9,894 research outputs found
Dark-Matter Harmonics Beyond Annual Modulation
The count rate at dark-matter direct-detection experiments should modulate
annually due to the motion of the Earth around the Sun. We show that
higher-frequency modulations, including daily modulation, are also present and
in some cases are nearly as strong as the annual modulation. These higher-order
modes are particularly relevant if (i) the dark matter is light, O(10) GeV,
(ii) the scattering is inelastic, or (iii) velocity substructure is present;
for these cases, the higher-frequency modes are potentially observable at
current and ton-scale detectors. We derive simple expressions for the harmonic
modes as functions of the astrophysical and geophysical parameters describing
the Earth's orbit, using an updated expression for the Earth's velocity that
corrects a common error in the literature. For an isotropic halo velocity
distribution, certain ratios of the modes are approximately constant as a
function of nuclear recoil energy. Anisotropic distributions can also leave
observable features in the harmonic spectrum. Consequently, the higher-order
harmonic modes are a powerful tool for identifying a potential signal from
interactions with the Galactic dark-matter halo.Comment: 40 pages, 10 figures; v2 refs added, minor improvements; v3 refs
added, minor improvements, JCAP versio
Distinguishing Dark Matter from Unresolved Point Sources in the Inner Galaxy with Photon Statistics
Data from the Fermi Large Area Telescope suggests that there is an extended
excess of GeV gamma-ray photons in the Inner Galaxy. Identifying potential
astrophysical sources that contribute to this excess is an important step in
verifying whether the signal originates from annihilating dark matter. In this
paper, we focus on the potential contribution of unresolved point sources, such
as millisecond pulsars (MSPs). We propose that the statistics of the
photons---in particular, the flux probability density function (PDF) of the
photon counts below the point-source detection threshold---can potentially
distinguish between the dark-matter and point-source interpretations. We
calculate the flux PDF via the method of generating functions for these two
models of the excess. Working in the framework of Bayesian model comparison, we
then demonstrate that the flux PDF can potentially provide evidence for an
unresolved MSP-like point-source population.Comment: 27 pages, 8 figures; v2, reference added and other minor change
Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy
We present a new method to characterize unresolved point sources (PSs),
generalizing traditional template fits to account for non-Poissonian photon
statistics. We apply this method to Fermi Large Area Telescope gamma-ray data
to characterize PS populations at high latitudes and in the Inner Galaxy. We
find that PSs (resolved and unresolved) account for ~50% of the total
extragalactic gamma-ray background in the energy range ~1.9 to 11.9 GeV. Within
10 of the Galactic Center with , we find that ~5-10%
of the flux can be accounted for by a population of unresolved PSs, distributed
consistently with the observed ~GeV gamma-ray excess in this region. The excess
is fully absorbed by such a population, in preference to dark-matter
annihilation. The inferred source population is dominated by near-threshold
sources, which may be detectable in future searches.Comment: 7+22 pages, 4+18 figures; v2, minor changes, new Pass 8 data analyzed
(conclusions unchanged); v3, PRL version, substantive improvements and
additional checks (conclusion unchanged
Local Electronic and Magnetic Studies of an Artificial La2FeCrO6 Double Perovskite
Through the utilization of element-resolved polarized x-ray probes, the
electronic and magnetic state of an artificial La2FeCrO6 double perovskite were
explored. Applying unit-cell level control of thin film growth on SrTiO3 (111),
the rock salt double perovskite structure can be created for this system, which
does not have an ordered perovskite phase in the bulk. We find that the Fe and
Cr are in the proper 3+ valence state, but, contrary to previous studies, the
element-resolved magnetic studies find the moments in field are small and show
no evidence of a sizable magnetic moment in the remanent state.Comment: 3 pages, 4 figure
Recommended from our members
Modulation effects in dark matter-electron scattering experiments
One of the next frontiers in dark-matter direct-detection experiments is to explore the MeV to GeV mass regime. Such light dark matter does not carry enough kinetic energy to produce an observable nuclear recoil, but it can scatter off electrons, leading to a measurable signal. We introduce a semianalytic approach to characterize the resulting electron-scattering events in atomic and semiconductor targets, improving on previous analytic proposals that underestimate the signal at high recoil energies. We then use this procedure to study the time-dependent properties of the electron-scattering signal, including the modulation fraction, higher-harmonic modes and modulation phase. The time dependence can be distinct in a nontrivial way from the nuclear scattering case. Additionally, we show that dark-matter interactions inside the Earth can significantly distort the laboratory-frame phase-space distribution of sub-GeV dark matter.MIT Department of Physics Pappalardo Program (Fellowship
The Patient's Guide to Psoriasis Treatment. Part 4: Goeckerman Therapy.
BackgroundThe Goeckerman regimen remains one of the oldest, most reliable treatment options for patients with moderate to severe psoriasis. Goeckerman therapy currently consists of exposure to ultraviolet B light and application of crude coal tar. The details of the procedure can be confusing and challenging to understand for the first-time patient or provider.ObjectiveTo present a freely available online guide and video on Goeckerman treatment that explains the regimen in a patient-oriented manner.MethodsThe Goeckerman protocol used at the University of California-San Francisco Psoriasis and Skin Treatment Center as well as available information from the literature were reviewed to design a comprehensive guide for patients receiving Goeckerman treatment.ResultsWe created a printable guide and video resource that covers the supplies needed for Goeckerman regimen, the treatment procedure, expected results, how to monitor for adverse events, and discharge planning.ConclusionThis new resource is beneficial for prospective patients planning to undergo Goeckerman treatment, healthcare providers, and trainees who want to learn more about this procedure. Online media and video delivers material in a way that is flexible and often familiar to patients
Model-based asymptotically optimal dispersion measure correction for pulsar timing
In order to reach the sensitivity required to detect gravitational waves,
pulsar timing array experiments need to mitigate as much noise as possible in
timing data. A dominant amount of noise is likely due to variations in the
dispersion measure. To correct for such variations, we develop a statistical
method inspired by the maximum likelihood estimator and optimal filtering. Our
method consists of two major steps. First, the spectral index and amplitude of
dispersion measure variations are measured via a time-domain spectral analysis.
Second, the linear optimal filter is constructed based on the model parameters
found in the first step, and is used to extract the dispersion measure
variation waveforms. Compared to current existing methods, this method has
better time resolution for the study of short timescale dispersion variations,
and generally produces smaller errors in waveform estimations. This method can
process irregularly sampled data without any interpolation because of its
time-domain nature. Furthermore, it offers the possibility to interpolate or
extrapolate the waveform estimation to regions where no data is available.
Examples using simulated data sets are included for demonstration.Comment: 15 pages, 15 figures, submitted 15th Sept. 2013, accepted 2nd April
2014 by MNRAS. MNRAS, 201
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