Conservative upper limits on WIMP annihilation cross section from Fermi-LAT gamma -rays

📝 Abstract

The spectrum of an isotropic extragalactic $\gamma $-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subtraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain $conservative$ upper limits on their annihilation cross section \sigmav. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, would {\sigmav} be inversely proportional to the WIMP velocity, very severe limits are derived for the velocity-independent part of the annihilation cross section.

💡 Analysis

The spectrum of an isotropic extragalactic $\gamma $-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subtraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain $conservative$ upper limits on their annihilation cross section \sigmav. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, would {\sigmav} be inversely proportional to the WIMP velocity, very severe limits are derived for the velocity-independent part of the annihilation cross section.

📄 Content

arXiv:1112.4171v1 [hep-ph] 18 Dec 2011 Conservative upper limits on WIMP annihilation cross section from Fermi-LAT γ-rays Fiorenza Donato Dipartimento di Fisica Teorica, Torino University and INFN, Via P. Giuria 1, 10122 Torino, Italy E-mail: donato@to.infn.it Francesca Calore II. Institute for Theoretical Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany Valentina De Romeri IFIC (CSIC - Universidad de Valencia), C/ Catedratico Jose Beltran 2, E-46980 Paterna (Valencia), Spain Abstract. The spectrum of an isotropic extragalactic γ-ray background (EGB) has been measured by the Fermi-LAT telescope at high latitudes. Two new models for the EGB are derived from the subtraction of unresolved point sources and extragalactic diffuse processes, which could explain from 30% to 70% of the Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely due to the annihilation of dark matter (DM) particles in the Galactic halo, we obtain conservative upper limits on their annihilation cross section ⟨σv⟩. Severe bounds on a possible Sommerfeld enhancement of the annihilation cross section are set as well. Finally, would ⟨σv⟩be inversely proportional to the WIMP velocity, very severe limits are derived for the velocity-independent part of the annihilation cross section.

1. The extragalactic γ-ray background The high latitude (|b| > 10o) γ-ray emission measured by Fermi-LAT [1], given its reduced contamination by galactic sources, is a powerful tool to set limits on the contribution of DM to the measured flux. The spectrum has been obtained after the subtraction from the data of the sources resolved by the telescope, the (indeed model dependent) diffuse galactic emission, the cosmic ray (CR) background in the detector and the solar γ-ray emission. For each low–flux source there may be a large number of unresolved point sources which have not been detected because of selection effects, or too low emission. Most of the unassociated high latitude sources are blazars, a class of Active Galactic Nuclei (AGNs), and they pile to the EGB with the largest flux [2]. Galactic resolved pulsars and Milli-Second Pulsars (MSPs) represent the second largest population in the Fermi-LAT catalog [3, 4] and they are expected to contribute significantly to the putative EGB. A non-negligible γ-ray flux seems to be guaranteed by unresolved normal star-forming galaxies [5]. Ultra-high energy CRs (UHECRs) may induce secondary electromagnetic cascades, originating neutrinos and γ-rays at Fermi-LAT energies [6]. Unresolved blazars and MSPs are believed to contribute at least few percent to the Fermi-LAT EGB, while predictions for star-forming galaxies and UHECRs are highly model dependent. In the following, we describe few classes of γ-ray emitters whose unresolved flux is firmly estimated in a non-negligible Fermi-LAT EGB percentage. In a conservative scenario (Model I), we will subtract AGN and MSPs to the Fermi-LAT EGB as derived in Ref. [1]. A more relaxed model (Model II) will be drawn by the further subtraction of a minimal flux from star-forming galaxies and CRs at the highest energies. The derivation of each contribution is described in details in [7]. 1.1. Models for the EGB The aim of this Section is to subtract from the Fermi-LAT EGB [1] additional contributions from unresolved sources at latitudes |b| > 10o. The contributions removed from the Fermi-LAT spectrum are minimal. In fact, the predictions that we will take into account for the relevant unresolved sources are the lowest ones according to the literature. For the Model I, we subtract from the Fermi-LAT EGB [1] the unresolved contributions for both BL Lacs and FSRQs, and MSPs. Model II for the EGB refers to the scenario where the additional contributions from star-forming galaxies and UHECRs add to explaining the Fermi-LAT EGB. For all the details, the interested reader is addressed to Ref. [7]. The ensuing fluxes are displayed in Fig. 1. At 100 MeV, Model II explains about 70% of the Fermi-LAT EGB, while above 1-2 GeV they count about 50% of the total. E (GeV) -1 10 1 10 2 10 ) -1 sr -1 s -2 dN/dE (GeV cm 2 E -9 10 -8 10 -7 10 -6 10 Fermi EGB Model II EGB Sum of contributions, Model II Sum of contributions, Model I Starforming galaxies UHECRs Figure 1. γ-ray spectrum for |b| > 10o latitudes. Fermi-LAT data points are displayed along with their power–law fit (solid black curve) [1]. Dots and long dashed-curve (yellow) correspond to the UHECRs and star-forming galaxies γ-ray fluxes, respectively. Dot-dashed (blue) curve: sum of BL Lacs, FSRQs and MSPs fluxes. Short-dashed (red): sum of all the unresolved components. Solid (green) curve is derived by subtracting all the contributions to the Fermi-LAT result (Model II). (GeV) χ m 2 10 3 10 4 10 ) -1 s 3 v> (cm σ < -26 10 -25 10 -24 10 -23 10 -22 10 -21 10 b b -τ

τ

µ + µ Fermi EGB Model I Model II Figure 2. Upper bounds on ⟨σv⟩from γ-ray in the high latitude galactic halo, as a function of the DM mass. F

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