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MAGIC GAMMA-RAY TELESCOPE OBSERVATION OF THE PERSEUS CLUSTER OF GALAXIES: IMPLICATIONS FOR COSMIC RAYS, DARK MATTER, AND NGC 1275
Stockholm University, Faculty of Science, Department of Physics.
2010 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 710, no 1, 634-647 p.Article in journal (Refereed) Published
Abstract [en]

The Perseus galaxy cluster was observed by the MAGIC Cherenkov telescope for a total effective time of 24.4 hr during 2008 November and December. The resulting upper limits on the gamma-ray emission above 100 GeV are in the range of 4.6–7.5 × 10^{−12} cm^{−2} s^{−1} for spectral indices from −1.5 to −2.5, thereby constraining the emission produced by cosmic rays, dark matter annihilations, and the central radio galaxy NGC 1275. Results are compatible with cosmological cluster simulations for the cosmic-ray-induced gamma-ray emission, constraining the average cosmic ray-to-thermal pressure to <4% for the cluster core region (<8% for the entire cluster). Using simplified assumptions adopted in earlier work (a power-law spectrum with an index of −2.1, constant cosmic ray-to-thermal pressure for the peripheral cluster regions while accounting for the adiabatic contraction during the cooling flow formation), we would limit the ratio of cosmic ray-to-thermal energy to ECR/Eth < 3%. Improving the sensitivity of this observation by a factor of about 7 will enable us to scrutinize the hadronic model for the Perseus radio mini-halo: a non-detection of gamma-ray emission at this level implies cosmic ray fluxes that are too small to produce enough electrons through hadronic interactions with the ambient gas protons to explain the observed synchrotron emission. The upper limit also translates into a level of gamma-ray emission from possible annihilations of the cluster dark matter (the dominant mass component) that is consistent with boost factors of ∼10^{4} for the typically expected dark matter annihilation-induced emission. Finally, the upper limits obtained for the gamma-ray emission of the central radio galaxy NGC 1275 are consistent with the recent detection by the Fermi-LAT satellite. Due to the extremely large Doppler factors required for the jet, a one-zone synchrotron self-Compton model is implausible in this case. We reproduce the observed spectral energy density by using the structured jet (spine-layer) model which has previously been adopted to explain the high-energy emission of radio galaxies.

Place, publisher, year, edition, pages
2010. Vol. 710, no 1, 634-647 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-42441DOI: 10.1088/0004-637X/710/1/634ISI: 000273850800049OAI: oai:DiVA.org:su-42441DiVA: diva2:346361
Note
30 authorsAvailable from: 2010-09-01 Created: 2010-08-31 Last updated: 2011-11-23Bibliographically approved
In thesis
1. Gamma-Ray Emission from Galaxy Clusters: DARK MATTER AND COSMIC-RAYS
Open this publication in new window or tab >>Gamma-Ray Emission from Galaxy Clusters: DARK MATTER AND COSMIC-RAYS
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The quest for the first detection of a galaxy cluster in the high energy gamma-ray regime is ongoing, and even though clusters are observed in several other wave-bands, there is still no firm detection in gamma-rays.

To complement the observational efforts we estimate the gamma-ray contributions from both annihilating dark matter and cosmic-ray (CR) proton as well as CR electron induced emission.

Using high-resolution simulations of galaxy clusters, we find a universal concave shaped CR proton spectrum independent of the simulated galaxy cluster. Specifically, the gamma-ray spectra from decaying neutral pions, which are produced by CR protons, dominate the cluster emission. Furthermore, based on our derived flux and luminosity functions, we identify the galaxy clusters with the brightest galaxy clusters in gamma-rays. While this emission is challenging to detect using the Fermi satellite, major observations with Cherenkov telescopes in the near future may put important constraints on the CR physics in clusters.

To extend these predictions, we use a dark matter model that fits the recent electron and positron data from Fermi, PAMELA, and H.E.S.S. with remarkable precision, and make predictions about the expected gamma-ray flux from nearby clusters. In order to remain consistent with the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures for cold dark matter halos. In addition, we find comparable levels of gamma-ray emission from CR interactions and dark matter annihilations without Sommerfeld enhancement.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2010. 117 p.
Keyword
Galaxy clusters, gamma-rays, cosmic-rays, dark matter
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-42453 (URN)978-91-7447-141-0 (ISBN)
Public defence
2010-10-01, FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted.Available from: 2010-09-09 Created: 2010-08-31 Last updated: 2010-09-03Bibliographically approved

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