187 research outputs found
Separating astrophysical sources from indirect dark matter signals
Indirect searches for products of dark matter annihilation and decay face the
challenge of identifying an uncertain and subdominant signal in the presence of
uncertain backgrounds. Two valuable approaches to this problem are (1) using
analysis methods which take advantage of different features in the energy
spectrum and angular distribution of the signal and backgrounds, and (2) more
accurate characterization of backgrounds, which allows for more robust
identification of possible signals. These two approaches are complementary and
can be significantly strengthened when used together. I review the status of
indirect searches with gamma rays using two promising targets, the Inner Galaxy
and the Isotropic Gamma-Ray Background. For both targets, uncertainties in the
properties of backgrounds is a major limitation to the sensitivity of indirect
searches. I then highlight approaches which can enhance the sensitivity of
indirect searches using these targets.Comment: 7 pages, 4 figures. Contributed to the National Academy of Sciences'
Dark Matter Sackler Colloquiu
Joint anisotropy and source count constraints on the contribution of blazars to the diffuse gamma-ray background
We place new constraints on the contribution of blazars to the large-scale
isotropic gamma-ray background (IGRB) by jointly analyzing the measured source
count distribution (logN-logS) of blazars and the measured intensity and
anisotropy of the IGRB. We find that these measurements point to a consistent
scenario in which unresolved blazars make less than 20% of the IGRB intensity
at 1-10 GeV while accounting for the majority of the measured anisotropy in
that energy band. These results indicate that the remaining fraction of the
IGRB intensity is made by a component with a low level of intrinsic anisotropy.
We determine upper limits on the anisotropy from non-blazar sources, adopting
the best-fit parameters of the measured source count distribution to calculate
the unresolved blazar anisotropy. In addition, we show that the anisotropy
measurement excludes some recently proposed models of the unresolved blazar
population.Comment: 7 pages, 4 figures. v2: new section (Sec.III) and 2 figures added.
Expanded discussions in the other sections. Results and conclusions
unchanged. New Section III is also a reply to the comment of Harding &
Abazajian arXiv:1204.3870 on this wor
Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT
The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi Large Area Telescope at Galactic latitudes |b|>30° in four energy bins spanning 1–50 GeV. At multipoles ℓ≥155, corresponding to angular scales ≲2°, angular power above the photon noise level is detected at >99.99% confidence level in the 1–2 GeV, 2–5 GeV, and 5–10 GeV energy bins, and at >99% confidence level at 10–50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles ℓ≥155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C_P/⟨I⟩^2=9.05±0.84×10^(-6) sr, while the energy dependence of C_P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ_s=2.40±0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background
In Silico Synchronization of Cellular Populations Through Expression Data Deconvolution
Cellular populations are typically heterogenous collections of cells at
different points in their respective cell cycles, each with a cell cycle time
that varies from individual to individual. As a result, true single-cell
behavior, particularly that which is cell-cycle--dependent, is often obscured
in population-level (averaged) measurements. We have developed a simple
deconvolution method that can be used to remove the effects of asynchronous
variability from population-level time-series data. In this paper, we summarize
some recent progress in the development and application of our approach, and
provide technical updates that result in increased biological fidelity. We also
explore several preliminary validation results and discuss several ongoing
applications that highlight the method's usefulness for estimating parameters
in differential equation models of single-cell gene regulation.Comment: accepted for the 48th ACM/IEEE Design Automation Conferenc
The Fermi Large Area Telescope on Orbit: Event Classification, Instrument Response Functions, and Calibration
The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy γ-ray telescope, covering the energy range from 20 MeV to more than 300 GeV. During the first years of the mission, the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the instrument response functions (IRFs), the description of the instrument performance provided for data analysis. In this paper, we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources
Signatures of LCDM substructure in tidal debris
In the past decade, surveys of the stellar component of the Galaxy have
revealed a number of streams from tidally disrupted dwarf galaxies and globular
clusters. Simulations of hierarchical structure formation in LCDM cosmologies
predict that the dark matter halo of a galaxy like the Milky Way contains
hundreds of subhalos with masses of ~10^8 solar masses and greater, and it has
been suggested that the existence of coherent tidal streams is incompatible
with the expected abundance of substructure. We investigate the effects of dark
matter substructure on tidal streams by simulating the disruption of a
self-gravitating satellite on a wide range of orbits in different host models
both with and without substructure. We find that the halo shape and the
specific orbital path more strongly determine the overall degree of disruption
of the satellite than does the presence or absence of substructure, i.e., the
changes in the large-scale properties of the tidal debris due to substructure
are small compared to variations in the debris from different orbits in a
smooth potential. Substructure typically leads to an increase in the degree of
clumpiness of the tidal debris in sky projection, and in some cases a more
compact distribution in line-of-sight velocity. Substructure also leads to
differences in the location of sections of debris compared to the results of
the smooth halo model, which may have important implications for the
interpretation of observed tidal streams. A unique signature of the presence of
substructure in the halo which may be detectable by upcoming surveys is
identified. We conclude, however, that predicted levels of substructure are
consistent with a detection of a coherent tidal stream from a dwarf galaxy.Comment: 15 pages, 13 figures, accepted for publication in ApJ. Matches
accepted versio
Robust identification of isotropic diffuse gamma rays from Galactic dark matter
Dark matter annihilation in Galactic substructure will produce diffuse
gamma-ray emission of remarkably constant intensity across the sky, making it
difficult to disentangle this Galactic dark matter signal from the
extragalactic gamma-ray background. We show that if Galactic dark matter
contributes a modest fraction of the measured emission in an energy range
accessible to the Fermi Gamma-ray Space Telescope, the energy dependence of the
angular power spectrum of the total measured emission could be used to
confidently identify gamma rays from Galactic dark matter substructure.Comment: 4 pages, 2 figures, added 1 reference, published in PR
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