our line of sight), as in the unified model
of active galactic nuclei (AGN). We find excess emission (compared with pure
recombination) in all resonance lines (np to 1s) up to the photoelectric edge,
demonstrating the importance of photoexcitation as well. We introduce a simple
model of a cone of plasma irradiated by the nuclear continuum; the line
emission we observe along our line of sight perpendicular to the cone is
produced through recombination/radiative cascade following photoionization and
radiative decay following photoexcitation. A remarkably good fit is obtained to
the H-like/He-like ionic line series, with inferred radial ionic column
densities consistent with recent observations of warm absorbers in Seyfert 1
galaxies. Previous Chandra imaging revealed a large (extending out to 500 pc)
ionization cone containing most of the X-ray flux, implying that the warm
absorber in NGC 1068 is a large-scale outflow. To explain the ionic column
densities, a broad, flat distribution in the logarithm of the ionization
parameter () is necessary, spanning --3. This
suggests either radially-stratified ionization zones or the existence of a
broad density distribution (spanning a few orders of magnitude) at each radius.Comment: 23 pages, 16 figures, ApJ (accepted). XSPEC local model "photo" is
available at http://xmm.astro.columbia.edu/research.htm
Uncovering cis Regulatory Codes Using Synthetic Promoter Shuffling
Revealing the spectrum of combinatorial regulation of transcription at individual promoters is essential for understanding the complex structure of biological networks. However, the computations represented by the integration of various molecular signals at complex promoters are difficult to decipher in the absence of simple cis regulatory codes. Here we synthetically shuffle the regulatory architecture â operator sequences binding activators and repressors â of a canonical bacterial promoter. The resulting library of complex promoters allows for rapid exploration of promoter encoded logic regulation. Among all possible logic functions, NOR and ANDN promoter encoded logics predominate. A simple transcriptional cis regulatory code determines both logics, establishing a straightforward map between promoter structure and logic phenotype. The regulatory code is determined solely by the type of transcriptional regulation combinations: two repressors generate a NOR: NOT (a OR b) whereas a repressor and an activator generate an ANDN: a AND NOT b. Three-input versions of both logics, having an additional repressor as an input, are also present in the library. The resulting complex promoters cover a wide dynamic range of transcriptional strengths. Synthetic promoter shuffling represents a fast and efficient method for exploring the spectrum of complex regulatory functions that can be encoded by complex promoters. From an engineering point of view, synthetic promoter shuffling enables the experimental testing of the functional properties of complex promoters that cannot necessarily be inferred ab initio from the known properties of the individual genetic components. Synthetic promoter shuffling may provide a useful experimental tool for studying naturally occurring promoter shuffling
Direct Search for Dark Matter - Striking the Balance - and the Future
Weakly Interacting Massive Particles (WIMPs) are among the main candidates
for the relic dark matter (DM). The idea of the direct DM detection relies on
elastic spin-dependent (SD) and spin-independent (SI) interaction of WIMPs with
target nuclei. In this review paper the relevant formulae for WIMP event rate
calculations are collected. For estimations of the WIMP-proton and WIMP-neutron
SD and SI cross sections the effective low-energy minimal supersymmetric
standard model is used. The traditional one-coupling-dominance approach for
evaluation of the exclusion curves is described. Further, the mixed spin-scalar
coupling approach is discussed. It is demonstrated, taking the high-spin Ge-73
dark matter experiment HDMS as an example, how one can drastically improve the
sensitivity of the exclusion curves within the mixed spin-scalar coupling
approach, as well as due to a new procedure of background subtraction from the
measured spectrum. A general discussion on the information obtained from
exclusion curves is given. The necessity of clear WIMP direct detection
signatures for a solution of the dark matter problem, is pointed out.Comment: LaTeX, 49 pages, 14 figures, 185 reference
Can a Dusty Warm Absorber Model Reproduce the Soft X-ray Spectra of MCG-6-30-15 and Mrk 766?
XMM-Newton RGS spectra of MCG-6-30-15 and Mrk 766 exhibit complex discrete
structure, which was interpreted in a paper by Branduardi-Raymont et al. (2001)
as evidence for the existence of relativistically broadened Lyman alpha
emission from carbon, nitrogen, and oxygen, produced in the inner-most regions
of an accretion disk around a Kerr black hole. This suggestion was subsequently
criticized in a paper by Lee et al. (2001), who argued that for MCG-6-30-15,
the Chandra HETG spectrum, which is partially overlapping the RGS in spectral
coverage, is adequately fit by a dusty warm absorber model, with no
relativistic line emission. We present a reanalysis of the original RGS data
sets in terms of the Lee et al. (2001) model, and demonstrate that spectral
models consisting of a smooth continuum with ionized and dust absorption alone
cannot reproduce the RGS spectra of both objects. The original relativistic
line model with warm absorption proposed by Branduardi-Raymont et al. (2001)
provides a superior fit to the RGS data, both in the overall shape of the
spectrum and in the discrete absorption lines. Limits on the amount of X-ray
absorption by dust particles are discussed. We also discuss a possible
theoretical interpretation for the putative relativistic Lyman alpha line
emission in terms of the photoionized surface layers of the inner regions of an
accretion disk.Comment: Replaced with accepted version. To appear in ApJ; tentatively
scheduled for the v596 Oct. 10, 2003 issu
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