3,486 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
Nuclear Cosmic Rays propagation in the Atmosphere
The transport of the nuclear cosmic ray flux in the atmosphere is studied and
the atmospheric corrections to be applied to the measurements are calculated.
The contribution of the calculated corrections to the accuracy of the
experimental results are discussed and evaluated over the kinetic energy range
10-10 GeV/n. The Boron (B) and Carbon (C) elements system is used as a
test case. It is shown that the required corrections become largely dominant at
the highest energies investigated. The results are discussed.Comment: Proc. of 30th International Cosmic Ray Conference, Merida, Mexico; 4
page
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
Rank ordering of questionnaire items using Wilksâs statistic: an example of application to three populations
Wilksâ test (1938) constitutes a major contribution to statistics and in practical applications. It is admittedly a classical method but even so it is of real interest because it is highly convenient to use. We hope to illustrate this in the following article by means of a study in the field of traffic psychology. In this demonstration, we start from a âcharacterization questionnaireâ filled in by three populations of drivers and we test the population effect working from categorical data and 3-way tables. The differentiating power of the items is then examined and ranked in decreasing value. Through this example, we thus show the scope of Wilksâstatistics and their highly general import with qualitative or category-specific data, compared with other techniques
The role of electron capture decay in the precision era of Galactic cosmic-ray data
Electron capture (EC) decay relies on attachment and stripping
cross-sections, that in turn, depend on the atomic number of the nucleus. We
revisit the impact of EC decay in the context of the high-precision cosmic-ray
fluxes measured by the AMS-02 experiment. We derive the solution of the
steady-state fluxes in a 1D thin disk model including EC decay. We compare our
results with relevant elemental and isotopic fluxes and evaluate the impact of
this process, given the precision of recent AMS-02, ACE-CRIS, SuperTIGER, and
Voyager data. We find this impact to be at the level or larger than the
precision of recently collected data for several species, e.g. Ga and
As, indicating that EC decay must be properly taken into account in the
calculation.Comment: 8 pages, 3 figures, Proceedings for International Cosmic Ray
Conference (ICRC) 202
Constructing irreducible polynomials recursively with a reverse composition method
We suggest a construction of the minimal polynomial of
over from the minimal polynomial for all positive integers whose prime factors divide . The
computations of our construction are carried out in . The key
observation leading to our construction is that for holds
where and
is a primitive -th root of unity in . The
construction allows to construct a large number of irreducible polynomials over
of the same degree. Since different applications require
different properties, this large number allows the selection of the candidates
with the desired properties
Estimates on Green functions of second order differential operators with singular coefficients
We investigate the Green functions G(x,x^{\prime}) of some second order
differential operators on R^{d+1} with singular coefficients depending only on
one coordinate x_{0}. We express the Green functions by means of the Brownian
motion. Applying probabilistic methods we prove that when x=(0,{\bf x}) and
x^{\prime}=(0,{\bf x}^{\prime}) (here x_{0}=0) lie on the singular hyperplanes
then G(0,{\bf x};0,{\bf x}^{\prime}) is more regular than the Green function of
operators with regular coefficients.Comment: 16 page
The effect of rotation and tidal heating on the thermal lightcurves of Super Mercuries
Short period (<50 days) low-mass (<10Mearth) exoplanets are abundant and the
few of them whose radius and mass have been measured already reveal a diversity
in composition. Some of these exoplanets are found on eccentric orbits and are
subjected to strong tides affecting their rotation and resulting in significant
tidal heating. Within this population, some planets are likely to be depleted
in volatiles and have no atmosphere. We model the thermal emission of these
"Super Mercuries" to study the signatures of rotation and tidal dissipation on
their infrared light curve. We compute the time-dependent temperature map at
the surface and in the subsurface of the planet and the resulting
disk-integrated emission spectrum received by a distant observer for any
observation geometry. We calculate the illumination of the planetary surface
for any Keplerian orbit and rotation. We include the internal tidal heat flow,
vertical heat diffusion in the subsurface and generate synthetic light curves.
We show that the different rotation periods predicted by tidal models
(spin-orbit resonances, pseudo-synchronization) produce different photometric
signatures, which are observable provided that the thermal inertia of the
surface is high, like that of solid or melted rocks (but not regolith). Tidal
dissipation can also directly affect the light curves and make the inference of
the rotation more difficult or easier depending on the existence of hot spots
on the surface. Infrared light curve measurement with the James Webb Space
Telescope and EChO can be used to infer exoplanets' rotation periods and
dissipation rates and thus to test tidal models. This data will also constrain
the nature of the (sub)surface by constraining the thermal inertia.Comment: 15 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Dark matter annihilation and decay profiles for the Reticulum II dwarf spheroidal galaxy
The dwarf spheroidal galaxies (dSph) of the Milky Way are among the most
attractive targets for indirect searches of dark matter. In this work, we
reconstruct the dark matter annihilation (J-factor) and decay profiles for the
newly discovered dSph Reticulum II. Using an optimized spherical Jeans analysis
of kinematic data obtained from the Michigan/Magellan Fiber System (M2FS), we
find Reticulum II's J-factor to be among the largest of any Milky Way dSph. We
have checked the robustness of this result against several ingredients of the
analysis. Unless it suffers from tidal disruption or significant inflation of
its velocity dispersion from binary stars, Reticulum II may provide a unique
window on dark matter particle properties.Comment: 5 pages, 4 figures. Match the ApJL accepted versio
Heavy Nuclei Enrichment of the Galactic Cosmic Rays at High Energy: Astrophysical Interpretation
A substantial increase of the mean logarithmic mass of galactic
cosmic rays {\em vs} energy has been observed . We study three effects that
could explain this trend i) different source spectra for protons and heavy
nuclei ii) a selective nuclear destruction in flight of heavies iii) a gradient
of the source number and chemical composition in the galactic disk.
We take advantage of the diffusive cosmic ray propagation model developed at
LAPTH to study specifically the geometrical aspects of the propagation and
extend it to high energy. Using a simple modeling of the spectral knee around
$10^{15}$ eV, a bump in appears. This feature is smoother when the
spectral index of protons is steeper than Fe's.
We analyze the effects of the rigidity dependence of the diffusion
coefficient and the scale height of the confinement halo and we show that is most sensitive to the first parameter. Pure geometrical effects are less
determining than the diffusion coefficient spectral index. Subsequently, we
conclude that the physics of cosmic ray confinement is the essential cause of
the heavy nuclei enrichment until eV.Comment: 26 pages, 7 PostScript figures. Astropart. Phys, accepted; misprint
in fig6 correcte
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