Modelling the flux distribution function of the extragalactic gamma-ray background from dark matter annihilation

Abstract

Abstract. The one-point function (i.e., the isotropic flux distribution) is a complementary method to (anisotropic) two-point correlations in searches for a gamma-ray dark matter annihilation signature. Using analytical models of structure formation and dark matter halo properties, we compute the gamma-ray flux distribution due to annihilations in extragalactic dark matter halos, as it would be observed by the Fermi Large Area Telescope. Combining the central limit theorem and Monte Carlo sampling, we show that the flux distribution takes the form of a narrow Gaussian of 'diffuse' light, with an 'unresolved point source' power-law tail as a result of bright halos. We argue that this background due to dark matter constitutes an irreducible and significant background component for point-source annihilation searches with galaxy clusters and dwarf spheroidal galaxies, modifying the predicted signal-to-noise ratio. A study of astrophysical backgrounds to this signal reveals that the shape of the total gamma-ray flux distribution is very sensitive to the contribution of a dark matter component, allowing us to forecast promising one-point upper limits on the annihilation cross section. We show that by using the flux distribution at only one energy bin, one can probe the canonical cross section required for explaining the relic density, for dark matter of masses around tens of GeV.

NPTFit: A Code Package for Non-Poissonian Template Fitting

“…Note that many of the details presented in this section have appeared in the earlier works of (Malyshev & Hogg 2011;Lee et al 2015Lee et al , 2016), however we have reproduced these here in order to have a single clear picture of the method.…”

“…This section reviews the NPTF, which was first presented in Lee et al (2016) and is described in more detail in Linden et al (2016) and Lisanti et al (2016) (see also Malyshev & Hogg 2011;Lee et al 2015;Zechlin et al 2016aZechlin et al , 2016b. The NPTF is used to fit a model  with parameters q to a data set d consisting of counts n p in each pixel p. The likelihood function for the NPTF is then simply…”

“…However, unresolved PSs-due to their potentially large number density-can be a leading and sometimes pesky source of flux across wavelengths. Recently, a novel analysis technique called the non-Poissonian template fit (NPTF) has been developed for characterizing populations of unresolved PSs at fluxes below the detection threshold for finding individually significant sources (Lee et al 2015(Lee et al , 2016. The technique expands upon the traditional fluctuation analysis technique (see, for example, (Miyaji & Griffiths 2002;Malyshev & Hogg 2011)), which analyzes the aggregate photoncount statistics of a data set to characterize the contribution from unresolved PSs, by additionally incorporating spatial information both for the distribution of unresolved PSs and for the potential sources of non-PS emission.…”

Unresolved Gamma-Ray Sky through its Angular Power Spectrum

“…Different populations of gamma-ray emitters induce anisotropies in the UGRB with different amplitudes and different angular and energy spectra [2]. A measurement of the gamma-ray angular power spectrum (APS) can therefore constrain the nature of the UGRB in a complementary way with respect to the intensity energy spectrum and the 1-point photon count probability distribution [3][4][5][6][7][8]. A different but related approach based on two-point statistics is the cross correlation of the gamma-ray sky with independent probes tracing the large scale structures of the Universe [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25].…”

Extragalactic diffuse γ-rays from dark matter annihilation: revised prediction and full modelling uncertainties

“…Twenty years later, the calculation was revisited [7,8] based on an improved understanding of structure formation in the ΛCDM cosmological paradigm: contrarily to previous estimates, the DGRB was found to be a promising entity to probe for signatures of DM annihilation. Many efforts followed to refine this calculation (see [1] for a comprehensive list of references), or to go beyond the simple average calculation to increase the sensitivity to the DM signal against astrophysical backgrounds (via the photon distribution function [9], searching for a small-scale anisotropy in the DGRB via auto-correlations [10] or cross-correlations with galaxy catalogues [11]). Based on these calculations and the analysis of four years of Fermi-LAT data [12,13], several authors [13][14][15][16] recently concluded that the limits on dark matter candidates derived from the DGRB are competitive with the best constraints on DM set by dwarf spheroidal galaxies [17,18], and that they are currently the best limits set by Fermi-LAT data at TeV masses.…”