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Searching for Gamma Rays from Galaxy Clusters with the Fermi Large Area Telescope: Cosmic Rays and Dark Matter
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this licentiate thesis, I report a search for GeV γ rays towards the location of Galaxy clusters. I mainly discuss the results of a search for cosmic-ray (CR) induced γ-ray emission but also briefly elaborate on a related study, searching for Dark Matter (DM)-induced γ-ray emission from Galaxy clusters. In addition, I provide a detailed discussion on the analysis tools that were used and discuss some additional tests that are not included in the papers this licentiate thesis is based on.

In a comprehensive search almost covering the entire sky, we find no statistically significant evidence for either DM or CR induced γ rays from galaxy clusters. Thus we report upper limits on CR quantities that exclude emission scenarios in which the maximum hadronic injection efficiency is larger than 21% and associated limits on the maximum CR-to-thermal pressure ratio, <XCR>. In addition, we update previous flux upper limits given a new set of modeling and taking the source extension into account. For a DM masses below 100 GeV, we exclude annihilation cross sections above ∼ 10−24 cm3 s−1 into bb. For decaying DM, we exclude decay times lower than 1027 s over the mass range of 20 GeV– 2 TeV.

Place, publisher, year, edition, pages
Stockholm: Stockholm University, 2013. , 57 p.
Keyword [en]
galaxy clusters, gamma rays, cosmic rays, dark matter, fermi-lat, likelihood analysis, joint likelihood
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-96231OAI: oai:DiVA.org:su-96231DiVA: diva2:665822
Presentation
2013-11-14, FA 32, Albanova University Center, Fysikum, Stockholm University, 13:15 (English)
Opponent
Supervisors
Available from: 2013-11-21 Created: 2013-11-15 Last updated: 2013-11-21Bibliographically approved
List of papers
1. A Combined Analysis of Clusters of Galaxies - Gamma Ray Emission from Cosmic Rays and Dark Matter
Open this publication in new window or tab >>A Combined Analysis of Clusters of Galaxies - Gamma Ray Emission from Cosmic Rays and Dark Matter
2011 (English)In: eConf Proceedings C110509, 2011Conference paper, Published paper (Refereed)
Abstract [en]

Multiwavelength observations suggest that clusters are reservoirs of vast amounts relativistic electrons and positrons that are either injected into and accelerated directly in the intra-cluster medium, or produced as secondary pairs by cosmic ray ions scattering on ambient protons. In these possible scenarios gamma rays are produced either through electrons upscattering low-energy photons or by decay of neutral pions produced by hadronic interactions. In addition, the high mass-to-light ratios in clusters in combination with considerable Dark Matter (DM) overdensities makes them interesting targets for indirect DM searches with gamma rays. The resulting signals are different from known point sources or from diffuse emission and could possibly be detected with the Fermi-LAT. Both WIMP annihilation/decay spectra and cosmic ray induced emission are determined by universal parameters, which make a combined statistical likelihood analysis feasible. We present initial results of this analysis leading to limits on the DM annihilation cross section or decay time and on the hadron injection efficiency.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-94983 (URN)
Conference
2011 Fermi Symposium, Roma, May 9-12
Available from: 2013-10-19 Created: 2013-10-19 Last updated: 2013-11-21Bibliographically approved
2. Search for cosmic-ray induced γ-ray emission in Galaxy Clusters
Open this publication in new window or tab >>Search for cosmic-ray induced γ-ray emission in Galaxy Clusters
(English)In: The Astrophysical JournalArticle in journal (Refereed) Submitted
Abstract [en]

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into   rays, that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended  -ray emission at the locations of 50 galaxy clusters in 4 years of Fermi-LAT data under the assumption of the universal cosmic-ray model proposed by Pinzke & Pfrommer (2010). We find an excess at a significance of  , which upon closer inspection is however correlated to individual excess emission towards three galaxy clusters: Abell 400, Abell 1367 and Abell 3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters we exclude hadronic injection efficiencies in simple hadronic models above 25% and establish limits on the cosmic-ray to thermal pressure ratio within the virial radius,    , to be below 0.012-0.014 depending on the morphological classification. In addition we derive new limits on the  -ray flux from individual clusters in our sample.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-94984 (URN)
Available from: 2013-10-19 Created: 2013-10-19 Last updated: 2013-11-21

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Astronomy, Astrophysics and Cosmology

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