522 research outputs found
A Pedagogical Discussion Concerning the Gravitational Energy Radiated by Keplerian Systems
We first discuss the use of dimensional arguments (and of the quadrupolar
emission hypothesis) in the derivation of the gravitational power radiated on a
circular orbit. Then, we show how to simply obtain the instantaneous power
radiated on a general Keplerian orbit by approximating it locally by a circle.
This allows recovering with a good precision, in the case of an ellipse, the
highly non trivial dependence on the eccentricity of the average power given by
general relativity. The whole approach is understandable by undergraduate
students.Comment: A simpler method has been used in the calculations, which requires
now only standard knowledge (the radius of curvature is defined by the normal
acceleration). Two figures have been added. Concerning the dimensional
analysis, the comparison with electromagnetism has been detaile
The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions
Recently obtained kinematic data has shown that the Large Magellanic Cloud
(LMC) possesses an old stellar halo. In order to further characterize the
properties of this halo, parametric King models are fit to the surface density
of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing
surveys, the model fits yield the center of their distribution at RA =
05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a
check the halo model is compared with RR Lyrae star counts in fields near the
LMC's periphery previously surveyed with photographic plates. These data,
however, require a cautious interpretation. Several topics regarding the LMC
stellar halo are discussed. First, the properties of the halo imply a global
mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10
M_sun for the LMC in good agreement with estimates based on the rotation curve.
Second, although the LMC's disk and halo are kinematically distinct, the shape
of the surface density profile of the halo is remarkably similar to that of the
young disk. For example, the best-fit exponential scale length for the RR Lyrae
stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMC's blue light.
In the Galaxy, the halo and disk do not resemble each other like this. Finally,
a local maximum in the LMC's microlensing optical depth due to halo-on-disk
stellar self-lensing is predicted. For the parameters of the stellar halo
obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is
large enough to possibly account for these two events, but not for all of the
observed microlensing.Comment: 11 pages, 1 figure, accepted to ApJ Letter
Theoretical uncertainties in extracting cosmic-ray diffusion parameters: the boron-to-carbon ratio
PAMELA and, more recently, AMS-02, are ushering us into a new era of greatly
reduced statistical uncertainties in experimental measurements of cosmic-ray
fluxes. In particular, new determinations of traditional diagnostic tools such
as the boron-to-carbon ratio (B/C) are expected to significantly reduce errors
on cosmic-ray diffusion parameters, with important implications for
astroparticle physics, ranging from inferring primary source spectra to
indirect dark matter searches. It is timely to stress, however, that the
conclusions obtained crucially depend on the framework in which the data are
interpreted as well as from some nuclear input parameters. We aim at assessing
the theoretical uncertainties affecting the outcome, with models as simple as
possible while still retaining the key dependencies. We compare different
semi-analytical, two-zone model descriptions of cosmic-ray transport in the
Galaxy. We test for the effect of a primary source contamination in the boron
flux by parametrically altering its flux, as well as for nuclear cross section
uncertainties. Our study on preliminary results from AMS-02 suggests that,
differently for instance from the leptonic case, realistic modelling of the
geometry of the Galaxy and of the source distribution are of minor importance
to correctly reproduce B/C data at high energies and thus, to a large extent,
for the extraction of diffusion parameters. The Ansatz on the lack of primary
injection of boron represents the most serious bias, and requires
multi-messenger studies to be addressed. If this uncertainty could be lifted,
nuclear uncertainties would still represent a serious concern, which degrade
the systematic error on the inferred parameters to the 20% level, or three
times the estimated experimental sensitivity. In order to reduce this, a new
nuclear cross section measurement campaign is probably required.Comment: 14 pages, 11 figures, 4 tables, published in A&
Which fraction of the measured cosmic-ray antiprotons might be due to neutralino annihilation in the galactic halo?
We analyze the data of low-energy cosmic-ray antiproton spectrum, recently
published by the BESS Collaboration, in terms of newly calculated fluxes for
secondary antiprotons and for a possible contribution of an exotic signal due
to neutralino annihilation in the galactic halo. We single out the relevant
supersymmetric configurations and discuss their explorability with experiments
of direct search for particle dark matter and at accelerators. We discuss how
future measurements with the Alpha Magnetic Spectrometer (AMS) on the Shuttle
flight may disentangle the possible neutralino-induced contribution from the
secondary one.Comment: 25 pages, ReVTeX, 18 figures (high resolution figures available upon
request
Antiproton and Positron Signal Enhancement in Dark Matter Mini-Spikes Scenarios
The annihilation of dark matter (DM) in the Galaxy could produce specific
imprints on the spectra of antimatter species in Galactic cosmic rays, which
could be detected by upcoming experiments such as PAMELA and AMS02. Recent
studies show that the presence of substructures can enhance the annihilation
signal by a "boost factor" that not only depends on energy, but that is
intrinsically a statistical property of the distribution of DM substructures
inside the Milky Way. We investigate a scenario in which substructures consist
of "mini-spikes" around intermediate-mass black holes. Focusing on
primary positrons and antiprotons, we find large boost factors, up to a few
thousand, that exhibit a large variance at high energy in the case of positrons
and at low energy in the case of antiprotons. As a consequence, an estimate of
the DM particle mass based on the observed cut-off in the positron spectrum
could lead to a substantial underestimate of its actual value.Comment: 13 pages, 9 figures, minor changes, version accepted for publication
in PR
Positrons from dark matter annihilation in the galactic halo: theoretical uncertainties
Indirect detection signals from dark matter annihilation are studied in the
positron channel. We discuss in detail the positron propagation inside the
galactic medium: we present novel solutions of the diffusion and propagation
equations and we focus on the determination of the astrophysical uncertainties
which affect the positron dark matter signal. We find dark matter scenarios and
propagation models that nicely fit existing data on the positron fraction.
Finally, we present predictions both on the positron fraction and on the flux
for already running or planned space experiments, concluding that they have the
potential to discriminate a possible signal from the background and, in some
cases, to distinguish among different astrophysical propagation models.Comment: 22 pages, 15 figures. A few comments and references adde
Positrons from dark matter annihilation in the galactic halo: uncertainties
Indirect detection signals from dark matter annihilation are studied in the
positron channel. We discuss in detail the positron propagation inside the
galactic medium: we present novel solutions of the diffusion and propagation
equations and we focus on the determination of the astrophysical uncertainties
which affect the positron dark matter signal. We show that, especially in the
low energy tail of the positron spectra at Earth, the uncertainty is sizeable
and we quantify the effect. Comparison of our predictions with current
available and foreseen experimental data are derived.Comment: 4 pages, 4 figures, Proc. of the 30th International Cosmic Ray
Conference, July 3 - 11, 2007, Merida, Yucatan, Mexico (ICRC07
Constraints on WIMP Dark Matter from the High Energy PAMELA data
A new calculation of the ratio in cosmic rays is compared to the
recent PAMELA data. The good match up to 100 GeV allows to set constraints on
exotic contributions from thermal WIMP dark matter candidates. We derive
stringent limits on possible enhancements of the WIMP \pbar flux: a =100 GeV (1 TeV) signal cannot be increased by more than a factor 6 (40)
without overrunning PAMELA data. Annihilation through the channel is
also inspected and cross-checked with data. This scenario is
strongly disfavored as it fails to simultaneously reproduce positron and
antiproton measurements.Comment: 5 pages, 5 figures, the bibliography has been updated, minor
modifications have been made in the tex
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