Searching for Dark Matter Annihilation in M87

= 246 GeV, freezingout in the early universe. Their relic density would be in agreement with the observed Ω dm h 2 = 0.1120 [1]. This is termed the “WIMP Miracle”. Moreover, stable neutral particles at the electroweak symmetry breaking scale are predicted by supersymmetric extensions of the Standard Model. In the present day universe, annihilation interactions in regions with high mass densities still occur and can lead to the production of energetic neutrinos, electrons, positrons and gamma rays which inverse-Compton scatter with starlight photon fields or the cosmic microwave background resulting in signals potentially detectable across the soft to hard X-ray energy band. Limits on this dark matter annihilation (DMA) emission have been studied thus far in the Galactic Center [2], extragalactic gamma ray background [3], clusters of galaxies [4] and by focusing on dwarf galaxies in the Milky Way halo [5,6]. Searching for signatures of secondary gamma rays and decay prod-ucts lends insight into the thermally averaged annihilation cross section and mass of the dark matter particle. The first radio galaxy detected in the TeV energy range, M87 is one of the best studied in its source class with a jet resolved by optical, radio and X-ray observations. This cD galaxy is the nearest radio galaxy emitting very high energy (VHE) gamma rays and is known for its proximity to the Earth (D = 16 Mpc) and exceptionally bright arcsecond scale jet [7]. M87 is an excellent laboratory in which to study dark matter because of its location in the central high mass density region of the Virgo cluster. In this work, the imprint of WIMP annihilation radiation from the halo structure of M87 in the form of prompt and inverse-Compton gamma rays is investigated, and dark matter particles are constrained, via a multiwavelength analysis of the spectral energy distribution of M87.

Observations from the Chandra X-ray Observatory, Fermi Large Area Telescope (Fermi-LAT), Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (MAGIC), and High Energy Stereoscopic System (H.E.S.S.) Cherenkov array are used in this analysis. In contrast to the model presented by Abdo et al. [7], historical data in the lowenergy range from the radio-to-optical regime which can be contaminated by dust and starlight and are thought to originate from different regimes in the source than the VHE- emission are not included in the fit but rather used as benchmarks that the emission should not exceed. Although noncontemporaneous, the data set represents a long-term average by excluding significant flaring events and thus represents the steady low state spectral energy distribution of high-energy emission in M87. The Synchrotron Self Compton (SSC) process occurs when electrons accelerated in the magnetized plasma jet emerging from the ergosphere of a central black hole reach ultrarelativistic speeds and emit synchrotron photons which then inverse-Compton scatter off the original source population of electrons. This generates a high-energy spectrum that extends into the TeV regime. The Synchrotron Self Compton model of Rüger et al. [8] is applied here to the high-energy data with a focus near the upper cut-off of the spectrum so that the Compton scattering cross section is treated appropriately in the Klein-Nishina regime. This model has relativistic electrons injected into a spherical emission region with a randomly oriented magnetic field, moving up the jet with relativistic speed. Taking a bulk Lorentz factor Γ = 2.3 (Doppler factor δ = 3.9) as in [7] we vary other parameters so the maximum Lorentz factor Γ max = 10 8 , magnetic field B = 3 G, normalization factor K = 10 6 cm -3 s -1 , and the radius of the emitting source R = 3.5 × 10 13 cm to obtain a differential slope of s = 2.2 for the injected electron power law distribution with exponential cut-off.

A dark matter model is then added to this SSC model. When dark matter neutralinos annihilate, they produce heavy quarks, leptons and W bosons. During their subsequent hadronization, they decay mainly into pions. Prompt pion emission is the emittance of very high energy gamma rays from the decay of neutral pions. The charged pions decay into electrons and positrons which can up-scatter cosmic microwave background photons through the inverse-Compton mechanism also to very high energies. This model is referred to here as SSC+DMA.

The differential flux of gamma rays produced by the decay of annihilation products in M87 is given by Equation (1). σ A ν is the thermally averaged annihilation cross section, m χ the mass of th

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