31,262 research outputs found
Contamination of stellar-kinematic samples and uncertainty about dark matter annihilation profiles in ultrafaint dwarf galaxies: the example of Segue I
The expected gamma-ray flux coming from dark matter annihilation in dwarf
spheroidal (dSph) galaxies depends on the so-called `J-factor', the integral of
the squared dark matter density along the line-of-sight. We examine the degree
to which estimates of J are sensitive to contamination (by foreground Milky Way
stars and stellar streams) of the stellar-kinematic samples that are used to
infer dark matter densities in `ultrafaint' dSphs. Applying standard kinematic
analyses to hundreds of mock data sets that include varying levels of
contamination, we find that mis-classified contaminants can cause J-factors to
be overestimated by orders of magnitude. Stellar-kinematic data sets for which
we obtain such biased estimates tend 1) to include relatively large fractions
of stars with ambiguous membership status, and 2) to give estimates for J that
are sensitive to specific choices about how to weight and/or to exclude stars
with ambiguous status. Comparing publicly-available stellar-kinematic samples
for the nearby dSphs Reticulum~II and Segue~I, we find that only the latter
displays both of these characteristics. Estimates of Segue~I's J-factor should
therefore be regarded with a larger degree of caution when planning and
interpreting gamma-ray observations. Moreover, robust interpretations regarding
dark matter annihilation in dSph galaxies in general will require explicit
examination of how interlopers might affect the inferred dark matter density
profile.Comment: 12 pages, 8 figures. New appendix A (joint light/dark matter
likelihood), results unchanged. Match accepted MNRAS versio
Spherical Jeans analysis for dark matter indirect detection in dwarf spheroidal galaxies - Impact of physical parameters and triaxiality
Dwarf spheroidal (dSph) galaxies are among the most promising targets for the
indirect detection of dark matter (DM) from annihilation and/or decay products.
Empirical estimates of their DM content - and hence the magnitudes of expected
signals - rely on inferences from stellar-kinematic data. However, various
kinematic analyses can give different results and it is not obvious which are
most reliable. Using extensive sets of mock data of various sizes (mimicking
'ultra-faint' and 'classical' dSphs) and an MCMC engine, here we investigate
biases, uncertainties, and limitations of analyses based on parametric
solutions to the spherical Jeans equation. For a variety of functional forms
for the tracer and DM density profiles, as well as the orbital anisotropy
profile, we examine reliability of estimates for the astrophysical J- and
D-factors for annihilation and decay, respectively. For large (N > 1000)
stellar-kinematic samples typical of 'classical' dSphs, errors tend to be
dominated by systematics, which can be reduced through the use of sufficiently
general and flexible functional forms. For small (N < 100) samples typical of
'ultrafaints', statistical uncertainties tend to dominate systematic errors and
flexible models are less necessary. We define an optimal strategy that would
mitigate sensitivity to priors and other aspects of analyses based on the
spherical Jeans equation. We also find that the assumption of spherical
symmetry can bias estimates of J (with the 95% credibility intervals not
encompassing the true J-factor) when the object is mildly triaxial (axis ratios
b/a = 0.8, c/a = 0.6). A concluding table summarises the typical error budget
and biases for the different sample sizes considered.Comment: 21 pages, 20 figures. Minor changes (several clarifications): match
the MNRAS accepted versio
A Supersymmetric Flipped SU(5) Intersecting Brane World
We construct an N=1 supersymmetric three-family flipped SU(5) model from type
IIA orientifolds on with D6-branes intersecting at
general angles. The spectrum contains a complete grand unified and electroweak
Higgs sector. In addition, it contains extra exotic matter both in
bi-fundamental and vector-like representations as well as two copies of matter
in the symmetric representation of SU(5).Comment: 17 pages, 3 tables, v2 published in Phys.Lett.
Stellar Mixing and the Primordial Lithium Abundance
We compare the properties of recent samples of the lithium abundances in halo
stars to one another and to the predictions of theoretical models including
rotational mixing, and we examine the data for trends with metal abundance. We
find from a KS test that in the absence of any correction for chemical
evolution, the Ryan, Norris, & Beers (1999} sample is fully consistent with
mild rotational mixing induced depletion and, therefore, with an initial
lithium abundance higher than the observed value. Tests for outliers depend
sensitively on the threshold for defining their presence, but we find a
1045% probability that the RNB sample is drawn from the rotationally mixed
models with a 0.2 dex median depletion (with lower probabilities corresponding
to higher depletion factors). When chemical evolution trends (Li/H versus Fe/H)
are treated in the linear plane we find that the dispersion in the RNB sample
is not explained by chemical evolution; the inferred bounds on lithium
depletion from rotational mixing are similar to those derived from models
without chemical evolution. We find that differences in the equivalent width
measurements are primarily responsible for different observational conclusions
concerning the lithium dispersion in halo stars. The standard Big Bang
Nucleosynthesis predicted lithium abundance which corresponds to the deuterium
abundance inferred from observations of high-redshift, low-metallicity QSO
absorbers requires halo star lithium depletion in an amount consistent with
that from our models of rotational mixing, but inconsistent with no depletion.Comment: 39 pages, 9 figures; submitted Ap
Inverse approach to Einstein's equations for fluids with vanishing anisotropic stress tensor
We expand previous work on an inverse approach to Einstein Field Equations
where we include fluids with energy flux and consider the vanishing of the
anisotropic stress tensor. We consider the approach using warped product
spacetimes of class . Although restricted, these spacetimes include many
exact solutions of interest to compact object studies and to cosmological
models studies. The question explored here is as follows: given a spacetime
metric, what fluid flow (timelike congruence), if any, could generate the
spacetime via Einstein's equations. We calculate the flow from the condition of
a vanishing anisotropic stress tensor and give results in terms of the metric
functions in the three canonical types of coordinates. A condition for perfect
fluid sources is also provided. The framework developed is algorithmic and
suited for the study and validation of exact solutions using computer algebra
systems. The framework can be applied to solutions in comoving and non-comoving
frames of reference, and examples in different types of coordinates are worked
out.Comment: 15 pages, matches version to appear in Phys.Rev.
Feedback Control of Quantum Transport
The current through nanostructures like quantum dots can be stabilized by a
feedback loop that continuously adjusts system parameters as a function of the
number of tunnelled particles . At large times, the feedback loop freezes
the fluctuations of which leads to highly accurate, continuous single
particle transfers. For the simplest case of feedback acting simultaneously on
all system parameters, we show how to reconstruct the original full counting
statistics from the frozen distribution.Comment: 4 pages, 2 figure
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