Searches for DM signals from the Galactic Centre region with H.E.S.S

📝 Abstract

The H.E.S.S. experiment, an array of four Imaging Atmospheric Cherenkov Telescopes, observes the Galactic Centre (GC) region in the very high-energy (VHE, E > 100 GeV) domain since 2003. The GC is believed to be the region with the highest Dark Matter (DM) density in our Galaxy, thus making it one of the primary targets for VHE gamma-ray observations, looking for signals from annihilation or decay of DM particles. The interpretation of the collected H.E.S.S. data with regard to possible DM signals is however complicated by the presence of several astrophysical sources of VHE gamma-ray radiation in this region. The current status of the search for DM signals from the GC region is reviewed.

💡 Analysis

The H.E.S.S. experiment, an array of four Imaging Atmospheric Cherenkov Telescopes, observes the Galactic Centre (GC) region in the very high-energy (VHE, E > 100 GeV) domain since 2003. The GC is believed to be the region with the highest Dark Matter (DM) density in our Galaxy, thus making it one of the primary targets for VHE gamma-ray observations, looking for signals from annihilation or decay of DM particles. The interpretation of the collected H.E.S.S. data with regard to possible DM signals is however complicated by the presence of several astrophysical sources of VHE gamma-ray radiation in this region. The current status of the search for DM signals from the GC region is reviewed.

📄 Content

arXiv:1106.2752v1 [astro-ph.HE] 14 Jun 2011 Searches for DM signals from the Galactic Centre region with H.E.S.S. Daniil NEKRASSOV∗ Max-Planck Instute for Nuclear Physics, P.O. Box 103980, D 69029 Heidelberg, Germany E-mail: daniil.nekrassov@mpi-hd.mpg.de for the H.E.S.S. Collaboration http://www.mpi-hd.mpg.de/hfm/HESS/ The H.E.S.S. experiment, an array of four Imaging Atmospheric Cherenkov Telescopes, observes the Galactic Centre (GC) region in the very high-energy (VHE, E > 100 GeV) domain since 2003. The GC is believed to be the region with the highest Dark Matter (DM) density in our Galaxy, thus making it one of the primary targets for VHE gamma-ray observations, looking for signals from annihilation or decay of DM particles. The interpretation of the collected H.E.S.S. data with regard to possible DM signals is however complicated by the presence of several astrophysical sources of VHE gamma-ray radiation in this region. The current status of the search for DM signals from the GC region is reviewed. 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/ Searches for DM signals from the Galactic Centre region with H.E.S.S. Daniil NEKRASSOV

1. Introduction H.E.S.S. (High Energy Stereoscopic System) is an array of four Imaging Atmospheric Cherenkov Telescopes (IACTs), located in the Khomas Highlands of Namibia at an altitude of 1800 m above sea level. Each telescope comprises 382 round mirrors, mounted in a Davies-Cotton design [1] with a total mirror area of ∼107 m and a focal length of 15.28 m [2]. The mirrors are collecting Cherenkov light, which is emitted by charged constituents of a particle cascade, initiated by the interaction of primary γ-rays in the Earth’s upper atmosphere. The light is subsequently reflected onto a camera, that is sensitive and fast enough to detect such weak Cherenkov flashes of 10 ns duration. The cameras consist of 960 highly sensitive photo-multiplier tubes (PMTs), which pro- vide the needed sensitivity and time resolution. In order to reduce light losses, Winston cones are installed in front of each PMT, focusing the incident light into the active volume of PMTs. The angular size of each PMT is 0.16◦[3], thus the total field of view of a H.E.S.S. camera accounts to 5◦, adapted for observations of extended γ-ray sources. Thanks to the stereoscopic technique allowing for an accurate reconstruction of the direction and energy of the primary gamma-rays as well as for an efficient rejection of the background induced by cosmic-ray interactions, the energy threshold is about 100 GeV at zenith and the angular resolution is around 0.1◦per γ-ray. The point source sensitivity is 2×10−13 cm−2 s−1 above 1 TeV for a 5σ detection in 25 hours [4]. The H.E.S.S. experiment participates in the indirect search for DM, which most probably consists of a new type of particles. Such particles are predicted, e.g., by currently existing theories beyond the Standard Model (SM) of particle physics (see, e.g., [5] and references therein), like the Supersymmetry (SUSY). In SUSY, the corresponding WIMP (Weakly Interacting Massive Particle) candidate for DM is the lightest neutral particle, most commonly the neutralino χ0, with the mass mχ between ∼100 GeV and a few TeV. The self-annihilation (or decay in models, where the R-parity is not conserved [5]) of such WIMPs, mediated by the weak interaction, would create SM particles in the final state, also including γ-rays, produced mostly in hadronization processes. Since the spectrum of such γ-rays would extend up to the WIMP mass, these should be in principle detectable in the VHE domain. For the case of self-annihilations, the expected γ-ray flux from a certain sky region can be calculated using the following relation: dΦ(∆Ω,Eγ) dEγ = 1 4π ⟨σv⟩ m2χ dN dEγ × ¯J∆Ω, where ¯J = 1 ∆Ω Z ∆Ω Z line-of-sight ρ2(r)ds. (1.1) Here, ⟨σv⟩is the velocity-weighted annihilation cross-section of the corresponding DM can- didate and dN dEγ is the spectrum of γ-rays that can be expected from such annihilations. Both terms entirely depend on the underlying particle physics model. ¯J is the averaged line-of-sight integral of the squared DM density ρ(r) and thus represents the astrophysical input in this relation. The latter term depends on the selected observation targets, of which one of the most promising is the Galactic Centre (GC) region, since to date, it is the closest confirmed region of high DM density (see [7], [8] for results from recent N-body simulations). Deep VHE γ-ray observations were per- formed by H.E.S.S. in this region and various studies of these data in the DM context are described in the following. 2 Searches for DM signals from the Galactic Centre region with H.E.S.S. Daniil NEKRASSOV
2. H.E.S.S. results on the GC region The GC region has been observed by the H.

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