Constraining dark matter signal from a combined analysis of Milky Way satellites using the Fermi-LAT

Constraining dark matter signal from a combined analysis of Milky Way satellites using the Fermi-LAT Maja Llena Garde∗ Stockholm University E-mail: maja.garde@fysik.su.se On behalf of the Fermi-LAT collaboration The Fermi LAT collaboration has recently presented constraints on the gamma-ray signal from annihilating dark matter using separate analyses of a number of dwarf spheroidal galaxies. Since the expected annihilation signal has the same physical properties regardless of the target (except for a normalization scale), it is possible to enhance the constraining power using a combined analysis, the initial results of which will be presented here. Identification of Dark Matter 2010 July 26 - 30 2010 University of Montpellier 2, Montpellier, France ∗Speaker. c ⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ arXiv:1102.5701v1 [astro-ph.HE] 28 Feb 2011 dSph stacking with the Fermi-LAT Maja Llena Garde 1. Introduction The Fermi Gamma-ray Space Telescope was launched on June 11, 2008. Its main instrument, the Large Area Telescope (Fermi-LAT), consists of 16 identical modules in a 4x4 array with each module comprised of a tracker for direction determination and a calorimeter for energy measure- ments [1]. The Fermi-LAT observes the entire sky every ∼3 hours (2 orbits) with a field of view covering ∼2.4 sr and a sensitive energy range extending from 20MeV to >300GeV. These features makes the Fermi-LAT a great instrument for dark matter (DM) searches. The leading DM candidate is a weakly interacting massive particle (WIMP), and this is the DM candidate we are primarily focusing on with the Fermi-LAT. The gamma-ray flux from self- annihilating WIMPs can be expressed as φWIMP(E,ψ) = J(ψ)×ΦPP(E) [2], where ΦPP(E) is the "particle physics factor" described by ΦPP(E) = 1 2 < σv > 4πm2 WIMP ∑ f dNf dE B f (1.1) and J(ψ) is the "astrophysical factor", or J-factor, described by J(ψ) = Z l.o.s. dl(ψ)ρ2(l(ψ)). (1.2) Here ⟨σv⟩is the WIMP annihilation cross section times relative velocity, mWIMP is the WIMP mass, ρ(r) is the dark matter density distribution, and ∑f dNf dE B f is the gamma ray spectrum generated per WIMP annihilation where the sum is over final states f with branching ratio B f . The particle physics factor has two spectral features: the continuum feature and the line feature (which is often referred to as the ”smoking gun”). Dwarf spheroidal galaxies (dSphs) are DM dominated systems since they have a high mass to light ratio. Many dSphs are closer than 100 kpc to the galactic center, and they have low background since most dSphs are expected to be free from other astrophysical gamma-ray sources and they have a small gas content. This makes them a good target for gamma-ray DM searches with the Fermi- LAT. However, the photon statistics for single dSphs are expected to be very low. The Fermi-LAT collaboration has recently presented results from a DM search in a number of dSphs [3]. Since the DM spectra are the same for all astrophysical sources, a combined analysis would improve the statistics. In this proceeding we present the preliminary results of a combined likelihood analysis of eight dSphs. 2. Analysis The combined likelihood used in this work is described by L(σv,mWIMP;obs) = N ∏ i Li(σv,mWIMP,C,bi;obs), (2.1) where σv and mWIMP are the common DM signal parameters (velocity averaged cross-section and mass), C are constants (e.g. branching fraction in our case), and b are individual parameters 2 dSph stacking with the Fermi-LAT Maja Llena Garde Name l b JNFW(×1019 GeV 2 cm5 ) Bootes I 358.08 69.62 0.16+0.35 −0.13 Coma Berenices 241.9 83.6 0.16+0.22 −0.08 Draco 86.37 34.72 1.20+0.31 −0.25 Fornax 237.1 −65.7 0.06+0.03 −0.03 Sculptor 287.15 −83.16 0.24+0.06 −0.06 Sextans 243.4 42.2 0.06+0.03 −0.02 Ursa Mayor II 152.46 37.44 0.58+0.91 −0.35 Ursa Minor 104.95 44.80 0.64+0.25 −0.18 Table 1: dSphs analyzed in this work. All positions and J-factors of the dSphs are taken from [3]. (isotropic and galactic diffuse background normalization, normalization of nearby sources). The main advantages of the combined likelihood are that the analysis can be individually optimized and that combined limits are more robust under individual background fluctuations and under individual astrophysical modelling uncertainties than individual limits. We have observed the eight dSphs listed in Table 1, which is the same subset analyzed in [3]. In this preliminary analysis we have used 21 month data from 2008-08-04 to 2010-05-12. We have used the diffuse event class which only contains the events with the highest gamma-like confidence, and we have chosen events ranging from 200MeV to 100GeV. We used the Fermi-LAT instrument responce function P6_V3_DIFFUSE. Our region of interest (ROI) is a region of 10 degrees radius centered on dSph location. Standard cuts removing Earth albedo photons have been made. The dSphs are modeled as DM point sources usin

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