9 research outputs found
Constraining Galactic dark matter with gamma-ray pixel counts statistics
Gamma-ray searches for new physics such as dark matter are often driven by
investigating the composition of the extragalactic gamma-ray background (EGB).
Classic approaches to EGB decomposition manifest in resolving individual point
sources and dissecting the intensity spectrum of the remaining unresolved
component. Furthermore, statistical methods have recently been proven to
outperform the sensitivity of classic source detection algorithms in finding
point-source populations in the unresolved flux regime. In this article, we
employ the 1-point photon count statistics of eight years of Fermi-LAT data to
resolve the population of extragalactic point sources and to decompose the
diffuse isotropic background contribution for Galactic latitudes |b|>30 deg. We
use three adjacent energy bins between 1 and 10 GeV. For the first time, we
extend the analysis to incorporate a potential contribution from annihilating
dark matter smoothly distributed in the Galaxy. We investigate the sensitivity
reach of 1-point statistics for constraining the thermally-averaged
self-annihilation cross section of dark matter, using different
template models for the Galactic foreground emission. Given the official
Fermi-LAT interstellar emission model, we set upper bounds on the DM
self-annihilation cross section that are comparable with the
constraints obtained by other indirect detection methods, in particular by the
stacking analysis of several dwarf spheroidal galaxies.Comment: 11 pages, 7 figures, 1 table; v2: major changes improving the
selection of the RO
Statistical Measurement of the Gamma-ray Source-count Distribution as a Function of Energy
Statistical properties of photon count maps have recently been proven as a
new tool to study the composition of the gamma-ray sky with high precision. We
employ the 1-point probability distribution function of 6 years of Fermi-LAT
data to measure the source-count distribution dN/dS and the diffuse components
of the high-latitude gamma-ray sky as a function of energy. To that aim, we
analyze the gamma-ray emission in five adjacent energy bands between 1 GeV and
171 GeV. It is demonstrated that the source-count distribution as a function of
flux is compatible with a broken power law up to energies of ~50 GeV. The index
below the break is between 1.95 and 2.0. For higher energies, a simple
power-law fits the data, with an index of 2.2^{+0.7}_{-0.3} in the energy band
between 50 GeV and 171 GeV. Upper limits on further possible breaks as well as
the angular power of unresolved sources are derived. We find that point-source
populations probed by this method can explain 83^{+7}_{-13}% (81^{+52}_{-19}%)
of the extragalactic gamma-ray background between 1.04 GeV and 1.99 GeV (50 GeV
and 171 GeV). The method has excellent capabilities for constraining the
gamma-ray luminosity function and the spectra of unresolved blazars.Comment: 6 pages, 2 figures, 2 tables; v2: minor changes, accepted for
publication in ApJ
Unveiling the Gamma-ray Source Count Distribution Below the FERMI Detection Limit with Photon Statistics
The source-count distribution as a function of their flux, dN/dS, is one of
the main quantities characterizing gamma-ray source populations. We employ
statistical properties of the Fermi Large Area Telescope (LAT) photon counts
map to measure the composition of the extragalactic gamma-ray sky at high
latitudes (|b|>30 deg) between 1 GeV and 10 GeV. We present a new method,
generalizing the use of standard pixel-count statistics, to decompose the total
observed gamma-ray emission into (a) point-source contributions, (b) the
Galactic foreground contribution, and (c) a truly diffuse isotropic background
contribution. Using the 6-year Fermi-LAT data set (P7REP), we show that the
dN/dS distribution in the regime of so far undetected point sources can be
consistently described with a power law of index between 1.9 and 2.0. We
measure dN/dS down to an integral flux of ~2x10^{-11} cm^{-2}s^{-1}, improving
beyond the 3FGL catalog detection limit by about one order of magnitude. The
overall dN/dS distribution is consistent with a broken power law, with a break
at 2.1^{+1.0}_{-1.3}x10^{-8} cm^{-2}s^{-1}. The power-law index
n_1=3.1^{+0.7}_{-0.5} for bright sources above the break hardens to
n_2=1.97+-0.03 for fainter sources below the break. A possible second break of
the dN/dS distribution is constrained to be at fluxes below 6.4x10^{-11}
cm^{-2}s^{-1} at 95% confidence level. The high-latitude gamma-ray sky between
1 GeV and 10 GeV is shown to be composed of ~25% point sources, ~69.3% diffuse
Galactic foreground emission, and ~6% isotropic diffuse background.Comment: 28 pages, 20 figures, 6 tables; v2: minor changes and additions,
accepted for publication in ApJ